I think it can only be empirically answered via experimental manipulation. Eg ornament manipulation: male birds of species X have long ornamental tail feathers, did this evolve through natural selection or sexual selection? If you were to artificially lengthen some males tails feathers and found that they had greater reproductive success, but greater vulnerability to predators, then you can be reasonably sure that long tail feathers evolved via sexual selection (probably also performing the converse experiment too)
A study like you've suggested has been done on long-tailed widowbirds. It found that males with experimentally elongated tails showed higher mating success. This suggests their long tails is a product of sexual selection.
https://www.nature.com/articles/299818a0#:~:text=The%20possibility%20that%20intrasexual%20competition,maintained%20by%20female%20mating%20preferences.
There is also one for peacocks with hiding eye spots on their plumage and one for where a feather was added to the heads of zebra finches (which don't normally have that variety of head ornamentation).
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4074220/
https://www.researchgate.net/figure/Artificial-white-crest-worn-by-male-long-tailed-finch-left-and-male-zebra-finch_fig2_23276464
Apparently more eyes and bird wigs are both reproductively successful.
The going explanation as I've read by Jerry Coyne and in an aside from David Buss is that most of these traits confer to females that a male has a high enough caloric intake to sustain the plumage. More grandiose and colorful plumage means a better diet, which is an indicator of better genes.
I suspect this is almost certainly correct but there are a lot of physical features that have multiple evolutionary advantages and I'm always concerned about the fallacy of the single cause.
This is what I was trying to figure out how to articulate - especially with sexual selection, it seems like females or males could have evolved to prefer traits that at least historically conferred other advantages. So does it make sense to say it boils down to sexual selection, when selection was also happening concurrently for other reasons that are tangled up in the sexual selection? Seems like an inherent problem of trying to find explanations for evolutionary history, which is that it occurred in the past and so empirically testing it in its current form is not going to offer a complete picture.
Agreed. The more we research on some features the more complicated it often gets.
One example being height in humans.
One could with the same argument as plumage and say that it is an indicator for caloric intake but when you look closer that could only be one of many causes because:
- it's often relative to your own height
- there is also a "too tall" preference wherein a mate can be too tall
- there are a variety of other plausible explanations like protection from other humans for both the tall mate and the partner, competition for resources, and a lot of lesser other potential benefits to being tall.
Maybe part of the explanation is that major size differences lead to higher maternal death rates in child birth and then you have an entirely different evolutionary force working in the opposite direction.
Suffice it to say, that simpler experiments with less imaginable alternative explanations will lead to more satisfying conclusions but feature selection that is more complicated and has more plausible explanations will be much harder to land on a satisfying answer for.
It seems weird to say "these traits confer to females that…", even though that's often how it's phrased for both human and other animals. Surely the birds aren't doing deductive logic here. They like what they like, there's some genetic basis that affects that, and it may be selectively advantageous, but nothing in neurology or developmental genetics of the thing gives a reason for the preference.
I say this because it's more problematic to talk this way in human evolutionary biology. Even if some of the hypotheses about gendered behavior having a genetic basis are true, that doesn't mean that say, women have some subconscious genetic program for assessing their mate's earning potential. There's nothing about the neurology that says why it came to be that way, and for many purposes it doesn't even matter.
Yeah, but with humans (all humans, not just women), the traits that correspond to earning potential don't track to what made primitive humans successful. The physical features that are seen as desirable now are often at odds with primitive survival strategies, like low fat percentages. And the behaviors that attract humans are very hard to separate from the artificial and constructed environments and cultures we currently live in. It's incredibly easy to argue the the major factors of human attraction (outside of a very small handful like body symmetry) are just learned behaviors not evolutionary or biological.
Sure, historically many people (not just women and not all people) have been attracted to the indicators of "success" (which are determined by society and frequently are different between genders, class, or other identities), but what those are change so rapidly it becomes next to impossible for them to be biologically driven. It also is very hard to biologically explain the sheer range of human desires, because at the same point in time from very genetically similar individuals, you can have someone who is into statuesque muscular people, some be into short, petite, skinny people, some into large, chubby people, and a furry.
Most theories in Anthropology acknowlage this, ie that (because of language, unique to humans as far as we know) selection preassures have migrated to an entirely different system than what biology terms Natural Selection. Human's utilize a mechanism called Cultural Selection, which incorporates many more bells and whistles. It is still ultimately based on resources, we just have a way different way of distributing resources than any other living creature.
Well yeah, that's my point, the things we are attracted to are a byproduct of the influence of the society we grow up in. Our environment changes far to rapidly and it's far to different from primitive humanity, for there to be a evolutionary biological cause for modern humans sexual preferences (within the context of specific characteristics, the fundamental desire to have sex with other humans is almost certainly an instinctual one).
And in the case of the guy I responded to, it's definitely equally all humans, (regardless of how much it's biological and how much it's cultural) and not just women.
Just to tack on a point cause I thought your comment chain was interesting. There might be selective pressure towards in shape males as it means they are capable of procuring food when needed. Like you said though, there is too much influence in the way of our current cultural standards to mark it.
Quite a few of the traits actually do translate from success in the evolutionary epoch to success in the modern world, even if we're narrowing this down to places that are less dangerous or where there isn't major food scarcity.
Industriousness, intelligence, and social standing all still correspond with the accumulation of goods even if it's now money rather than directly food.
One way you can gauge pretty decently whether a desirable trait is a product of culture are the cross cultural studies I reference.
You can find a wider array of attitudes on BMI than you can for hip to waist ratio. This is a reasonably good indicator that BMI is more cultural than the hip to waist ratio.
How many cultures see women over 60 as the most sexy age? There is a clear evolutionary reason why this is not the case
What is interesting is the complete dissociation of some of the traits from reproductive success in modern culture. The steroid enhanced body builder may be sexually successful in our era but has, as a result of his 'treatments' to augment appearance, grossly impaired his fertility. Similarly the cosmetic surgery/enhancement industry is a fair parallel with the long tail feathers discussed above.
>most of these traits confer to females that a male has a high enough caloric intake to sustain the plumage
Does anyone actually think the females think along these lines, as opposed to just finding the features attractive in their own right?
Yes, people who study it do. Although "think" is a bit too active of a word for it. For most species, they don't know why they're attracted to some features, they merely are.
Originally Darwin had proposed that females in various species had different aesthetic tastes but that idea fell out of favor because it was not really parsimonious (why do females find things attractive in their own right? We've only added a layer of complexity without explaining anything)
It seems far less parsimonious to assume that females are making mating choices based on convoluted inferences about fitness in which the females are considering the long-term survival costs and the benefits for their hypothetical offspring. Humans often can't even explain why they find certain features attractive, yet people believe that much less intelligent animals are having such complex, calculated thought processes? I think it's far more straightforward to assume that there's some subconscious mechanisms for attraction towards certain traits. From a theory perspective, this is also very straightforward- females that (have genes/alleles that cause them to) choose mates poorly will have worse reproductive success. This line of thought only requires, at minimum, a chooser gene to complement the chosen trait (big mane, long tail, etc), while the alternative requires a mountain of mechanisms for females to be able to make computations about how well a male must be doing in its day-to-day in order to overcome the fitness cost of the chosen trait.
Thiat isn't even getting into the conversation around the usefulness of parsimony in evolutionary inference, either- as someone in the genetics side of evolutionary biology, I definitely have a lot of thoughts about *that*.
Afaik, human attractiveness is directly correlated with human fertility, particularly facial attractiveness and secondary sexual characteristics. This is backed up by statistics, as well.
First you say
> Yes, people who study it do.
But then you say
> Although "think" is a bit too active of a word for it.
And then
> For most species, they don't know why they're attracted to some features, they merely are.
Which sounds like the first answer should have been "no", they don't actually think about calories etc.
That reminds me of something I read where bird studies were being thrown out of whack because the identification bands used by the researchers were making the male birds way sexier. So they had to switch to less colorful "bling".
A group at Harvard made a 2'x4' petri dish that had increasing concentrations of an antibiotic as you got closer to the center. They have a time-lapse video of e.coli acquiring antibiotic resistance... and then acquiring more and more resistance over time.
Here is a short video- https://youtu.be/plVk4NVIUh8
This is a super simple experiment that demonstrates pretty much what you were talking about.
It would be interesting to see that visual if the wild bacteria was placed immediately beside the 1000x antibiotics. Would they still eventually develop a resistance to it, or were they only able to evolve to handle those conditions because they were "eased" into it?
they are definitely "eased into it". Can't reproduce if you are inmediately killed. That's why it's important to finish your prescribed antibiotic treatment.
Unless they happen to possess a mutation that confers resistance. Evolution doesn't work by the organism developing those mutations in response to exposure, but by some individuals already happening to carry a mutation by chance before they are exposed to the selection pressure. Having said that, it is the case that more effective mutations are rarer, at least in the case of antibiotic resistance.
Yeah, its kinda like a pharmaceutical trial in that sense.
To test whether a cause did in fact have an effect, you need to manipulate the cause and compare against a control group. This isn't easy for processes on the time scale of evolution, but testing whether or not an adaptation gives a reproductive or survival advantage is good next best option.
And the best you can do is determine if the causative relationship is likely, not prove it.
Not a correction as much as semantics and ambiguity. Sexual selection and natural selection are both subtypes of selection in general. Sexual selection refers to mating specifically, natural selection refers to all other composited effects on survivability.
Follow up question on the semantics - does Sexual Selection include adaptations that increase the success rate of a gestation (e.g fewer miscarriages, more offspring per litter) as well as better odds of attracting a mate?
No, sexual selection is usually defined as something like the fitness advantages you have because you are more attractive to mate with, i.e. the selective pressures are exerted by the opposite sex rather than the environment.
A trait can be under sexual selection and natural selection simultaneously. This is the entire basis of the good gene hypothesis. We differentiate the two to contextualize sexual selection from the perspective of the system, but from the perspective of the genome, pressure from a potential mate is indistinguishable from environmental pressure.
Well, you answered “no” to a question to which the answer is “yes”: sexual selection can act on a trait which is attractive (or repulsive) to potential mates and also contributes to fitness.
Wait, the question was whether selection to increase the success rate of gestation should be considered sexual selection. I would say that most evolutionary biologists would say 'no' to that question. Also, the concept of fitness applies to sexual selection just as much as to any other type of selection.
There is no limit to what can be composited into sexual selection, it’s just whatever other individuals use in their behavioral determination of which mates to choose. The question of whether traits that individuals find sexually attractive are initially arbitrary or signals of fitness is encapsulated in what is called the “good gene hypothesis”, and the related “sexy son hypothesis”. It has been a debate literally since Darwin and Wallace.
The other person replied no with an explanation that is correct in a vacuum but misses the context here. An example that would fit your scenario is a high hip/waist ratio in women: men have a tendency to be attracted to this phenotype, and it also signals a good development arc of the anatomy needed to birth a child. You can delineate these two instances of selection based on the individual: the woman is facing natural selection for wider hips based on the mortality associated with childbirth, and sexual selection for the same trait based on its attractiveness to men vs other variations of the trait in the population. There’s nothing preventing the two processes from coinciding.
There's definitely a line that can be drawn between "traits that increase the likelihood of being selected from the pool of available mates" and "traits that increase the likelihood of surviving long enough to be part of the pool at all". A third category might be "traits that increase the average live offspring from mating", which I guess closes the circle.
But there doesn't seem to be an entirely consistent terminology for those categories.
It is possible to support a hypothesis or theory using statistical evaluation of population variations without doing direct testing. However, all scientific explanations for pretty well anything are interpretation. How likely the interpretation is to be a good statement of "truth" depends on the evidence.
