Tops and turnips

This interesting paper has attracted some attention from the Intelligent Design websites like ENV and UD. And well it might, because it has reversed the usual Darwinian understanding of evolutionary expansion as an increasingly divergent tree, and replaced it with something more like a turnip. “The turnip of life” … doesn’t quite have the same ring, does it?

The basic idea was to take around 1000 clades dating from the Cambrian to the present (the “Phanerozoic”), and study the pattern of forms that have arisen from them over time. The classic picture is that the pattern should normally appear like an inverted cone, or maybe a spinning top, with forms becoming increasingly diverse over time. That’s not what they found.

The paper, unfortunately, is behind a paywall, but you can download the supportive evidence as well as the abstract. The researchers found a spread of patterns from “top heavy” clades as per the classical picture, to “bottom heavy” turnip-like patterns, in which rapid divergence occurs at the bottom of the clade, but with diminishing diversity thereafter. It also found some clades with no particular bias over time.

But when the results were corrected for the five mass-extinction events (such as the K–Pg event that did for the dinosaurs), in which anything up to 75% of all species and many higher groups disappeared, bottom-heavy clades outnumber top-heavy ones by three to one. In other words, unless a clade is cut off in its prime, it’s likely to show most diversification early on, rather than later.

The authors say that their figures cannot distinguish between effects due to environmental factors (ie selection) and developmental effects (primarily mutations). But in fact they also point out that there are no sudden expansions of existing fauna to fill ecological niches soon after major extinction events, which leads them to conclude that the major factor is the effect of new “evolutionary breakthroughs” (for example, legs on tetrapods, birds’ ability to fly and so on) which provide a new range of opportunities to exploit new lifestyles.

This is significant because, in classic Neodarwinism and in many minds even today, the key arbiter of change is natural selection. A constant pool of almost infinite genetic and phenotypic variation is assumed to exist, much as in Darwin’s original theory, when gradualist variation was supposed to have no real limits. Indeed, in classical population genetics mutation is downplayed to the point where much evolution can happen just from gene-frequency changes from selection, recombination and drift; mutation just being the way life tops up evolution’s fuel tank.

But if the study is right about the predominance of bottom-heavy clades, and about the reason for it – the emergence of evolutionary innovation – then the rate-limiting step that defines the most important part of the process of evolution is not selection, but variation. Natural selection, then, is “held up” by the lack of the “right” sort of mutation. Somebody has to invent flying, or the other major innovations of a particular clade, before natural selection can set to work to tweak it.

Or to put it another way, evolution doesn’t gradually produce wings or flagellae – instead the production of wings and flagellae drives evolution. It’s quite a different theory, you’ll agree.

Furthermore, of course, a clade is just as big as you want to make it. At one extreme, a clade centred on LUCA would be as extensive as the biosphere. On the other hand clades of, say, woodpeckers or hamsters might be beyond the resolution of the fossil record, compared to those included in the study. Yet it might be that the bottom-heavy picture applies even at such small scales as well, in which case the role of natural selection would look even more restricted, and micro-evolution would just be macro-evolution by mutation in small letters. That’s a bit of a reversal of the usual claim.

One further point of interest is why clades start losing diversity as time goes by, in the absence of major extinction events. The only mechanism for de-diversification is extinction of forms. It would seem either that competition comes along in the form of a taxon with an innovation beyond the adaptive capability of existing species, showing another limitation of evolution’s abilities, or that there is some kind of genetic degeneration inherent to life-forms, as suggested by John Sanford in his genetic entropy theory. Which would beg the question of where the new, genetically re-invigorated, clade came from.

Either way it seems to be the environment that’s the given, and the organism that does the heavy lifting of evolutionary change, rather than the reverse. So the original, old question remains: what leads to the sudden development of significant and useful new features in living things?

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About Jon Garvey

Training in medicine (which was my career), social psychology and theology. Interests in most things, but especially the science-faith interface. The rest of my time, though, is spent writing, playing and recording music.
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