Thursday, November 9, 2017

What we can learn from the birds and why there are birds

Evolution is a fact of life, but there are many different interpretations of how it works. There is the persistent classically Darwinian view, in which natural selection explains everything as a deterministic 'force'--clearly the kind of imagery Darwin himself had.  This is nowadays focused around genes as the metaphor for the competing deterministic causal factors that are responsible.  We know that even clearly adaptive traits we see today evolved through earlier stages of adaptation that may have had nothing to do with current functions.

We know now that this is a deeply important factor about the origins of the major functional traits of organisms, but also that life is complex and chance plays a major role in its dynamics.  In one sense this means selection cannot literally be force-like: it must have some 'probabilistic' aspects, even if there isn't a fixed probability, or probability process like coin-flipping, at work.  That aspect, due to competing selection and so on, is more like a series of one-off effects.  At the same time, the fast fox doesn't always catch the fleeing rabbit, so that even if selection is favoring 'fast' genes, there is an element of what would appear afterwards to have been probabilism in the change of fast-gene frequencies.

Every organism is subjected to functional challenges on all of its traits, all of the time, so that even if natural selection acted as a force (which it cannot really precisely be), which adaptive functions among this array of competing constraints win out will be affected by chance, because from trait A's viewpoint, the relative impact of selection on the other traits will always be changing.

We also know that there is complex genetic control of complex functions, and this involves gene duplication and multiple more or less equivalent pathways to similar outcomes.  So any given gene's effect on the trait will be affected by the other redundant genes it carries.

There is still a widespread, almost ritualistic view of evolution, informally at least, in terms of the genes 'for' some trait whose favorable variation was driven essentially in a deterministic, force-like way to replace other genetic alternatives in their species.  This can easily be seen even among biologists, who should know better, and especially in the biomedical community, in which at least some pratctitioners have actually been taught the premises of evolution at a serious level--beyond, for example, what is often purveyed in medical schools. 

A typical habit is to today's functions and traits, and the past's traits (only rarely the past's genes as well), and to extrapolate from then to now, using reasoning--typically informal reasoning--to connect the dots with steady lines, the way we treat objects falling to earth or planets orbiting stars.

However, much of this is because evolutionary change is highly subject to time-compression that both reflects and is caused by these assumptions.  The 'million' aspect of a million years is skipped over as if it were just a few days.  Yet, we are wholly aware of the immense timescales that apply to most evolutionary changes in complex functions, like, say, our brainpower or our upright posture.  One way to try, at least, to unhitch ourselves from these illusory lapses into physics-like determinism, is to look at things over a much more vast time scale, for which we actually have evidence.

An instructive case
It is probably impossible for us to really grasp the meaning of evolution's timescale.  That's the enormous value of mathematical modeling, if it is used properly.  Our 'ancient modern human' ancestors in the fossil record existed around 100,000 years ago, or arguably much less.  Our species has occupied the world since then, but even much of that well within the last 20,000 or so years (only around 12,000 in the Americas).

But we have some really good evidence of things on spans of times a thousand times as long--that is, on the order of 100,000,000 (a hundred million) years.  This example has to do with the evolution of flight.  A very fine discussion of feathered dinosaurs can be heard on the podcast of the BBC Radio 4 program "In Our Time", that can be downloaded as a podcast or listened to online; here is the link:

How did dinosaurs or their precursors develop the complexly rearranged bodies, and the feathered exteriors that were required for flight and the evolution of birds?  What adaptations occurred and when, and can we know why?  Major recent fossil finds, largely in China, have opened these questions for much closer examination than was possible when the first bird fossil, archaeopteryx, was found in Europe in around 1861, right after Darwin's Origin of Species (1859).

This BBC discussion, even expressed implicitly in a strong selectionistic viewpoint, shows the subtleties of the issues, when 100 million years is the span and large the number of specimens.  If you listen carefully, you can see the many nuances, small changes, rudimentary beginnings and so on that were involved--and the nature of speculation and attempts to guess at the nature of the reasons for the existence of these small steps that eventually led to feathered flight--but that, in themselves, were mainly unrelated to flight.  It is a sobering lesson in evolutionary interpretation, and even this discussion necessarily lapses into speculation.

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