For reasons we'll go into, it's not entirely clear where this view comes from, but it's often justified as a form of materialism, as if Genesis According to Darwin is true and therefore selection is the proper riposte to Genesis According to God. But it's highly misplaced in science, and the assumption obscures an actual discussion. Let's agree that if we'll stay within the confines of materialism--that is, no mysticism and that the laws of physical nature must not just be consistent with, but must also ultimately explain, everything biological--we can examine the nature of strongly determinstic Darwinism.
So let's consider the proposition that every trait is due to natural selection. We'll use an example. First we need a trait. Ok, here's a picture of my thumb (not used out of vanity over its undoubted beauty!). Is 'it' a 'trait'? If it is, of course, we can speak about 'its' evolution. If it's not a trait, then what is it and how can we understand it?
|Thumbs up...or angles out....or....?|
On second thought, maybe the whole thumb is not a 'trait'. Maybe it's the angle, θ, the measure of the bend where the second phalanx starts. Or is the trait that my thumb has a second phalanx? Or that I can make that angle? Or is the trait the length, x, of the second phalanx? Or the angle between the thumb and the first finger? How do we decide? All of these traits or parts of traits have in some way evolved together, so it's hard to say when one aspect begins and another one ends, that is, what is the 'trait'.
But let's say we make our decision on that question. Then, if whatever we decide our trait must, by the usual assumption, have evolved by natural selection, we want to be able to say what it evolved 'for'. And to do that, we must show that the trait we're assuming resulted from natural selection somehow had direct fitness (survival or fertility) effects, because, since to selectionistic explanations chance is not an option, differential fitness is the mechanism by which natural selection works. If our trait is θ, that means that if my ancestors had had an intrapollectial angle (pollex means thumb in Latin, and therefore using the word makes it 'science') that was--what?--greater than my θ, or was it less than my θ, or was it enough that it wasn't exactly my θ?, then they didn't have any kids and I wouldn't be here!
But wait! What if the trait was the length of the distal pollectial segment, x, that mattered. Those ancestors with more stubby thumb-ends are fossils without issue (that means, by the way, that when paleontologists dig them up they can't interpret what they were adapted for, since the death was because they weren't adaptive). Or maybe those with gross thumbs whose last digit was a tad too long (how much is too much--one millimeter, say?) died out. The trouble of course is that even my adaptive distant ancestors are dead, and how can we say whether they were 'fully' adaptive and died after having many kids, or were just caught by a lion when they wandered carelessly away from the campfire to take a pee one night (we don't need to consider whether, perhaps gender-related, the thumb trait was involved in that latter process).
Now given my important pollectial triangle (here we consider θ and x to have jointly been the critical 'trait', though what justification there is for that is an open issue), our task is to understand what natural selection molded it for.
The obvious thing is to look at what we actually do with our thumbs (that requires a triangular configuration like mine). Most anthropologists will argue that our thumbs are for tool use--at least, it says that in textbooks about thumbs, so it must be true. But I know that my deep ancestors didn't have pens and pencils or space-bars to tap, so it must have been for throwing stones in annual sporting events where the winner impressed the girls and, well, you know....
What about holding a rake in the fall when I clear my yard of leaves---it couldn't have been for that because my ancestors evolved in Africa where you don't have to rake leaves in the fall because it's tropical and there is no fall! No, I have to think of uses that aren't just modern exploitations of a trait that was strongly selected in the past for something they did in the past. This is a bit now-centered, because in the past the use of the traits they had then might not have been for what they did then, but what their ancestors did in their distant past.
Anyway, I suppose since the trait must have had important fitness-related function, if it is assumed to have evolved by natural selection, it may have had a role in sexual stimulation of some sort. One can let one's imagination run wild, but we needn't go that far. As I was saying, other than for modern uses, what I use my pollectial angle for is for getting itchy specks of dust out and sleep-grains of the corner of my eye. Without that angle, θ, I wouldn't easily be able to do it. Now on the dusty African savannah, when sharp-eyed spotting of prey and aiming of spears or hunting-stones was vital, the eye-clearing use of thumbs is an obvious explanation.