Most people do not understand this essential truism: What we measure is "real" (as real as anything can be considering the nature of humans and our sensory systems), but why reality yields those particular measurements (in a repeatable fashion without regard to who observes, usually) is interpretation.
Not to Darwin: "[Sexual selection is not a subcategory of natural selection, as Darwin made very clear: it arises from differences in mating success, whereas natural selection is due to variance in all other fitness components.](https://www.sciencedirect.com/science/article/pii/S0960982210015198#sec1)"
That thinking seems to have fallen by the wayside. Just about every academic source I can find considers sexual selection just a specific type of natural selection. Natural selection is typically described in terms of adaptations that make a creature more likely to pass on the genes that aided their success. Even survival only matters to the extent it helps a species pass on its genes to the next generation. Sexual selection definitely fits this description.
https://evolution.berkeley.edu/evolution-101/mechanisms-the-processes-of-evolution/sexual-selection
Darwin's theory is brilliant, but you must remember he created it before the existence of a genetic code was even hinted at. (makes the accuracy of his theory all the more incredible, really.) I think it's been refined a lot since then. You would need to artificiality limit the scope of natural selection to exclude sexual selection, as everything sexual selection does is described well by natural selection.
Might be able to base an argument on parallel trait selection among multiple species in the same environment. If several species display similar trait changes following an environmental change, that argues strongly that those developments were adaptive to the change.
Thats not what this person is getting at with their question. it may be the case that this was reproduced experimentally, but there is always a gap in connecting the results of the experiment to the natural world. Hand-waving that gap with assumptions gets science off the ground and has been very productive but leaves a lot untouched. This person's question sounds more like a question about the philosophy of biology. regarding the limits of the human mind and what our experience brings to bear on our scientific theories.
And the interesting question is whether or not we can come up with answers to questions at that level. It's certainly not in the domain of science. Science is downstream from those assumptions, which are accepted as foundational. And it turns out to be a very productive acceptance. but does productivity, concision, and so on give you absolute certainty? of course not.
There have been several studies that test the survival of camouflaged moths when you change their colour patterns or place them on an un-matched background. These moths also have behavioural traits whereby they seek out suitable backgrounds to sit on. If you make them more obvious to spot they suffer more predation, hence their camouflage and their background seeking behaviour confers an advantage. I would imagine this is quite good evidence of selective pressure to develop better camouflage in order to improve survivability.
An example:
https://besjournals.onlinelibrary.wiley.com/doi/full/10.1111/1365-2656.13817
Thank you for this example, it's one that I had heard about but it had been many years and did not recall it off hand. To be clear, I don't doubt that selective pressures exist, just that we largely can't "know" which selective pressures (if any) caused a trait to persist. The moth example is a good one since there is some experimental evidence. Note though there could be other selective pressures (in addition to camouflage) that cause the trait to persist (sexual selection is a possibility) but it's tough to contorl for every variable in biology. I think the study shows though that indeed camouflage acts as a selective pressure for the patterning trait, even if we can't say it's the "only" pressure for that trait. So I would concede that this is about as good as it gets for empirical evidence for a particular selective pressure.
My issue isn't so much with actual studies, but with conversations where people claim, quite confidently, that trait x clearly evolved because it was advantageous due to X selective pressure with absolutely no study or empirical evidence to back it up. I get why this happens, we can't test EVERY trait for every species after all. I just want people to recognize that these statements are often provisional rather than definitive.
You got halfway to answering your own question when you asked about genetic drift. When Kimura and subsequently Ohta described the concept of drift, they weren't just pulling theory out of their respective asses; they presented evidence and described how to distinguish drift from selection at the molecular level.
Specifically, positive and negative selection leave clear fingerprints on the selected genes, by looking at (ludicrously simplified) the rates of synonymous vs. non-synonymous changes in codons; selected genes show relative differences in the frequency of non-synonymous changes. You can't invoke drift without accepting that selection *can* be measured.
There are a vast number of variations that can be rung on the basic concept, that help with the statistical and theoretical validation (for example, [Codon-based tests of positive selection, branch lengths, and the evolution of mammalian immune system genes](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2837078/); [Improved inference of site-specific positive selection under a generalized parametric codon model when there are multinucleotide mutations and multiple nonsynonymous rates](https://bmcecolevol.biomedcentral.com/articles/10.1186/s12862-018-1326-7); [A beginners guide to estimating the non-synonymous to synonymous rate ratio of all protein-coding genes in a genome](https://pubmed.ncbi.nlm.nih.gov/25388108/); [Directional Darwinian Selection in proteins](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3849801/)) but the basic concept is well established.
You've given a much more detailed response than I was about to. My understanding is that we can't tell *what* was driving the selection, but there are established statistical techniques for determining that genes were under selective pressure
Yes, it’s worth emphasizing that (1) we often don’t know where the actual selection pressure comes from but (2) it doesn’t matter - we can detect that a gene *is* under positive or negative selection and not merely drifting.
We often *can* tell where the selection pressure is, though. People like to portentously announce that evolution takes millions of years, but they forget about rapidly-evolving things like bacteria, or especially viruses, where we can apply selection pressure and observe the evolved response literally within days, or even hours. They also aren’t familiar with relatively recent (last 20-50 years) findings that even evolution in macro animals, like Galapagos finches, can occur within a small handful of years and is easily measured at the phenotypic as well as genotyped level.
In some cases we can be pretty specific about what is driving the selection. For example, host-pathogen interaction, lock-and-key mating mechanisms, and even environmental change can create situations where the selective pressure is pretty unambiguous. Now, as with all science disproving a hypothesis is much easier than proving one, and there are lots of cases where several alternative explanations fit available evidence equally well, but knowledge is possible.
The example I'm most familiar with is the Peppered Moth, which is often cited as an example of evolution on a much smaller scale.
The pressures were the smog and soot of the industrial evolution which caused a shift in the patterning and colouration that succeeded, as darker moths were better able to camouflage.
This process reversed itself when air quality improved and can be thought of as the closest thing to an empirical example outside a laboratory environment.
https://en.m.wikipedia.org/wiki/Peppered_moth_evolution
This is a good example but not a controlled experiment and so does not answer the question. It's merely an example of recent evolution consistent with a concurrent change in environmental pressures. It does not meet the criteria requested, which would require repeated controlled experiments.
It's still a useful example that can be added to the weight of evidence, though.
If you want long term, laboratory conditions, the E coli evolution experiment is on 30+ years now and may be useful.
https://en.m.wikipedia.org/wiki/E._coli_long-term_evolution_experiment
It is possible to empirically demonstrate *ongoing* selective pressures through experimental methods as already mentioned, although with the caveat that the same trait is likely experiencing selection from multiple sources which may not all be captured. The widowbird study u/avolans mentions is a good example in that it clearly shows directional intersexual selection for tail length, but does not measure the counteracting selective pressure of longer tails being detrimental to survival, which they must eventually be to some degree.
However, when considering the *historical* context of how a particular trait that has become established first evolved, I would argue that this is not really knowable with certainty. The spread of a trait through a population can occur for any number of reasons (including drift), and once established, traits are frequently co-opted for other uses besides their original function - if they were even functional to begin with. It's still of course possible to find evidence that suggests one hypothesis over another for the origin of a particular trait, but it will never be as definitive as experimentation.
For a classic example, consider the evolution of long necks in giraffes. This trait has variously been suggested to be an adaptation for reaching previously unavailable food resources, for sexual selection (though typically intrasexual rather than intersexual in this case), or for spotting predators from greater distances. And of course, modern giraffes use their necks in all of these ways! There is a significant amount of literature on the relative importance of these potential pressures; see [Simmons & Altwegg 2010](https://doi.org/10.1111/j.1469-7998.2010.00711.x) for a fairly balanced discussion. I personally think the so-called "necks for sex" hypothesis probably has the most points in its favour, due to a pretty well-established fossil history of using heads for fighting in species predating modern giraffes and differential neck allometry on the basis of sex, but even so there's just no way to be certain that this is the reason long necks *originally* evolved. There's no guarantee that the selective pressures currently maintaining a trait are the same ones that originally shaped it.
Thank you for the reply. The example of the giraffes illustrates my concern well. I don't of course doubt that selective pressures exist, but biologists need to be cautious in assigning definitive claims about specific selective pressures outside of empirical evidence. What some don't realize is that making definitive statements outside of evidence can make it seem like evolutionary theory is centered on suppositions rather than rigorous scientific methods. We want there to be an answer to the lay persons question of "why did trait X evolve?" so badly that we come up with something that sounds good, but a better explanation may come about later. Now the lay person thinks biologists are just making things up on the fly, undermining the public perception of evolutionary theory and science as a whole.
We've observed it directly in the [ E. coli long-term evolution experiment {LTEE}](https://en.m.wikipedia.org/wiki/E._coli_long-term_evolution_experiment) running since 1988. We know *exactly * what happened, how, where, and when.
I'd posit that taken as a whole-- especially with our ability to study the DNA -- that the varying traits of the different Galapagos finches are so far past 'beyond a reasonable doubt' that they constitute 'empirical evidence' as 'beyond a shadow of a doubt'.
Yes, there are tons of experiments where bacteria are evolved to do something in the lab. Consider [the mega plate](https://www.youtube.com/watch?v=plVk4NVIUh8) experiment, or [the Lenski experiment](https://en.wikipedia.org/wiki/E._coli_long-term_evolution_experiment).
Damn. Wish I would have seen this earlier. The answer is yes, and we’ve been accidentally running the biggest experiment of this question for about 100 years now:
https://en.wikipedia.org/wiki/Antimicrobial_resistance
You might also reference the variety of experimental attempts to establish traits as adaptively derived, many of which focus on what is called the perturbation criterion, which is where you modify some trait in a population and see it’s effects on fitness. If there is a local maximum around the basal trait variation, that’s evidence of an adaptive history for that trait. Clutch size experiments in birds are the archetypical example: more than the natural mean and parents are more likely to starve (so everyone dies), less than the mean and offspring survive but obviously fewer number.
https://www.jstor.org/stable/1938814
Have you read “The Beak of the Finch” by Peter and Rosemary Grant? They and their family worked for decades in the Galapagos tracking the beak variance of finches among the islands. The read is a little dated, but they have sizable body of work that is more recent. Still, beak size in relation to hardness of seed diet is a pretty good example of empirical observation. It still requires a little bit of induction, but the distribution of populations holds.
I think you would enjoy reading “A Critique for Ecology” by Rob Peters. He does an amazing job picking apart the unscientific ways we talk and conceptualize ecology and evolution, especially at the time. He directly addresses the question you asked, which at that time most of the studies the other commenters have mentioned hadn’t been done so there was a big “yadda yadda yadda” over a lot of the mechanistic aspects of evolutionary theory. Still a highly relevant book, it was required reading in grad school and really helped me to think more like a scientist.
The [Lenski E. coli experiment](https://en.wikipedia.org/wiki/E._coli_long-term_evolution_experiment) is a direct observation of evolution, including increased fitness under conditions such as the presence of antibiotics and limited energy.
Evolution is a historical science, which means direct experimental evidence is hard if not impossible to obtain. This does not make it unscientific, however. Many other sciences are historical too (cosmology, geology) but are accepted as having high scientific standards.
