I enjoyed a little innocent mischief on Joshua Swamidass’s Peaceful Science website last week. In response to someone reporting the news of ancient human finds in China (tools 2.2 million years old), somebody else (George) suggested that, since it has been hypothesized that the epicanthic folds of Asians might be an adaptation to cold, this would accord with a population that had had to go through several ice ages.
I replied in the spirit of another thread, on neutral evolution. Remember that neutral evolution states that most changes in evolution are near-neutral and not acted upon by natural selection, which mainly acts as “purifying selection” in eradicating highly deleterious changes. Nobody seems to put money on precisely how much adaptive selection actually occurs. For example, Josh suggested on that thread that most of the differences between peregrine falcons and hobbies would be neutral, not adaptive.
In that spirit, then, I suggested that epicanthic folds are more likely to be just a near-neutral matter with no biological significance, and all the stories about adapatations to extreme cold (or extreme heat – both have been proposed!) are just that – stories of the Just So variety.
But the story didn’t end there. George fished out a reference to a key gene for epicanthic folds actually being thought to be an adaptation to increase lactation in low Vitamin D conditions (in an ancient population in Beringia, near the Bering Strait). This invokes pleiotropy, the fact that genes often code for two, or many, entirely disparate functions. That would make epicanthic folds what Stephen Jay Gould called a “spandrel” – an accidental feature caused by some other adaptive change and along for the ride, the adaptation in this case to do with lactation, and the spandrel the more obvious epicanthic folds.
One might add that there’s no reason that the lactation function might not itself be a spandrel of some third, unguessed, function, or that the whole linked genetic network might consist only of neutral changes fixed by drift. It’s all possible.
In an old discussion of neutral theory on Larry Moran’s blog, respected biologist Joe Felsenstein tried to strike a balance:
…the high level of adaptation of living systems (a level they have to have, else they could not survive and reproduce) must have been the result of natural selection. Mutational processes alone and/or genetic drift alone just could not have made a bird that flies or a fish that swims, not ever.
Note the logic here – even if neutral theory appears overwhelmingly dominant scientifically, adaptive selection must occur because nothing else explains the amazing functionality of the living world. But remember Darwin’s original proposal (paraphrased): “Things look wisely designed, but I think I can prove a natural alternative to special creation that would do the job.” The argument loses all its force if it runs, “Although adaptive natural selection cannot be clearly demonstrated, it must occur.” No – if natural selection cannot be demonstrated, then special creation is still available for hire… or else one must find some other theory leading to adaptation, which is both an observed fact and a logical necessity. And apart from outliers like Austin Hughes plasticty-relaxation-mutation, I know of none.
The demonstration of adaptive selection is indeed problematic at several levels, not least the rarity of truly beneficial mutations, the wildly different estimates up to 1 in a million or more showing that they’re not really measurable. That looks bad for a mechanism that is supposed to account for everything in nature that works well.
Be that as it may, various genetic markers are used to identify genes (not traits) under selection. However, as the late Austin Hughes also showed:
Contrary to a widespread impression, natural selection does not leave any unambiguous “signature” on the genome, certainly not one that is still detectable after tens or hundreds of millions of years. To biologists schooled in Neo-Darwinian thought processes, it is virtually axiomatic that any adaptive change must have been fixed as a result of natural selection. But it is important to remember that reality can be more complicated than simplistic textbook scenarios.
Given pleiotropy – with genes coding for sometimes hundred of functions, or even more with alternative splicing – then even if a gene could be demonstrated to be “under selection”, it doesn’t tell you that oriental eyes are advantageous, rather than lactation with less need for Vitamin D, or slightly altered height, or any one or more of the other linked functions. Given that there are those suggesting that every gene has some role in every function, that too is unhelpful in explaining traits.
Now that brings us back to Just So stories, the sine qua non of adaptive evolution. Darwin attempted to show how individual instances of adaptation matched possible processes of natural selection, but it’s important to remember that virtually no examples have been proven. Even textbook instances like the melanotic peppered moth (here and here), relatively simple as a trait both genetically and phenotypically, and with a clear proposal as to environmental cause, has been disputed for over a century, and may not be finally settled even now.
This did not matter great deal in the heyday of Darwinism. You have a bird of paradise? Slap a sexual selection story on it, and it’s good enough an explanation to be satisfying, at least until you discover how often the females mate outside the species, or even the genus. Fast cheetahs chase Thompson’s gazelle? It must be an arms race, until you find that cheetahs evolved hunting entirely different species and are too genetically inbred to adapt.
Time has shown all manner of these stories to be wrong (eg this piece of mine). But one could always invent another, and eventually, one supposed, one of them would prove right somehow. Meanwhile, it was the principle of the thing – if your particular scenario was wrong, or even if all your scenarios were wrong, something else of the same kind would be true, because natural selection watched over all. After all, you can argue about the causes of the First World War, but in the end you know the best team won, by definition.
So epicanthic folds are not due to cold, or to heat? No matter: natural selection means they serve some adaptive function, whether that be sexual allure, keeping out dust, or more imaginative things. But if adaptive evolution can no longer be assumed in any particular case, then what happens to our impression that we understand, in general, why all those “endless forms most beautiful” are so well adapted (which, as per Felsenstein, they have to be or else they could not survive or reproduce)?
It is, in effect, thrown to the winds. Why is a polar bear white? Obviously an adaptation to camouflage it. But somebody points out how easy polar bears are to see in the snow… so maybe it’s about thermal regulation. But in fact there’s no reason why that should be. Whiteness might be a spandrel linked to Vitamin A metabolism. Or, most likely under neutral theory, it just happened and wasn’t deleterious enough to become extinct, becoming fixed in the population fortuitously.
Well, we’ve discovered the prevalence of neutral mutations. We’ve discovered pleiotropy. We’ve held the faith that adaptive natural selection must sometimes, at least, operate, though there appears to be no consensus on when, and how one decides. We’ve even discovered other things like niche construction, in which the polar bear, finding itself white and so ostracised by brown grizzlies, migrates to the ice where there is less competition, or where it some other unspecifiable advantage. But if the explanation for the cream bear could be any of these, have we done the slightest thing to explain what Darwin set out to explain by proposing his theory, that is why each creature is so wonderfully what it is?
Kipling’s Just So stories appeared at a time when humour, like society, rejoiced in order. The way the elephant child got his trunk is entirely logical, and in fact could almost be predicted under the premise that he was poking his nose into where a crocodile lived.
Humour more recently, though, follows the pattern of MontY Python’s Flying CirCUS. The laughs are in the disconnectedness and irrationality. We pan out from the dinner-suited TV presenter’s desk to find he is actually in a desert, not a studio. And so on. The humor lies in the meaninglessness of the thing – nobody expects a giant boot to end the Liberty Bell March, and nobody expects the Spanish Inquisition. Uncertainty is great for laughs – but it’s not so good as a theory to understand life. If the elephant got his trunk by the crocodile’s stretching it, or because long trunks are accompanied by good digestion, or because they just happened by neutral drift, then we can’t even construct a plausible Just So story.
In fact, when science jettisoned universal adaptive natural selection in favour of today’s near-neutral theory, it greatly weakened the observational axion of Darwin that species are wonderfully adapted to their environment. Or rather, it didn’t weaken the observation, but the theoretical basis explaining the obervation, as Joe Felsenstein objected. The guy who understands the theory but doesn’t observe nature may, it’s true, deny the adaptation – but in that case we should criticise his myopia, not doubt our own knowledge of nature.
But under today’s best theory, we just can’t know how the elephant got his trunk. Maybe it was by special creation after all.