A number of posts have appeared at Uncommon Descent on the topic of macroevolution. Comments here have been appended to other threads, but I thought it an appropriate subject for its own thread.
The posts start here with a link to chemist James M Tour’s blog, on which he posted some personal musings on the creation-evolution debate. Numerous follow-on posts have appeared on UD subsequently, in a rather recursive comments-becoming-posts-spawning-more-comments-that-become-posts manner. I won’t detail them all, but they comprise the bulk of UD threads between 18th and 22nd February.
Tour admits his lack of credentials in the subject, but fundamentally expresses doubts that microevolution (which he accepts) leads to macroevolution. The issue has taken a bizarre turn since, apparently, a couple of UD regulars have offered to stump up costs for Nick Matzke to have lunch with Tour in a meeting that will be witnessed by one of them (it’s his dollar!) but, at Tour’s request, will not be recorded or discussed externally. A personal tutorial. Matzke’s mission, should he choose to accept it, is to prove to Tour’s satisfaction that the extrapolation is justified – that macroevolution is sufficiently explained by iterating the small degrees of microevolution.
I think the problem often boils down to people’s mental construct of one modern group turning into another. A dog could become a fox at a pinch, but a cat … ? Instead, place the forms side by side and draw lines of genetic continuity to a common ancestor. At the node, according to theory, that ancestral population split, not into two families, but into two species. Formally, microevolution is evolution at or below the level of species, while macroevolution deals with broader patterns. This requires a precise understanding of the biological role of species – because a species in sexual forms is not a mere convenience of classification, but is a true biological entity: the breeding population. It also requires an understanding of the role of recombination – in such a population, this enables genes to travel quasi-independently, on units that are smaller than the entire genome. But when recombination between two populations stops, and the barrier has a biological basis, they have become two species, irreversibly. They stop sharing genes, and so drift apart.
At any moment in time, the entire ‘definition’ of a species is held within the genomes of its currently living members. And of course, that is a movable feast. Gene frequencies constantly, unavoidably and often irreversibly shift in a recombining population. The statistical inevitability of this is readily demonstrated mathematically, in computer models, and (over periods unavoidably shorter than the working life of the average scientist) in the wild. Two ends of shifting frequencies are of particular significance: the day a new gene arises, and the day a gene disappears. This could of course be the same day, but with a baseline frequency that can readily be calculated, a proportion of new genes will not be rapidly extinguished, but will ‘sweep to fixation’ – they will become the sole representative at their locus, and it is the existing gene that has been extinguished. Evolution has no ‘memory’, and cannot distinguish old from new. Now, the number of generations in which this is expected to happen typically exceeds the lifetime of a scientist, but if we can demonstrate the idealised behaviour of populations (akin to, and using some of the same maths as, diffusion), and model it in computers, what is it about real populations that renders extrapolation unjustified?
This is, remember, still within the region of change dubbed ‘microevolution’, which Tour accepts. And even ark-friendly Creationists accept that and a little bit more, since proliferation of species – a macroevolutionary pattern – is invoked to explain species numbers. If two populations cannot interbreed, their genes cannot recombine – however recent their ancestry, they have moved into the part of the continuum marked ‘macroevolution’. At its lowest level, then, macroevolution is hardly bigger than microevolution. If populations can change, split populations can diverge. If that divergence is proposed to be limited as a general rule, there needs to be either a clear universal mechanism that stops such change proceeding indefinitely, or definite and widespread discontinuities in the historic pattern we see in fossils or in commonality between modern forms.
One possible ‘anchor’ is that real populations don’t just have a genotype, they have a phenotype. That tends to be the focus of all attention. It is the change in gross form in the fossil record, the morphological divergence among taxonomic classes, that really impresses. And here, it all looks superficially discrete, even to the point where, for some people, that morphology presents a barrier, a drag on indefinite genetic change. While they might accept the logic of iterative genetic amendment multiplying up into major change (think changing a book a letter at a time), the bodies of two current species cannot be connected by a genetic continuum to a common ancestor because there cannot be viable intermediates (think the demand that all ‘transitional’ books must be worth reading). But this amounts to making a judgement about the space of viable organisms, without doing any investigation. Evolutionists are wrong to assume that the space between any two species is fully connected via viable ancestors to a common ancestor, because … well, what? Where do we place the barrier between reasonable and unreasonable extrapolation? We certainly aren’t generally challenged to prove that two individual humans are connected by an audited genetic continuum. Nor that, for example, any two dogs are. Or dogs and foxes. Or dogs and cats? Dogs and rats? Ah well, now … the bigger it gets, the more stringent the demands. Is there nothing we can do to assess the claim of genetic continuity?
Early attempts to investigate the patterns were obliged to concentrate on phenotype. They had little choice; they had no idea how genotype was implemented. Those morphologies converged on nested hierarchies of characters. The perfectly reasonable assumption was that a character resemblance – canine teeth, say – was shared between all dogs because they got it from a common ancestor. And between dogs and wolves because they did also. And between dogs and cats because … well, all of a sudden, some people insist that this explanation be discarded RIGHT NOW … at this remove, extrapolation is a matter of metaphysical bias, not reasonable inference. But no sound rationale is offered to make such a dichotomy between close-branching relationship and more distant ‘common design’ between suppposedly genetically discontinuous groups.
We now have the luxury undreamt of by the early taxonomists, of direct examination of the molecular basis of phenotype – and more, of genotype that has no apparent phenotype at all beyond existing. Precisely how, in Tour’s ‘chemical’ terms, genotype becomes phenotype is not deeply understood – but that it does cannot be reasonably doubted. And we see no discontinuity. The genetic differences between close branches (accepted as common descent) differ only in degree, not in type, from those among more distant ‘macroevolutionary’ categories. The deeper you go, the more scrambled the relationship signal, but this is hardly at odds with the assumption of steady divergence – it is entirely consistent with it. At the limit, the signal would degrade entirely, even in a true genetic continuum, but even so for the most part, we are somewhere short of that point.
It could be argued that genetic continuity does not prove that the change was unguided – and indeed, it does not. But the process which appears to proceed without guidance now, inexorably changing and diverging populations, you’d expect to be in operation wherever and whenever DNA replication is occurring. All you need in principle to achieve all taxonomic categories above the species (discrete categories that are human constructs, not ‘natural’ groups based upon biological interaction) is a process of bifurcation and continuing divergence beyond the node, and viability of all intermediate forms. The appearance of genetic continuity is either deceit, or it indicates that a branching process has been in operation, and all intermediates must have been viable. We don’t need to know the precise steps, nor the selective environment (with or without Intent), to conclude that macroevolutionary divergence is real, and demands no fundamental source of variation beyond that supplied by generational change. With no evidential support at the genetic or the morphological level for an hypothesis of discontinuity between higher taxa, there is no good reason to adopt one.