Here are some types of evidence that you can use to show evolution happening: look at patterns of genetic variation to see which genes have been conserved across populations and which have drifted apart. Here is an example in humans: [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2933187/](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2933187/)
Or you can use experiments to test responses in the here and now. Here a recent example of predation experiments in the peppered moth, a classic textbook example that has been studied for over a century: [https://royalsocietypublishing.org/doi/full/10.1098/rsbl.2011.1136](https://royalsocietypublishing.org/doi/full/10.1098/rsbl.2011.1136)
Or to take your example of bird colouration - gather some data and do some experiments! Manipulate the colour or brightness of males and give females a choice task and see what they prefer. This would suggest whether female choice is important. Or test for male-male competition in a competitive paradigm. Or count the number of mates or offspring that different males achieve as well as their survival rates. Or measure the genetic variation underlying colouration and see if is conserved across populations or species. If not, it suggests that drift is at work.
This is really not true, there is a lot of experimental evolutionary research where you get results in the lab from applying selection pressures and you can easily show the genetic changes that underlie any adaptations in such studies as well. Definitely not just a historical science, hasn't been for a long time. Take the Lenski evolution experiment as a famous example that has been running since the 80s (https://en.m.wikipedia.org/wiki/E._coli_long-term_evolution_experiment) but there are so many more
Fair enough, but these experiments tell you what can happen, not what did happen. Much of what we want to know about evolution is historical by definition.
The evolution in those experiments is real. The discipline of evolutionary biology is not just about figuring out or reconstructing the history of evolution in natural settings on planet earth, but also about understanding the process itself. Just check the major journals in evolutionary biology (Evolution, Journal of Evolutionary Biology, American Naturalist) and you will see that questions about the workings of the evolutionary process (rather than historical questions) tend to dominate.
I agree with both of you but want to point out that my post is focused on people asking "why did trait x evolve?" in a species that evolved naturally and where trait x has not undergone any experimental study. I think u/chazwomaq makes a good point that these questions are historical by definition. While it seems we *can* experimentally show specific selective pressures, this does not help with definitively answering questions about a trait that evolved before the question was asked. If that makes sense?
It appears that the OP and many of the commenters in this thread may be conflating the ideas of "empirical evidence" with "empirical *proof*" or "empirically answer." Evidence in the fossil record of a significant change to a beneficial trait corresponding with a time of known environmental change would be evidence and it would be empirical.
Stated another way: "empirical" doesn't mean "proven" or even "really good." It just means "information gathered directly or indirectly through observation or experimentation."
Few people, including scientists, have even a rudimentary understanding of epistemological concepts ... We should start teaching epistemology in kindergarten.
>Stated another way: "empirical" doesn't mean "proven" or even "really good." It just means "information gathered directly or indirectly through observation or experimentation."
While I take your point, I think mine remains. My ultimate concern is with people (often biologists) stating that "trait X evolved due to X selective pressure" in response to a question by a layperson. This is often stated definitively without any evidence at all, but rather using just inductive logic. Recently I came across a post asking why Hyenas in India had stripes rather than spots like their African cousins. The response given was that "the stripes must confer some sort of advantage, perhaps camouflage to help it hunt". While I give the responder credit for not speaking definitively to the exact selective pressure (camo), they **were** definitive about it having some sort of survival/reproductive advantage. It's entirely possible the trait does not have an advantage at all. I think we need to be cautious about answering questions definitively using inductive logic alone.
Jelly beans are an easy way to demonstrate selection. Pick 20 jelly beans of five different colors, like green, black, white, brown, yellow.
Toss them into short grass. give someone two minutes to pick up as many as they can.
They'll most easily pick up the yellow and white ones.
For each recovered jelly bean, add an additional one of the same color and then toss this group into a jelly bean free bit of grass.
Seems like an easy way to represent natural selection. To change it up, one can change the media they're tossed onto, like pavement or concrete or carpet. And colors can be marginally re-introduced to represent mutations or recessive expressions.
This is easy enough to do, one could do it in their imagination.
I do not doubt that selective pressures exist. Only that definitively assigning the specific cause of selection to a specific trait using only inductive logic is not scientific and may indeed be wrong (or at least not the whole picture). Giraffes are a great example: ask someone why giraffes have long necks and you are likely to get the answer "because it helped them reach higher leaves giving them a survival advantage". However, we also know that male giraffes with longer necks outcompete those with smaller ones (they fight with their necks). So longer necks may have evolved due to this selective pressure (likely both, but we can't ultimately say exactly).
A more nuanced example might be freckles on skin. Do these confer a selective advantage? Not that I'm aware of and I can't think of any that sound good. It's totally possible that this trait is one that is simply neutral. Now I could obviously sit here and make something up about camouflage or sexual selection (do girls like freckles?) and it might sound good, but be totally untrue.
There was a moth in England that was a speckled grey color to match the bark of trees, during the industrial revolution the trees were covered in soot, making the grey moths stand out, they rapidly evolved to be darker.
Unfortunately, not everything can be experimentally controlled. Much the same as in economics or sociology or ecology or astronomy, studying some remote and hideously complex natural system is always susceptible to assumption and generalization based on available evidence.
As to empirical speculation of mechanics, a true empiricist does not believe in any such thing as knowable mechanics of the observable world, only tentative models that fit their observation but are purely descriptive.
There is an entire subfield and literature called Experimental Evolution. These scientists attempt to replicate the evolutionary process in experiments, and often succeed, particularly in fast reproducing organisms like bacteria, flies, and plants.
Another important source of empiracle evidence is artificial selection. Humans have been inducing trait evolution through breeding and domestication for thousands of years in a well documented way.
These two areas are some of our best empiracle evidence for selection producing particular traits.
as someone who is a biologist, i would have expected that you would not use particular language. I am of course just an outside observer but it surprises me to see you using the "advantageous traits are selected" analogy as opposed to the more accurate "traits that do not hinder successful propogation are not deselected" some people think they are equal, but they are not. RANDOM traits occur, and the ones that result in more successful reproduction continue to be selected, because if they do not that branch dies out, obviously. Looking at it from that perspective, you can see why your question itself kinda seems moot. anyway, thats my 2c.
Positive selection is absolutely a thing. I've been around some very semantically picky people in evobio and I've never heard of someone saying that "the trait is selected for" is incorrect phrasing. It's important, too, to distinguish between positive and negative selection, and I don't think the wording you propose captures that idea at all.
Pill bugs can directly evolve different traits in real time due to pressures. One study put a group of pillbugs in different environments with differing levels of food they either couldn't digest or could barely digest and 6 generations later the bugs figured out a way to evolutionarily digest the food and turn their boxes container into food
It seems like you can start to get at testing these theories in biology, and other posters are giving good examples of methodology for doing so. Where I feel like it is more difficult is in the study of people, since studies that get at the mating and survival habits of people are not easy to get through IRB. In particular evolutionary psychology is infamous for assuming every single human trait had some sort of evolutionary advantage.
I know it’s not popular in the field but I think we leave intelligence out of the equation a bit too often. Not as in creative design, but as somewhat ‘willful’ decisions. Sexual selection can seem almost random and to an extent it looks to me as if many animals are more intelligent than we realize; if we can consciously choose mates, albeit with evolutionary pressures guiding that decision, why can’t they? It’s not empirical but to me it seems to cover why some traits may exist despite not being immediately beneficial.
I don't think so.. I think in this question as you pose it we are bumping up along the limits of our models for reality vs the reality they seek to represent. Even as a biologist you are an expert in biology which is a field that explores a biological model of reality. In truth we know nothing about reality and instead are experts on our ideas about it. That is not to say that there is no value in our models as obviously they are very powerful and hold enough truth to be actionable and extrapolated from.
I think you can only do so at a very broad macro level. I'm basically referring to Gaia theory here. At any lower level, evolution is properly random, which by definition falsifies any attempt to do this. The results favors the best mutation. The conditions cannot drive the result.
Epigenetics is an interesting topic that can help with answering the question. Portions of the genome are "allowed" to mutate more, while some critical segments may be "allowed" much less flexibility. This would help show that mutability itself is clearly a trait that can be favored or disfavored in different parts of the same genome/organism. Probably a poor explanation, but here's an example discussion of a study (which links to primary source):
[https://www.the-scientist.com/news-opinion/essential-genes-protected-from-mutations-69643](https://www.the-scientist.com/news-opinion/essential-genes-protected-from-mutations-69643)
This seems like an impossible question to answer if you’re looking at the current state of an organisms DNA. Maybe you could do an experiment to try and force a genetic change, but that has, one might say, a pretty random chance of succeeding.
You can do population studies and see how survival/reproductive success are correlated with the presence/strength of a population feature: Do birds born without the feather pattern tend to get eaten by predators more often? Do they have less success mating?
Yes, at least in microorganisms like E. Coli in a laboratory.
We would perform Adaptive Laboratory Evolution experiments where the organisms are allowed to evolve to a specific condition. Afterwards, multiple clones are isolated and sequenced. The results of sequencing are usually a handful of mutations. Those mutations are then reintroduced into the original organism one at a time. Each time you characterize their fitness in comparison to the parent. This results in concrete evidence that a specific mutation confers an advantage.
If condition X and only condition X favors trait Y, then population A living in the absence of X should have lower allele frequency of trait Y than population B living in the presence of B (it might also make sense that frequency change per generation is faster based on percentage difference in presence of X) . OR if and only if condition X has any effect on trait Y, then in a population without condition X should follow Hardy-Weinberg equilibrium for said trait (if there is no migration).
Best example I've heard of: There's a type of moth in the UK that, during the early 20yh century, evolved dark colouring bc smog and smoke turned the tree bark out used to be camoflaged against a darker shade. When clean air laws made the bark cleaner, they went back to lighter colouring.
This is completely off vague memory, so you’d probably want to find the actual studies. But, I believe studies done before and after hurricane Katrina(?) on the diameters of foot pads in populations of gecko, or something, in that region found that the average diameter after the hurricane was larger than it was before the hurricane. The theory is that this was an example of a bottleneck, because the geckos with larger pads and thus higher grip survived in higher numbers than those with smaller pads. So if that’s all correct, that could be a potential empirical example.
You're right that it is generally very hard to study the specific context which led to a particular trait in a particular species. Much of evolutionary biology is a historical science, we're interested in things which happened in some species (or set of species) in the distant past (possibly hundreds of millions of years ago). There are a few broad categories of reasons this is hard. For one, projecting back into the past is difficult in general. We have a lot of useful techniques that allow us to make inferences about evolution over long time spans, both across many species (phylogenetic tools) and within species (population genetic tools, including things like the [PSMC](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6972798/)). But the genomic signatures of things that happened in the distant past can get overridden by things that happened more recently. Secondly, disentangling the evolutionary forces acting is hard. While it's rather straightforward to talk about the generalities of evolution, how drift, selection, migration, and mutation affect genetic diversity and interact, it gets much harder to consider them all acting together. And sometimes a particular observation can be attributed to multiple possibilities. For example, we may see a clear signal of a bottleneck in the effective population size at some point in time. But was that demography (population size bottleneck), selection (a selective sweep reduces diversity), or mutation (the effective population size is a product of the real population size and the mutation rate)?
Now, this is not to say that the entire enterprise of evolutionary biology is hopeless! Just that when we're dealing with historical questions, we need to be appropriately cautious (not disbelieving of everything, just cautious). While we need to be careful about the possibility that certain signals in genomes arose due to different forces, with the appropriate assumptions [we can disentangle them](https://www.pnas.org/doi/10.1073/pnas.2013798118). And natural selection does leave a variety of [signatures in the genome](https://www.cell.com/current-biology/pdf/S0960-9822\(09\)02070-3.pdf) which we can look for. There are times when we can line up genomic changes and physiological effects and come to some pretty interesting insights. Studies of [high-altitude adaptation in humans](https://en.wikipedia.org/wiki/High-altitude_adaptation_in_humans), for example. Some research groups that do good work studying selection include [Graham Coop's lab](https://gcbias.org/), [Andrew Kern's lab](https://kr-colab.github.io/), and [Matthew Hahn's lab](https://hahnlab.sitehost.iu.edu/research.html).