There is the annoying little question of why not everyone's thumbs have exactly the same triangle--side length or angle. If this is an adaptive trait formed by natural selection long ago, which it must have been since humans around the world have pollectial triangles (and so did Neandertals and even Denisovans!), then why is it still so variable?
Well, I'm getting tired of all this speculation. This is especially unnerving because I can think of other aspects of thumbs that might be considered as traits, or the trait may be a complex of traits with their own individual selective reasons. Or, really disturbing, these assembled aspects may have had different functions (that affected reproductive success) during the million years in which the trait(s) were adaptively evolving.
Now selectionist axioms don't provide room for probability in reproductive success. That's why earlier I said that if my ancestors' value of θ was even a bit different from mine, then they must not have reproduced; if some tolerance was allowed, then fitness would be probabilistic, and that means chance, which is not allowed: the trait has to be functionally deterministic. That's because once you admit probabilities (genetic drift), you open the door to all sorts of less satisfying, less simple, and less clear-cut explanations--and Nature and the Times will shun your papers. Part of the fear is that if one says a trait's presence is for 'random' reasons, it opens the door to creationism or, worse, lack of some explanation that's simple enough for undergraduates to understand or the research to be reported in the NY Times. It raises the specter of a 747 arising randomly out of a pile of scrap metal.
This is a silly non sequitur, since nobody (not even neutralists, who somehow can conceive of aspects of traits that arose for no adaptive reason at all) denies the role of various sorts of constraint, including classical natural selection, even when invoking genetic drift. But it's just as silly to think that every tiny aspect of every trait has to be here because of specific natural selection in the usual sense. Darwin rationalized various aspects of traits, such as correlations among them, basically as having developmental-constraint reasons and so on. We know of such things in better molecular terms today.
More importantly, it is the definition of what counts as a 'trait' that we should be thinking more seriously about. Organisms evolve as organisms, not just as sets of traits, and to the extent that a trait is in the eye of the definer, speculations about what it's 'for' may also be just-so stories in the mind of the definer. So is the use of selection as an axiom: if something (whatever that be) is assumed to be here because of selection, then to say that it's here because of selection is a tautology. The care one must take to avoid such tautological thinking has been known for many decades. There are serious questions about evolution's mechanism that usually are swept under the rug because they point out inconvenient complexity that does not lead to convenient publishable answers. Who decides where a 'trait' s/he can define is or was a unitary phenomenon during its evolution? And who decides how precise the hypothesized adaptation was? And who decides how much of its variation may have been tolerated or even itself adaptive?
Trait micro-definitions, arbitrary post-hoc trait definition, and so on show by example how hard it is to legitimately decide what to include. And if, as most of the evidence both theoretical and observational suggests, typical selective advantages are on the order of about one per thousand (you have 10001 kids with a good thumb-angle vs 1000 without such an angle), that is almost meaningless besides being literally impossible to prove. It certainly means that traits with socially sensitive implications when interpreted in strong selectionist terms are, evolutionarily, essentially meaningless, as is the very notion that selection like that is a determinative 'force'. Many such traits probably exist, and most traits may be like that, if we could even decide how to define them. They should be deprived of their societal implications or value judgments, or much of the scientific surety with which the explanations are too-often couched. Even mate-choice criteria, that could be very close to fitness, if the trait is used only now and then because there really wasn't much choice in human ancestral populations or kinship rules specified who you mated with, may mean little locally even if over long term the scenario is true.
The point about thumbs may seem like a kind of reductio ad absurdam, but is it? Viewed from some time or vantage points, structures whose adaptive origins may seem obvious might be as problematic as my thumb from some other frames of reference. For thousands of generations it could be that, say, the angle or size of the iliac crest would seem as trivial a 'trait' as my thumb, while today it seems fundamental to upright posture.
I must confess that my patience is running low (that's one of my traits!), so I am simply ignoring the wrinkles in the thumb. They may actually be the trait that evolved adaptively, though 'for' what is beyond me. Probably I'm just tiring. You can provide the explanation without my help, I'm sure.