@jongarvey’s most recent article is a good read, and is likely more correct than most popular work on evolution. He took serious the corrective of The Neutral Theory of Evolution, and it shows here, in a good way.
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.
That is likely right. Though, of course, some things will be positively selected. Without more information though, the default null hypothesis should be neutrality.
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. …
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.
Yes it is possible. We because of pleiotropy unexpected things are connected. Because of our amazing abilities, its not even clear what physical characteristics are actually advantageous. After all, we can just reshape our environment at will. It is not clear, for example, Asiatic ethnicities have much true adaptive advantages over African ethnicities for asian.
Yes, there are some good counter examples. E.g. lactose resistance (farming), hemoglobin resistant to malaria (africa), hemoglobin tuned for high altitude (himalayas). These seem, however, to be the exception rather than the rule. Most adaptive features we so adaptive we all got them. Despite our instincts, most “racial” differences are superficial and have little to do with adaptation. We are largely just neutral variations on the same theme.
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.
Well we know that both happen. So this is not that crazy @jongarvey. The exact path of selection and sequence and stepwise function, however, is very difficult to untangle.
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.
Exactly right, which makes this a non-sequitur (even though it proceeds ) :
No – if natural selection cannot be demonstrated, then special creation is still available for hire…
This gets to another point. Even when selection is important, we may not be able to detect it. It turns out it is very difficult to detect signatures of moderate to weak selection. So just because something cannot be demonstrated to be selected, does not mean in fact it was not selected.
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.
Well that is just a math error =). If beneficial mutations are 1:10^6, how many do we expect in a single generation in a population of 10,000, with about 100 mutations per birth? Turns out we expect 1 beneficial mutation per generation. What if there are 1 million people? We’d expect a whopping 100 mutations each generation. Remember, positive selection is going to work to fix these mutations much more rapidly than neutral evolution. Even if half of them are lost, that is quite a lot of beneficial change.
How many non-beneficial mutations were there? Almost a million times more! Do you see the paradox? In one sense, most mutations are neutral. However, that does not mean that beneficial mutations are rare at the population level. Population size serves to amplify the likelihood of coming across beneficial mutations. However, the number of neutral mutations will always totally overwhelm the number of beneficial mutations.
The math here is critical to make sense of this. Do you get it @jongarvey?
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.
Very true, though its not really an all-to-all relationship, and down in the weeds and contorted explanations like this might be true, and also the explanations that molecular evolutionary biology is concerned with.
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.
A better way of putting this is that species may be well adapted to their environment, but this does not explain much of their particulars. It turns out there are several ways to be “well-adapted” and the precise reason we see what we see in a given species might have very little to do with adaptation, even though it very well may be “well adapted.”
Darwinism turns to have very poor explanatory ability for this reason. Neutral theory however, and common descent, do have high explanatory ability. The reason why birds fly with feathers, whales have hair, bats lactate, and human hair goes gray has much more to do with common descent and neutral changes than adaptation.
As every population geneticist with an ounce of competence will tell you, Darwinism is dead and some version of neutral theory reigns uncontested, only refined. This has been the real revolution in evolutionary science.