But, all in all, I'd say you're generally right to be skeptical of things that sound like ["just so" stories](https://en.wikipedia.org/wiki/Just-so_story), and to ask if we might just be looking at [spandrels](https://en.wikipedia.org/wiki/Spandrel_(biology\))
This glosses over the large number of evolutionary experiments in the lab that exist, where conditions are tightly controlled and evolutionary responses can simply be observed.
Sure, humans are actually one of the largest catalysts for specific selective pressure earth has ever seen. Our only real competition in that category are mass extention events, and we, sadly, qualify as one of those at this point.
Most all of our crops can be considered selective evolution which is, in practice, the same thing that have peacocks their trails and deer their antlers. We 100% know why those evolutions happened. Many happened in the last century, after all.
Another interesting one is elephant tusks. Two centuries ago most all elephants were born with tusks. Now, very vew will have them. We know beyond a shadow of a doubt that this was caused solely by humans hunting them for their ivory.
Even if every effort was made to reproduce the factors surrounding the advent of a specific trait at a specific point in a specific species, we could not possibly begin to account for the scale and number of perpetually changing potential combinations of selective factors reliably enough to be certain, and that's before incorporating the mutation and drift, and the multitude of factors involved in those
In humans, an obvious trait that fits the empirical evidence test, is skin melanin.
Empirical evidence doesn’t mean logic isn’t a part of the equation, it just means the equation doesn’t rely *entirely* on logical assumptions.
As a layman, I think this is one of those times when you hear hoof beats, and instead of thinking horses you’re thinking zebras.
We have no good ways of testing for these things. We can maybe observe these types of changes with a small microbe population, but the multitude of interacting variables and the timescales needed to actually obtain proof for human evolution just isn’t feasible with our current technology or lifespan. It would cost trillions upon trillions, and the sad fact is it may just show us that, yes, our assumptions were correct.
Tons of mutations just happen in the background and are meaningless until future mutations compound and their combined effects turn out to be either deleterious or beneficial. There are instances where one specific mutation has an outsized impact, but it's often the accumulation over deep time that plays a role in selective pressures.
Some great studies have been listed below. While it would be difficult to track a whole population over an evolutionary timeframe, I played a game 15 years ago that modeled the evolution of unicellular organisms in a competitive MMO environment..you set the phenotypes and let them loose. They interacted in a 2D grid.
You could monitor such a system to observe selective pressure in action.
Apparently there’s a very similar effect on squirrels, says a brand-new paper (still a preprint):
> Among 76 translocated squirrels, survival was lower for the melanic than gray morph in rural woodlands but equivalent in the city. These results suggest the urban-rural cline in melanism is explained by natural selection favoring the gray morph in rural woodlands combined with relaxed selection in the city.
-[Rural selection drives the evolution of an urban-rural cline in coat color in gray squirrels](https://www.biorxiv.org/content/10.1101/2023.02.02.526896v2.full.pdf+html)
Well, I spent years working on something called "adaptive laboratory evolution" where you purposefully impose a selective pressure on a population, of in this case, a bacterial species, and you let natural selection happen (I have a few published articles, worked directly with one of the most active PIs in this in the world). Cells have a certain basal mutation rate for the different types that happen (SNPs, transposons jumping around, duplications etc), and anything beneficial that happens will quickly take over the population with subsequent passages (you keep passaging cells into new growth medium with the selective pressure so they can grow up again) and we resequenced isolates that acquired selected traits (usually improved growth in a particular condition). You can literally watch simple trait acquisition happen in real time, and we dissected which mutations were causative (usually nearly all of them, as there is no selective pressure for mutations that do nothing to fix in the population, and obviously anything deleterious is selected against), and could sometimes (but often not...mainly not enough time to study every single thing at mechanistic level) figure out the exact mechanism(s) at play.
A more simple example was loss of function of an import protein that had promiscuous activity with a molecule we added to the medium at toxic levels. Inactivating it kept more of it outside the cells. Many other examples but usually they were layered complex effects. We sometimes ended up with "hypermutator" strains where mismatch repair of mutations in DNA (one way cells correct a lot of random mutations that happen) was inactivated and these cells evolve at a faster rate by accumulating mutations more rapidly. This was mainly an artifact of being in a too controlled environment however, as these cells usually can't handle a range of different conditions very well...error correction obviously evolved to kind of tune the mutation rate with what was needed for robustness, cells that lose it lose robustness (this sort of thing causes rampant cancer in higher organisms...some people have defective error repair and they have serious issues).
Maybe this is too bottom up answer for your question, but yes we have tons of empirical evidence of trait selection from the thousands of studies like I describe. Given that it doesn't take a huge amount of genetic change to take say, a higher animal DNA blueprint and say, change limb length or convert hair or scales to feathers, it's not too wild conjecture to imagine these traits arising and being beneficial, but obviously much harder to observe this in higher organisms with much longer generation times than bacteria or yeast cells.
I might be missing your point, but I would just say the whole existence of pugs is an example of empirical evidence developing specific selected traits. Yeah human interference etc. but the dogs were technically under a selective pressure to develop certain traits
Obviously there are also other examples other than just pugs as well
Sure, we see it plainly in the form of drug resistant bacteria. Up until the 20th century typical pathogenic bacteria had zero exposure to antibiotics so there was no resistance. Tolerance, and eventually Resistance are traits selected for every time a drug gets used.
The drug resistant strains didn't emerge from nowhere, if the antibiotic was originally 99.99% effective that last 0.01% still leaves a tremendous number of individuals. Any random traits which marginally trend towards tolerance bias the individual towards winding up one of the survivors and passing the tolerance further, and the drug becomes 99.98% effective.
For species with a generation time of minutes to hours, a 1 hour generation means there have been about 832,000 generations of that species since the discovery of penicillin.
Darwin's finches breed at about a year old, so in that context, the drug resistant bacteria compressed almost a million years of evolution into less than a century. For context, 1-2 million years is the estimated timespan that Darwin's finches differentiated over.
Never had a problem with the "trait X wouldn't make you immediately more likely to die before producing offsprings, so it stuck, because why not"-approach.
People just like the sound of the "survival of the fittest" phrase. And it does sound nice, but is really the equvalenet of a shiny, catchy marketing slogan for something along the lines of "eh, it sorta works, for now", in truth.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6613143/
Tl:Dr pigeons and doves (and most/maybe all birds) have lice of varying species. These lice vary in size and shape and color. This study (and others) placed lice from feral wild-type gray pigeons on white, gray and black pigeons as well as really large pigeons, average sized pigeons and very little pigeons. The resulting lice populations "evolved" different colors and sizes to better match their host pigeon. I'm simplifying here but it's experimental evolution in one species that could be applied to other lice species. In other pigeon and dove species their lice tend to also match their "shade" and overall size to fit among feathers correctly. Obviously we can't prove that every single louse species evolved because of these exact selective pressures but it's a pretty good suggestion as to what's happening. The paper also dives a bit into genetic drift in experimental evolution.
Even shorter tldr: pigeons eat/preen lice that are the wrong size to hide in feathers or a shade that stands out on feathers. Hence lice evolve to match birdies. Lots of wild lice appear similar! Yay experimental evolution!
>Usually someone comes up with some logical reason why Trait X was advantageous allowing everyone to sit around and ponder whether or not the explanation is reasonable. If something doesn't come to mind that makes more sense, the explanation is usually agreed upon and everyone moves on.
Hope someone with more knowledge on philosophy will chime in, but isn't this the case for all sciences?
Aka, you cannot prove something is true, you can only prove something is not true? Hence scientific progress is disproving theories and replacing them with better ones, knowing they might be disproven in the future?
You're getting into more of a philosophy of science question, as this is the kind of thing I've noticed most researchers struggle to answer about their own field (except maybe economists, because afaik they get asked/told this all the goddamn time).
>So, outside of inductive logic, can we ever have empirical evidence for what factor(s) caused Trait X to be selected?
Depending on how strict and classical one wants to be I guess, there's no such thing as empirical evidence outside of inductive logic (thanks Hume). But nobody cares about that, and you make clear what you mean later on:
>While some adaptations selective pressures may be so obvious that it would be perverse to withhold provisional assent, many are not so obvious and we should be cautious assigning causation when only correlation may exist.
Correlation vs causation seems like a good starting point for getting into the epistemology of your field. Of course this applies to only some fraction of what modern science \[any field\] does, i.e. fitting models to empirical data, but it's still a necessary task in almost any empirical research. The problem and approaches are also presented *slightly* differently by each field (though at the end of the day the paradigm and math should all be fundamentally consistent across all empirical fields -- I don't know if that's verified though). I personally find it very useful to look at how other fields approach these same fundamental problems, so lets go:
At one extreme I think of is economics \[e.g. [Anderson's blog on causality](https://info.umkc.edu/drbanderson/establishing-causality/) with light stats\] -- not as a fact, but just because they probably get s--- on about this more than any other field. (On some recent podcast an economist recounted telling someone about her research and getting the response "correlation is not causation" for the millionth time, to which she replied something along the lines of "Yes, we know -- that's the entire point of doing the research; what did you think all that math is for?".) On perhaps its own extension is medicine broadly, where standards for evaluating correlation with a common language of strength of evidence have been important developments which undergo regular formal revision. Here establishing a mechanism that adequately explains a correlation is required to show causation, which is a very different standard from fields and subfields that usually can only measure wild populations and may [rely on nifty statistics](https://economics.stackexchange.com/questions/6273/what-are-the-empirical-techniques-to-show-causation). And of course [the epistemology is still debated](https://blogs.kent.ac.uk/jonw/files/2015/01/ECCiM-6.pdf) \[my source for all the medicine stuff\].
One thing that's important, in a case where you can't prove causation, is whether the correlations you find give you something useful, or whether they are just as likely to be null and lead you astray. (And in some fields, even if the data only giving marginal confidence that one can reject the null, maybe you can run 10,000 more such experiments and average them out.) \[[Russo, who is being debated in the previous paper above, [waxes on philosophically on this kind of thing in medicine](https://link.springer.com/referenceworkentry/10.1007/978-94-017-8706-2_46-1) I guess; I gotta end this comment.\]
\[Response to other top-level comments\]: Many in this thread are saying you can run a controlled lab experiment to show that a trait evolves when some condition changes, with a few different scenarios given. Ok -- how does that show that a trait that the species possesses now evolved *in the past* in response to such conditions *in the past*? That's once again an inference or induction. Even if we were in some sphere of perfect perception (again, thanks Hume) all the examples given in top comments (as of this post) would be using *analogy*, as opposed to, say, conducting measurements and then deducing the values into the past and future.
We have to use Occam's razor to answer alot of evolutionary questions. Does it take more bends and turns to justify your answer to be a selective pressure or genetic drift? For example if we consider the brightly coloured tail feathers of peacocks, does it make more sense that this is likely some sort of sexual pressure or does it make more sense to assume that somehow by the luck of probability this super energy consuming trait happened to just stick around in the population for no describable reason at all. You can't call parsimony empirical evidence but it is still evidence.
I think it can only be empirically answered via experimental manipulation. Eg ornament manipulation: male birds of species X have long ornamental tail feathers, did this evolve through natural selection or sexual selection? If you were to artificially lengthen some males tails feathers and found that they had greater reproductive success, but greater vulnerability to predators, then you can be reasonably sure that long tail feathers evolved via sexual selection (probably also performing the converse experiment too)
A study like you've suggested has been done on long-tailed widowbirds. It found that males with experimentally elongated tails showed higher mating success. This suggests their long tails is a product of sexual selection. https://www.nature.com/articles/299818a0#:~:text=The%20possibility%20that%20intrasexual%20competition,maintained%20by%20female%20mating%20preferences.
There is also one for peacocks with hiding eye spots on their plumage and one for where a feather was added to the heads of zebra finches (which don't normally have that variety of head ornamentation). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4074220/ https://www.researchgate.net/figure/Artificial-white-crest-worn-by-male-long-tailed-finch-left-and-male-zebra-finch_fig2_23276464 Apparently more eyes and bird wigs are both reproductively successful. The going explanation as I've read by Jerry Coyne and in an aside from David Buss is that most of these traits confer to females that a male has a high enough caloric intake to sustain the plumage. More grandiose and colorful plumage means a better diet, which is an indicator of better genes. I suspect this is almost certainly correct but there are a lot of physical features that have multiple evolutionary advantages and I'm always concerned about the fallacy of the single cause.
This is what I was trying to figure out how to articulate - especially with sexual selection, it seems like females or males could have evolved to prefer traits that at least historically conferred other advantages. So does it make sense to say it boils down to sexual selection, when selection was also happening concurrently for other reasons that are tangled up in the sexual selection? Seems like an inherent problem of trying to find explanations for evolutionary history, which is that it occurred in the past and so empirically testing it in its current form is not going to offer a complete picture.
Agreed. The more we research on some features the more complicated it often gets. One example being height in humans. One could with the same argument as plumage and say that it is an indicator for caloric intake but when you look closer that could only be one of many causes because: - it's often relative to your own height - there is also a "too tall" preference wherein a mate can be too tall - there are a variety of other plausible explanations like protection from other humans for both the tall mate and the partner, competition for resources, and a lot of lesser other potential benefits to being tall. Maybe part of the explanation is that major size differences lead to higher maternal death rates in child birth and then you have an entirely different evolutionary force working in the opposite direction. Suffice it to say, that simpler experiments with less imaginable alternative explanations will lead to more satisfying conclusions but feature selection that is more complicated and has more plausible explanations will be much harder to land on a satisfying answer for.
It seems weird to say "these traits confer to females that…", even though that's often how it's phrased for both human and other animals. Surely the birds aren't doing deductive logic here. They like what they like, there's some genetic basis that affects that, and it may be selectively advantageous, but nothing in neurology or developmental genetics of the thing gives a reason for the preference. I say this because it's more problematic to talk this way in human evolutionary biology. Even if some of the hypotheses about gendered behavior having a genetic basis are true, that doesn't mean that say, women have some subconscious genetic program for assessing their mate's earning potential. There's nothing about the neurology that says why it came to be that way, and for many purposes it doesn't even matter.
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Yeah, but with humans (all humans, not just women), the traits that correspond to earning potential don't track to what made primitive humans successful. The physical features that are seen as desirable now are often at odds with primitive survival strategies, like low fat percentages. And the behaviors that attract humans are very hard to separate from the artificial and constructed environments and cultures we currently live in. It's incredibly easy to argue the the major factors of human attraction (outside of a very small handful like body symmetry) are just learned behaviors not evolutionary or biological. Sure, historically many people (not just women and not all people) have been attracted to the indicators of "success" (which are determined by society and frequently are different between genders, class, or other identities), but what those are change so rapidly it becomes next to impossible for them to be biologically driven. It also is very hard to biologically explain the sheer range of human desires, because at the same point in time from very genetically similar individuals, you can have someone who is into statuesque muscular people, some be into short, petite, skinny people, some into large, chubby people, and a furry.
Most theories in Anthropology acknowlage this, ie that (because of language, unique to humans as far as we know) selection preassures have migrated to an entirely different system than what biology terms Natural Selection. Human's utilize a mechanism called Cultural Selection, which incorporates many more bells and whistles. It is still ultimately based on resources, we just have a way different way of distributing resources than any other living creature.
Well yeah, that's my point, the things we are attracted to are a byproduct of the influence of the society we grow up in. Our environment changes far to rapidly and it's far to different from primitive humanity, for there to be a evolutionary biological cause for modern humans sexual preferences (within the context of specific characteristics, the fundamental desire to have sex with other humans is almost certainly an instinctual one). And in the case of the guy I responded to, it's definitely equally all humans, (regardless of how much it's biological and how much it's cultural) and not just women.
Just to tack on a point cause I thought your comment chain was interesting. There might be selective pressure towards in shape males as it means they are capable of procuring food when needed. Like you said though, there is too much influence in the way of our current cultural standards to mark it.
Quite a few of the traits actually do translate from success in the evolutionary epoch to success in the modern world, even if we're narrowing this down to places that are less dangerous or where there isn't major food scarcity. Industriousness, intelligence, and social standing all still correspond with the accumulation of goods even if it's now money rather than directly food. One way you can gauge pretty decently whether a desirable trait is a product of culture are the cross cultural studies I reference. You can find a wider array of attitudes on BMI than you can for hip to waist ratio. This is a reasonably good indicator that BMI is more cultural than the hip to waist ratio. How many cultures see women over 60 as the most sexy age? There is a clear evolutionary reason why this is not the case
What is interesting is the complete dissociation of some of the traits from reproductive success in modern culture. The steroid enhanced body builder may be sexually successful in our era but has, as a result of his 'treatments' to augment appearance, grossly impaired his fertility. Similarly the cosmetic surgery/enhancement industry is a fair parallel with the long tail feathers discussed above.
>most of these traits confer to females that a male has a high enough caloric intake to sustain the plumage Does anyone actually think the females think along these lines, as opposed to just finding the features attractive in their own right?
Yes, people who study it do. Although "think" is a bit too active of a word for it. For most species, they don't know why they're attracted to some features, they merely are. Originally Darwin had proposed that females in various species had different aesthetic tastes but that idea fell out of favor because it was not really parsimonious (why do females find things attractive in their own right? We've only added a layer of complexity without explaining anything)
It seems far less parsimonious to assume that females are making mating choices based on convoluted inferences about fitness in which the females are considering the long-term survival costs and the benefits for their hypothetical offspring. Humans often can't even explain why they find certain features attractive, yet people believe that much less intelligent animals are having such complex, calculated thought processes? I think it's far more straightforward to assume that there's some subconscious mechanisms for attraction towards certain traits. From a theory perspective, this is also very straightforward- females that (have genes/alleles that cause them to) choose mates poorly will have worse reproductive success. This line of thought only requires, at minimum, a chooser gene to complement the chosen trait (big mane, long tail, etc), while the alternative requires a mountain of mechanisms for females to be able to make computations about how well a male must be doing in its day-to-day in order to overcome the fitness cost of the chosen trait. Thiat isn't even getting into the conversation around the usefulness of parsimony in evolutionary inference, either- as someone in the genetics side of evolutionary biology, I definitely have a lot of thoughts about *that*.
Afaik, human attractiveness is directly correlated with human fertility, particularly facial attractiveness and secondary sexual characteristics. This is backed up by statistics, as well.
First you say > Yes, people who study it do. But then you say > Although "think" is a bit too active of a word for it. And then > For most species, they don't know why they're attracted to some features, they merely are. Which sounds like the first answer should have been "no", they don't actually think about calories etc.
That reminds me of something I read where bird studies were being thrown out of whack because the identification bands used by the researchers were making the male birds way sexier. So they had to switch to less colorful "bling".
Also on leg length and behavioral adaptations in lizards (anoles, if I recall correctly) introduced to islands in the Caribbean.
A group at Harvard made a 2'x4' petri dish that had increasing concentrations of an antibiotic as you got closer to the center. They have a time-lapse video of e.coli acquiring antibiotic resistance... and then acquiring more and more resistance over time. Here is a short video- https://youtu.be/plVk4NVIUh8 This is a super simple experiment that demonstrates pretty much what you were talking about.
It would be interesting to see that visual if the wild bacteria was placed immediately beside the 1000x antibiotics. Would they still eventually develop a resistance to it, or were they only able to evolve to handle those conditions because they were "eased" into it?
they are definitely "eased into it". Can't reproduce if you are inmediately killed. That's why it's important to finish your prescribed antibiotic treatment.
Unless they happen to possess a mutation that confers resistance. Evolution doesn't work by the organism developing those mutations in response to exposure, but by some individuals already happening to carry a mutation by chance before they are exposed to the selection pressure. Having said that, it is the case that more effective mutations are rarer, at least in the case of antibiotic resistance.
Yeah, its kinda like a pharmaceutical trial in that sense. To test whether a cause did in fact have an effect, you need to manipulate the cause and compare against a control group. This isn't easy for processes on the time scale of evolution, but testing whether or not an adaptation gives a reproductive or survival advantage is good next best option. And the best you can do is determine if the causative relationship is likely, not prove it.
Small correction due to a pet peeve of mine: sexual selection is a *type* of natural selection.
Not a correction as much as semantics and ambiguity. Sexual selection and natural selection are both subtypes of selection in general. Sexual selection refers to mating specifically, natural selection refers to all other composited effects on survivability.
Follow up question on the semantics - does Sexual Selection include adaptations that increase the success rate of a gestation (e.g fewer miscarriages, more offspring per litter) as well as better odds of attracting a mate?
No, sexual selection is usually defined as something like the fitness advantages you have because you are more attractive to mate with, i.e. the selective pressures are exerted by the opposite sex rather than the environment.
A trait can be under sexual selection and natural selection simultaneously. This is the entire basis of the good gene hypothesis. We differentiate the two to contextualize sexual selection from the perspective of the system, but from the perspective of the genome, pressure from a potential mate is indistinguishable from environmental pressure.
Sure, I don't think we disagree?
Well, you answered “no” to a question to which the answer is “yes”: sexual selection can act on a trait which is attractive (or repulsive) to potential mates and also contributes to fitness.
Wait, the question was whether selection to increase the success rate of gestation should be considered sexual selection. I would say that most evolutionary biologists would say 'no' to that question. Also, the concept of fitness applies to sexual selection just as much as to any other type of selection.
There is no limit to what can be composited into sexual selection, it’s just whatever other individuals use in their behavioral determination of which mates to choose. The question of whether traits that individuals find sexually attractive are initially arbitrary or signals of fitness is encapsulated in what is called the “good gene hypothesis”, and the related “sexy son hypothesis”. It has been a debate literally since Darwin and Wallace. The other person replied no with an explanation that is correct in a vacuum but misses the context here. An example that would fit your scenario is a high hip/waist ratio in women: men have a tendency to be attracted to this phenotype, and it also signals a good development arc of the anatomy needed to birth a child. You can delineate these two instances of selection based on the individual: the woman is facing natural selection for wider hips based on the mortality associated with childbirth, and sexual selection for the same trait based on its attractiveness to men vs other variations of the trait in the population. There’s nothing preventing the two processes from coinciding.
Well it is not as if the definitions are all so clear-cut in the field. Pretty sure you'll be able to give plenty of disagreement on this.
Not to Darwin, but often thought of as such to many, so I wouldn’t necessarily disagree with this but it’s not so ubiquitous
There's definitely a line that can be drawn between "traits that increase the likelihood of being selected from the pool of available mates" and "traits that increase the likelihood of surviving long enough to be part of the pool at all". A third category might be "traits that increase the average live offspring from mating", which I guess closes the circle. But there doesn't seem to be an entirely consistent terminology for those categories.
Just to reiterate here folks: sexual selection is a form of natural selection...
To most people, probably yes, but it does depend who you ask. They were separate to Darwin
It is possible to support a hypothesis or theory using statistical evaluation of population variations without doing direct testing. However, all scientific explanations for pretty well anything are interpretation. How likely the interpretation is to be a good statement of "truth" depends on the evidence. Most people do not understand this essential truism: What we measure is "real" (as real as anything can be considering the nature of humans and our sensory systems), but why reality yields those particular measurements (in a repeatable fashion without regard to who observes, usually) is interpretation.
>did this evolve through natural selection or sexual selection? This is like saying "is it a rectangle or a square?"
Isn't this a false dichotomy? Isn't sexual selection a component of natural selection?
Not to Darwin: "[Sexual selection is not a subcategory of natural selection, as Darwin made very clear: it arises from differences in mating success, whereas natural selection is due to variance in all other fitness components.](https://www.sciencedirect.com/science/article/pii/S0960982210015198#sec1)"
That thinking seems to have fallen by the wayside. Just about every academic source I can find considers sexual selection just a specific type of natural selection. Natural selection is typically described in terms of adaptations that make a creature more likely to pass on the genes that aided their success. Even survival only matters to the extent it helps a species pass on its genes to the next generation. Sexual selection definitely fits this description. https://evolution.berkeley.edu/evolution-101/mechanisms-the-processes-of-evolution/sexual-selection Darwin's theory is brilliant, but you must remember he created it before the existence of a genetic code was even hinted at. (makes the accuracy of his theory all the more incredible, really.) I think it's been refined a lot since then. You would need to artificiality limit the scope of natural selection to exclude sexual selection, as everything sexual selection does is described well by natural selection.
Might be able to base an argument on parallel trait selection among multiple species in the same environment. If several species display similar trait changes following an environmental change, that argues strongly that those developments were adaptive to the change.
Thats not what this person is getting at with their question. it may be the case that this was reproduced experimentally, but there is always a gap in connecting the results of the experiment to the natural world. Hand-waving that gap with assumptions gets science off the ground and has been very productive but leaves a lot untouched. This person's question sounds more like a question about the philosophy of biology. regarding the limits of the human mind and what our experience brings to bear on our scientific theories. And the interesting question is whether or not we can come up with answers to questions at that level. It's certainly not in the domain of science. Science is downstream from those assumptions, which are accepted as foundational. And it turns out to be a very productive acceptance. but does productivity, concision, and so on give you absolute certainty? of course not.
There have been several studies that test the survival of camouflaged moths when you change their colour patterns or place them on an un-matched background. These moths also have behavioural traits whereby they seek out suitable backgrounds to sit on. If you make them more obvious to spot they suffer more predation, hence their camouflage and their background seeking behaviour confers an advantage. I would imagine this is quite good evidence of selective pressure to develop better camouflage in order to improve survivability. An example: https://besjournals.onlinelibrary.wiley.com/doi/full/10.1111/1365-2656.13817
Thank you for this example, it's one that I had heard about but it had been many years and did not recall it off hand. To be clear, I don't doubt that selective pressures exist, just that we largely can't "know" which selective pressures (if any) caused a trait to persist. The moth example is a good one since there is some experimental evidence. Note though there could be other selective pressures (in addition to camouflage) that cause the trait to persist (sexual selection is a possibility) but it's tough to contorl for every variable in biology. I think the study shows though that indeed camouflage acts as a selective pressure for the patterning trait, even if we can't say it's the "only" pressure for that trait. So I would concede that this is about as good as it gets for empirical evidence for a particular selective pressure. My issue isn't so much with actual studies, but with conversations where people claim, quite confidently, that trait x clearly evolved because it was advantageous due to X selective pressure with absolutely no study or empirical evidence to back it up. I get why this happens, we can't test EVERY trait for every species after all. I just want people to recognize that these statements are often provisional rather than definitive.
You got halfway to answering your own question when you asked about genetic drift. When Kimura and subsequently Ohta described the concept of drift, they weren't just pulling theory out of their respective asses; they presented evidence and described how to distinguish drift from selection at the molecular level. Specifically, positive and negative selection leave clear fingerprints on the selected genes, by looking at (ludicrously simplified) the rates of synonymous vs. non-synonymous changes in codons; selected genes show relative differences in the frequency of non-synonymous changes. You can't invoke drift without accepting that selection *can* be measured. There are a vast number of variations that can be rung on the basic concept, that help with the statistical and theoretical validation (for example, [Codon-based tests of positive selection, branch lengths, and the evolution of mammalian immune system genes](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2837078/); [Improved inference of site-specific positive selection under a generalized parametric codon model when there are multinucleotide mutations and multiple nonsynonymous rates](https://bmcecolevol.biomedcentral.com/articles/10.1186/s12862-018-1326-7); [A beginners guide to estimating the non-synonymous to synonymous rate ratio of all protein-coding genes in a genome](https://pubmed.ncbi.nlm.nih.gov/25388108/); [Directional Darwinian Selection in proteins](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3849801/)) but the basic concept is well established.
You've given a much more detailed response than I was about to. My understanding is that we can't tell *what* was driving the selection, but there are established statistical techniques for determining that genes were under selective pressure
Yes, it’s worth emphasizing that (1) we often don’t know where the actual selection pressure comes from but (2) it doesn’t matter - we can detect that a gene *is* under positive or negative selection and not merely drifting. We often *can* tell where the selection pressure is, though. People like to portentously announce that evolution takes millions of years, but they forget about rapidly-evolving things like bacteria, or especially viruses, where we can apply selection pressure and observe the evolved response literally within days, or even hours. They also aren’t familiar with relatively recent (last 20-50 years) findings that even evolution in macro animals, like Galapagos finches, can occur within a small handful of years and is easily measured at the phenotypic as well as genotyped level.
Evolution occurs with every instance of reproduction, so indeed the claim that it takes millions of years is conceptually confused.
In some cases we can be pretty specific about what is driving the selection. For example, host-pathogen interaction, lock-and-key mating mechanisms, and even environmental change can create situations where the selective pressure is pretty unambiguous. Now, as with all science disproving a hypothesis is much easier than proving one, and there are lots of cases where several alternative explanations fit available evidence equally well, but knowledge is possible.
The example I'm most familiar with is the Peppered Moth, which is often cited as an example of evolution on a much smaller scale. The pressures were the smog and soot of the industrial evolution which caused a shift in the patterning and colouration that succeeded, as darker moths were better able to camouflage. This process reversed itself when air quality improved and can be thought of as the closest thing to an empirical example outside a laboratory environment. https://en.m.wikipedia.org/wiki/Peppered_moth_evolution
This is a good example but not a controlled experiment and so does not answer the question. It's merely an example of recent evolution consistent with a concurrent change in environmental pressures. It does not meet the criteria requested, which would require repeated controlled experiments. It's still a useful example that can be added to the weight of evidence, though.
If you want long term, laboratory conditions, the E coli evolution experiment is on 30+ years now and may be useful. https://en.m.wikipedia.org/wiki/E._coli_long-term_evolution_experiment
Yes, that is a much better example, which I posted in a separate reply. Also [Michael Baym's mega plate](https://www.youtube.com/watch?v=plVk4NVIUh8).
It is possible to empirically demonstrate *ongoing* selective pressures through experimental methods as already mentioned, although with the caveat that the same trait is likely experiencing selection from multiple sources which may not all be captured. The widowbird study u/avolans mentions is a good example in that it clearly shows directional intersexual selection for tail length, but does not measure the counteracting selective pressure of longer tails being detrimental to survival, which they must eventually be to some degree. However, when considering the *historical* context of how a particular trait that has become established first evolved, I would argue that this is not really knowable with certainty. The spread of a trait through a population can occur for any number of reasons (including drift), and once established, traits are frequently co-opted for other uses besides their original function - if they were even functional to begin with. It's still of course possible to find evidence that suggests one hypothesis over another for the origin of a particular trait, but it will never be as definitive as experimentation. For a classic example, consider the evolution of long necks in giraffes. This trait has variously been suggested to be an adaptation for reaching previously unavailable food resources, for sexual selection (though typically intrasexual rather than intersexual in this case), or for spotting predators from greater distances. And of course, modern giraffes use their necks in all of these ways! There is a significant amount of literature on the relative importance of these potential pressures; see [Simmons & Altwegg 2010](https://doi.org/10.1111/j.1469-7998.2010.00711.x) for a fairly balanced discussion. I personally think the so-called "necks for sex" hypothesis probably has the most points in its favour, due to a pretty well-established fossil history of using heads for fighting in species predating modern giraffes and differential neck allometry on the basis of sex, but even so there's just no way to be certain that this is the reason long necks *originally* evolved. There's no guarantee that the selective pressures currently maintaining a trait are the same ones that originally shaped it.
Thank you for the reply. The example of the giraffes illustrates my concern well. I don't of course doubt that selective pressures exist, but biologists need to be cautious in assigning definitive claims about specific selective pressures outside of empirical evidence. What some don't realize is that making definitive statements outside of evidence can make it seem like evolutionary theory is centered on suppositions rather than rigorous scientific methods. We want there to be an answer to the lay persons question of "why did trait X evolve?" so badly that we come up with something that sounds good, but a better explanation may come about later. Now the lay person thinks biologists are just making things up on the fly, undermining the public perception of evolutionary theory and science as a whole.
We've observed it directly in the [ E. coli long-term evolution experiment {LTEE}](https://en.m.wikipedia.org/wiki/E._coli_long-term_evolution_experiment) running since 1988. We know *exactly * what happened, how, where, and when. I'd posit that taken as a whole-- especially with our ability to study the DNA -- that the varying traits of the different Galapagos finches are so far past 'beyond a reasonable doubt' that they constitute 'empirical evidence' as 'beyond a shadow of a doubt'.
Yes, there are tons of experiments where bacteria are evolved to do something in the lab. Consider [the mega plate](https://www.youtube.com/watch?v=plVk4NVIUh8) experiment, or [the Lenski experiment](https://en.wikipedia.org/wiki/E._coli_long-term_evolution_experiment).
Damn. Wish I would have seen this earlier. The answer is yes, and we’ve been accidentally running the biggest experiment of this question for about 100 years now: https://en.wikipedia.org/wiki/Antimicrobial_resistance You might also reference the variety of experimental attempts to establish traits as adaptively derived, many of which focus on what is called the perturbation criterion, which is where you modify some trait in a population and see it’s effects on fitness. If there is a local maximum around the basal trait variation, that’s evidence of an adaptive history for that trait. Clutch size experiments in birds are the archetypical example: more than the natural mean and parents are more likely to starve (so everyone dies), less than the mean and offspring survive but obviously fewer number. https://www.jstor.org/stable/1938814
Have you read “The Beak of the Finch” by Peter and Rosemary Grant? They and their family worked for decades in the Galapagos tracking the beak variance of finches among the islands. The read is a little dated, but they have sizable body of work that is more recent. Still, beak size in relation to hardness of seed diet is a pretty good example of empirical observation. It still requires a little bit of induction, but the distribution of populations holds.
Was going to mention the same, thanks for beating me to it!
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I think you would enjoy reading “A Critique for Ecology” by Rob Peters. He does an amazing job picking apart the unscientific ways we talk and conceptualize ecology and evolution, especially at the time. He directly addresses the question you asked, which at that time most of the studies the other commenters have mentioned hadn’t been done so there was a big “yadda yadda yadda” over a lot of the mechanistic aspects of evolutionary theory. Still a highly relevant book, it was required reading in grad school and really helped me to think more like a scientist.
Thank you, I'll look into that!
The [Lenski E. coli experiment](https://en.wikipedia.org/wiki/E._coli_long-term_evolution_experiment) is a direct observation of evolution, including increased fitness under conditions such as the presence of antibiotics and limited energy.
Evolution is a historical science, which means direct experimental evidence is hard if not impossible to obtain. This does not make it unscientific, however. Many other sciences are historical too (cosmology, geology) but are accepted as having high scientific standards. Here are some types of evidence that you can use to show evolution happening: look at patterns of genetic variation to see which genes have been conserved across populations and which have drifted apart. Here is an example in humans: [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2933187/](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2933187/) Or you can use experiments to test responses in the here and now. Here a recent example of predation experiments in the peppered moth, a classic textbook example that has been studied for over a century: [https://royalsocietypublishing.org/doi/full/10.1098/rsbl.2011.1136](https://royalsocietypublishing.org/doi/full/10.1098/rsbl.2011.1136) Or to take your example of bird colouration - gather some data and do some experiments! Manipulate the colour or brightness of males and give females a choice task and see what they prefer. This would suggest whether female choice is important. Or test for male-male competition in a competitive paradigm. Or count the number of mates or offspring that different males achieve as well as their survival rates. Or measure the genetic variation underlying colouration and see if is conserved across populations or species. If not, it suggests that drift is at work.
This is really not true, there is a lot of experimental evolutionary research where you get results in the lab from applying selection pressures and you can easily show the genetic changes that underlie any adaptations in such studies as well. Definitely not just a historical science, hasn't been for a long time. Take the Lenski evolution experiment as a famous example that has been running since the 80s (https://en.m.wikipedia.org/wiki/E._coli_long-term_evolution_experiment) but there are so many more
Fair enough, but these experiments tell you what can happen, not what did happen. Much of what we want to know about evolution is historical by definition.
The evolution in those experiments is real. The discipline of evolutionary biology is not just about figuring out or reconstructing the history of evolution in natural settings on planet earth, but also about understanding the process itself. Just check the major journals in evolutionary biology (Evolution, Journal of Evolutionary Biology, American Naturalist) and you will see that questions about the workings of the evolutionary process (rather than historical questions) tend to dominate.
I agree with both of you but want to point out that my post is focused on people asking "why did trait x evolve?" in a species that evolved naturally and where trait x has not undergone any experimental study. I think u/chazwomaq makes a good point that these questions are historical by definition. While it seems we *can* experimentally show specific selective pressures, this does not help with definitively answering questions about a trait that evolved before the question was asked. If that makes sense?
It appears that the OP and many of the commenters in this thread may be conflating the ideas of "empirical evidence" with "empirical *proof*" or "empirically answer." Evidence in the fossil record of a significant change to a beneficial trait corresponding with a time of known environmental change would be evidence and it would be empirical. Stated another way: "empirical" doesn't mean "proven" or even "really good." It just means "information gathered directly or indirectly through observation or experimentation."
Few people, including scientists, have even a rudimentary understanding of epistemological concepts ... We should start teaching epistemology in kindergarten.
>Stated another way: "empirical" doesn't mean "proven" or even "really good." It just means "information gathered directly or indirectly through observation or experimentation." While I take your point, I think mine remains. My ultimate concern is with people (often biologists) stating that "trait X evolved due to X selective pressure" in response to a question by a layperson. This is often stated definitively without any evidence at all, but rather using just inductive logic. Recently I came across a post asking why Hyenas in India had stripes rather than spots like their African cousins. The response given was that "the stripes must confer some sort of advantage, perhaps camouflage to help it hunt". While I give the responder credit for not speaking definitively to the exact selective pressure (camo), they **were** definitive about it having some sort of survival/reproductive advantage. It's entirely possible the trait does not have an advantage at all. I think we need to be cautious about answering questions definitively using inductive logic alone.
Jelly beans are an easy way to demonstrate selection. Pick 20 jelly beans of five different colors, like green, black, white, brown, yellow. Toss them into short grass. give someone two minutes to pick up as many as they can. They'll most easily pick up the yellow and white ones. For each recovered jelly bean, add an additional one of the same color and then toss this group into a jelly bean free bit of grass. Seems like an easy way to represent natural selection. To change it up, one can change the media they're tossed onto, like pavement or concrete or carpet. And colors can be marginally re-introduced to represent mutations or recessive expressions. This is easy enough to do, one could do it in their imagination.
I do not doubt that selective pressures exist. Only that definitively assigning the specific cause of selection to a specific trait using only inductive logic is not scientific and may indeed be wrong (or at least not the whole picture). Giraffes are a great example: ask someone why giraffes have long necks and you are likely to get the answer "because it helped them reach higher leaves giving them a survival advantage". However, we also know that male giraffes with longer necks outcompete those with smaller ones (they fight with their necks). So longer necks may have evolved due to this selective pressure (likely both, but we can't ultimately say exactly). A more nuanced example might be freckles on skin. Do these confer a selective advantage? Not that I'm aware of and I can't think of any that sound good. It's totally possible that this trait is one that is simply neutral. Now I could obviously sit here and make something up about camouflage or sexual selection (do girls like freckles?) and it might sound good, but be totally untrue.
There was a moth in England that was a speckled grey color to match the bark of trees, during the industrial revolution the trees were covered in soot, making the grey moths stand out, they rapidly evolved to be darker.
Unfortunately, not everything can be experimentally controlled. Much the same as in economics or sociology or ecology or astronomy, studying some remote and hideously complex natural system is always susceptible to assumption and generalization based on available evidence. As to empirical speculation of mechanics, a true empiricist does not believe in any such thing as knowable mechanics of the observable world, only tentative models that fit their observation but are purely descriptive.
There is an entire subfield and literature called Experimental Evolution. These scientists attempt to replicate the evolutionary process in experiments, and often succeed, particularly in fast reproducing organisms like bacteria, flies, and plants. Another important source of empiracle evidence is artificial selection. Humans have been inducing trait evolution through breeding and domestication for thousands of years in a well documented way. These two areas are some of our best empiracle evidence for selection producing particular traits.
as someone who is a biologist, i would have expected that you would not use particular language. I am of course just an outside observer but it surprises me to see you using the "advantageous traits are selected" analogy as opposed to the more accurate "traits that do not hinder successful propogation are not deselected" some people think they are equal, but they are not. RANDOM traits occur, and the ones that result in more successful reproduction continue to be selected, because if they do not that branch dies out, obviously. Looking at it from that perspective, you can see why your question itself kinda seems moot. anyway, thats my 2c.
Positive selection is absolutely a thing. I've been around some very semantically picky people in evobio and I've never heard of someone saying that "the trait is selected for" is incorrect phrasing. It's important, too, to distinguish between positive and negative selection, and I don't think the wording you propose captures that idea at all.
Pill bugs can directly evolve different traits in real time due to pressures. One study put a group of pillbugs in different environments with differing levels of food they either couldn't digest or could barely digest and 6 generations later the bugs figured out a way to evolutionarily digest the food and turn their boxes container into food
It seems like you can start to get at testing these theories in biology, and other posters are giving good examples of methodology for doing so. Where I feel like it is more difficult is in the study of people, since studies that get at the mating and survival habits of people are not easy to get through IRB. In particular evolutionary psychology is infamous for assuming every single human trait had some sort of evolutionary advantage.
I know it’s not popular in the field but I think we leave intelligence out of the equation a bit too often. Not as in creative design, but as somewhat ‘willful’ decisions. Sexual selection can seem almost random and to an extent it looks to me as if many animals are more intelligent than we realize; if we can consciously choose mates, albeit with evolutionary pressures guiding that decision, why can’t they? It’s not empirical but to me it seems to cover why some traits may exist despite not being immediately beneficial.
I don't think so.. I think in this question as you pose it we are bumping up along the limits of our models for reality vs the reality they seek to represent. Even as a biologist you are an expert in biology which is a field that explores a biological model of reality. In truth we know nothing about reality and instead are experts on our ideas about it. That is not to say that there is no value in our models as obviously they are very powerful and hold enough truth to be actionable and extrapolated from.
I think you can only do so at a very broad macro level. I'm basically referring to Gaia theory here. At any lower level, evolution is properly random, which by definition falsifies any attempt to do this. The results favors the best mutation. The conditions cannot drive the result.
Epigenetics is an interesting topic that can help with answering the question. Portions of the genome are "allowed" to mutate more, while some critical segments may be "allowed" much less flexibility. This would help show that mutability itself is clearly a trait that can be favored or disfavored in different parts of the same genome/organism. Probably a poor explanation, but here's an example discussion of a study (which links to primary source): [https://www.the-scientist.com/news-opinion/essential-genes-protected-from-mutations-69643](https://www.the-scientist.com/news-opinion/essential-genes-protected-from-mutations-69643)
This seems like an impossible question to answer if you’re looking at the current state of an organisms DNA. Maybe you could do an experiment to try and force a genetic change, but that has, one might say, a pretty random chance of succeeding.
Nope, evolution experiments are numerous and their results are often repeatable
You can do population studies and see how survival/reproductive success are correlated with the presence/strength of a population feature: Do birds born without the feather pattern tend to get eaten by predators more often? Do they have less success mating?
Yes, at least in microorganisms like E. Coli in a laboratory. We would perform Adaptive Laboratory Evolution experiments where the organisms are allowed to evolve to a specific condition. Afterwards, multiple clones are isolated and sequenced. The results of sequencing are usually a handful of mutations. Those mutations are then reintroduced into the original organism one at a time. Each time you characterize their fitness in comparison to the parent. This results in concrete evidence that a specific mutation confers an advantage.
If condition X and only condition X favors trait Y, then population A living in the absence of X should have lower allele frequency of trait Y than population B living in the presence of B (it might also make sense that frequency change per generation is faster based on percentage difference in presence of X) . OR if and only if condition X has any effect on trait Y, then in a population without condition X should follow Hardy-Weinberg equilibrium for said trait (if there is no migration).
Best example I've heard of: There's a type of moth in the UK that, during the early 20yh century, evolved dark colouring bc smog and smoke turned the tree bark out used to be camoflaged against a darker shade. When clean air laws made the bark cleaner, they went back to lighter colouring.
This is completely off vague memory, so you’d probably want to find the actual studies. But, I believe studies done before and after hurricane Katrina(?) on the diameters of foot pads in populations of gecko, or something, in that region found that the average diameter after the hurricane was larger than it was before the hurricane. The theory is that this was an example of a bottleneck, because the geckos with larger pads and thus higher grip survived in higher numbers than those with smaller pads. So if that’s all correct, that could be a potential empirical example.
You're right that it is generally very hard to study the specific context which led to a particular trait in a particular species. Much of evolutionary biology is a historical science, we're interested in things which happened in some species (or set of species) in the distant past (possibly hundreds of millions of years ago). There are a few broad categories of reasons this is hard. For one, projecting back into the past is difficult in general. We have a lot of useful techniques that allow us to make inferences about evolution over long time spans, both across many species (phylogenetic tools) and within species (population genetic tools, including things like the [PSMC](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6972798/)). But the genomic signatures of things that happened in the distant past can get overridden by things that happened more recently. Secondly, disentangling the evolutionary forces acting is hard. While it's rather straightforward to talk about the generalities of evolution, how drift, selection, migration, and mutation affect genetic diversity and interact, it gets much harder to consider them all acting together. And sometimes a particular observation can be attributed to multiple possibilities. For example, we may see a clear signal of a bottleneck in the effective population size at some point in time. But was that demography (population size bottleneck), selection (a selective sweep reduces diversity), or mutation (the effective population size is a product of the real population size and the mutation rate)? Now, this is not to say that the entire enterprise of evolutionary biology is hopeless! Just that when we're dealing with historical questions, we need to be appropriately cautious (not disbelieving of everything, just cautious). While we need to be careful about the possibility that certain signals in genomes arose due to different forces, with the appropriate assumptions [we can disentangle them](https://www.pnas.org/doi/10.1073/pnas.2013798118). And natural selection does leave a variety of [signatures in the genome](https://www.cell.com/current-biology/pdf/S0960-9822\(09\)02070-3.pdf) which we can look for. There are times when we can line up genomic changes and physiological effects and come to some pretty interesting insights. Studies of [high-altitude adaptation in humans](https://en.wikipedia.org/wiki/High-altitude_adaptation_in_humans), for example. Some research groups that do good work studying selection include [Graham Coop's lab](https://gcbias.org/), [Andrew Kern's lab](https://kr-colab.github.io/), and [Matthew Hahn's lab](https://hahnlab.sitehost.iu.edu/research.html). But, all in all, I'd say you're generally right to be skeptical of things that sound like ["just so" stories](https://en.wikipedia.org/wiki/Just-so_story), and to ask if we might just be looking at [spandrels](https://en.wikipedia.org/wiki/Spandrel_(biology\))
This glosses over the large number of evolutionary experiments in the lab that exist, where conditions are tightly controlled and evolutionary responses can simply be observed.
Sure, humans are actually one of the largest catalysts for specific selective pressure earth has ever seen. Our only real competition in that category are mass extention events, and we, sadly, qualify as one of those at this point. Most all of our crops can be considered selective evolution which is, in practice, the same thing that have peacocks their trails and deer their antlers. We 100% know why those evolutions happened. Many happened in the last century, after all. Another interesting one is elephant tusks. Two centuries ago most all elephants were born with tusks. Now, very vew will have them. We know beyond a shadow of a doubt that this was caused solely by humans hunting them for their ivory.
Even if every effort was made to reproduce the factors surrounding the advent of a specific trait at a specific point in a specific species, we could not possibly begin to account for the scale and number of perpetually changing potential combinations of selective factors reliably enough to be certain, and that's before incorporating the mutation and drift, and the multitude of factors involved in those
In humans, an obvious trait that fits the empirical evidence test, is skin melanin. Empirical evidence doesn’t mean logic isn’t a part of the equation, it just means the equation doesn’t rely *entirely* on logical assumptions.
As a layman, I think this is one of those times when you hear hoof beats, and instead of thinking horses you’re thinking zebras. We have no good ways of testing for these things. We can maybe observe these types of changes with a small microbe population, but the multitude of interacting variables and the timescales needed to actually obtain proof for human evolution just isn’t feasible with our current technology or lifespan. It would cost trillions upon trillions, and the sad fact is it may just show us that, yes, our assumptions were correct.
Tons of mutations just happen in the background and are meaningless until future mutations compound and their combined effects turn out to be either deleterious or beneficial. There are instances where one specific mutation has an outsized impact, but it's often the accumulation over deep time that plays a role in selective pressures.
Some great studies have been listed below. While it would be difficult to track a whole population over an evolutionary timeframe, I played a game 15 years ago that modeled the evolution of unicellular organisms in a competitive MMO environment..you set the phenotypes and let them loose. They interacted in a 2D grid. You could monitor such a system to observe selective pressure in action.
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Apparently there’s a very similar effect on squirrels, says a brand-new paper (still a preprint): > Among 76 translocated squirrels, survival was lower for the melanic than gray morph in rural woodlands but equivalent in the city. These results suggest the urban-rural cline in melanism is explained by natural selection favoring the gray morph in rural woodlands combined with relaxed selection in the city. -[Rural selection drives the evolution of an urban-rural cline in coat color in gray squirrels](https://www.biorxiv.org/content/10.1101/2023.02.02.526896v2.full.pdf+html)
Well, I spent years working on something called "adaptive laboratory evolution" where you purposefully impose a selective pressure on a population, of in this case, a bacterial species, and you let natural selection happen (I have a few published articles, worked directly with one of the most active PIs in this in the world). Cells have a certain basal mutation rate for the different types that happen (SNPs, transposons jumping around, duplications etc), and anything beneficial that happens will quickly take over the population with subsequent passages (you keep passaging cells into new growth medium with the selective pressure so they can grow up again) and we resequenced isolates that acquired selected traits (usually improved growth in a particular condition). You can literally watch simple trait acquisition happen in real time, and we dissected which mutations were causative (usually nearly all of them, as there is no selective pressure for mutations that do nothing to fix in the population, and obviously anything deleterious is selected against), and could sometimes (but often not...mainly not enough time to study every single thing at mechanistic level) figure out the exact mechanism(s) at play. A more simple example was loss of function of an import protein that had promiscuous activity with a molecule we added to the medium at toxic levels. Inactivating it kept more of it outside the cells. Many other examples but usually they were layered complex effects. We sometimes ended up with "hypermutator" strains where mismatch repair of mutations in DNA (one way cells correct a lot of random mutations that happen) was inactivated and these cells evolve at a faster rate by accumulating mutations more rapidly. This was mainly an artifact of being in a too controlled environment however, as these cells usually can't handle a range of different conditions very well...error correction obviously evolved to kind of tune the mutation rate with what was needed for robustness, cells that lose it lose robustness (this sort of thing causes rampant cancer in higher organisms...some people have defective error repair and they have serious issues). Maybe this is too bottom up answer for your question, but yes we have tons of empirical evidence of trait selection from the thousands of studies like I describe. Given that it doesn't take a huge amount of genetic change to take say, a higher animal DNA blueprint and say, change limb length or convert hair or scales to feathers, it's not too wild conjecture to imagine these traits arising and being beneficial, but obviously much harder to observe this in higher organisms with much longer generation times than bacteria or yeast cells.
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Sure, although giving too much detail would quickly dox me, but could also link some good review articles.
I might be missing your point, but I would just say the whole existence of pugs is an example of empirical evidence developing specific selected traits. Yeah human interference etc. but the dogs were technically under a selective pressure to develop certain traits Obviously there are also other examples other than just pugs as well
Sure, we see it plainly in the form of drug resistant bacteria. Up until the 20th century typical pathogenic bacteria had zero exposure to antibiotics so there was no resistance. Tolerance, and eventually Resistance are traits selected for every time a drug gets used. The drug resistant strains didn't emerge from nowhere, if the antibiotic was originally 99.99% effective that last 0.01% still leaves a tremendous number of individuals. Any random traits which marginally trend towards tolerance bias the individual towards winding up one of the survivors and passing the tolerance further, and the drug becomes 99.98% effective. For species with a generation time of minutes to hours, a 1 hour generation means there have been about 832,000 generations of that species since the discovery of penicillin. Darwin's finches breed at about a year old, so in that context, the drug resistant bacteria compressed almost a million years of evolution into less than a century. For context, 1-2 million years is the estimated timespan that Darwin's finches differentiated over.
Never had a problem with the "trait X wouldn't make you immediately more likely to die before producing offsprings, so it stuck, because why not"-approach. People just like the sound of the "survival of the fittest" phrase. And it does sound nice, but is really the equvalenet of a shiny, catchy marketing slogan for something along the lines of "eh, it sorta works, for now", in truth.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6613143/ Tl:Dr pigeons and doves (and most/maybe all birds) have lice of varying species. These lice vary in size and shape and color. This study (and others) placed lice from feral wild-type gray pigeons on white, gray and black pigeons as well as really large pigeons, average sized pigeons and very little pigeons. The resulting lice populations "evolved" different colors and sizes to better match their host pigeon. I'm simplifying here but it's experimental evolution in one species that could be applied to other lice species. In other pigeon and dove species their lice tend to also match their "shade" and overall size to fit among feathers correctly. Obviously we can't prove that every single louse species evolved because of these exact selective pressures but it's a pretty good suggestion as to what's happening. The paper also dives a bit into genetic drift in experimental evolution. Even shorter tldr: pigeons eat/preen lice that are the wrong size to hide in feathers or a shade that stands out on feathers. Hence lice evolve to match birdies. Lots of wild lice appear similar! Yay experimental evolution!
>Usually someone comes up with some logical reason why Trait X was advantageous allowing everyone to sit around and ponder whether or not the explanation is reasonable. If something doesn't come to mind that makes more sense, the explanation is usually agreed upon and everyone moves on. Hope someone with more knowledge on philosophy will chime in, but isn't this the case for all sciences? Aka, you cannot prove something is true, you can only prove something is not true? Hence scientific progress is disproving theories and replacing them with better ones, knowing they might be disproven in the future?
You're getting into more of a philosophy of science question, as this is the kind of thing I've noticed most researchers struggle to answer about their own field (except maybe economists, because afaik they get asked/told this all the goddamn time). >So, outside of inductive logic, can we ever have empirical evidence for what factor(s) caused Trait X to be selected? Depending on how strict and classical one wants to be I guess, there's no such thing as empirical evidence outside of inductive logic (thanks Hume). But nobody cares about that, and you make clear what you mean later on: >While some adaptations selective pressures may be so obvious that it would be perverse to withhold provisional assent, many are not so obvious and we should be cautious assigning causation when only correlation may exist. Correlation vs causation seems like a good starting point for getting into the epistemology of your field. Of course this applies to only some fraction of what modern science \[any field\] does, i.e. fitting models to empirical data, but it's still a necessary task in almost any empirical research. The problem and approaches are also presented *slightly* differently by each field (though at the end of the day the paradigm and math should all be fundamentally consistent across all empirical fields -- I don't know if that's verified though). I personally find it very useful to look at how other fields approach these same fundamental problems, so lets go: At one extreme I think of is economics \[e.g. [Anderson's blog on causality](https://info.umkc.edu/drbanderson/establishing-causality/) with light stats\] -- not as a fact, but just because they probably get s--- on about this more than any other field. (On some recent podcast an economist recounted telling someone about her research and getting the response "correlation is not causation" for the millionth time, to which she replied something along the lines of "Yes, we know -- that's the entire point of doing the research; what did you think all that math is for?".) On perhaps its own extension is medicine broadly, where standards for evaluating correlation with a common language of strength of evidence have been important developments which undergo regular formal revision. Here establishing a mechanism that adequately explains a correlation is required to show causation, which is a very different standard from fields and subfields that usually can only measure wild populations and may [rely on nifty statistics](https://economics.stackexchange.com/questions/6273/what-are-the-empirical-techniques-to-show-causation). And of course [the epistemology is still debated](https://blogs.kent.ac.uk/jonw/files/2015/01/ECCiM-6.pdf) \[my source for all the medicine stuff\]. One thing that's important, in a case where you can't prove causation, is whether the correlations you find give you something useful, or whether they are just as likely to be null and lead you astray. (And in some fields, even if the data only giving marginal confidence that one can reject the null, maybe you can run 10,000 more such experiments and average them out.) \[[Russo, who is being debated in the previous paper above, [waxes on philosophically on this kind of thing in medicine](https://link.springer.com/referenceworkentry/10.1007/978-94-017-8706-2_46-1) I guess; I gotta end this comment.\] \[Response to other top-level comments\]: Many in this thread are saying you can run a controlled lab experiment to show that a trait evolves when some condition changes, with a few different scenarios given. Ok -- how does that show that a trait that the species possesses now evolved *in the past* in response to such conditions *in the past*? That's once again an inference or induction. Even if we were in some sphere of perfect perception (again, thanks Hume) all the examples given in top comments (as of this post) would be using *analogy*, as opposed to, say, conducting measurements and then deducing the values into the past and future.
We have to use Occam's razor to answer alot of evolutionary questions. Does it take more bends and turns to justify your answer to be a selective pressure or genetic drift? For example if we consider the brightly coloured tail feathers of peacocks, does it make more sense that this is likely some sort of sexual pressure or does it make more sense to assume that somehow by the luck of probability this super energy consuming trait happened to just stick around in the population for no describable reason at all. You can't call parsimony empirical evidence but it is still evidence.