This post is a repsonse to Hoot's critique of a article posted by Schraf in the
Good Scientists Gone Bad -- Dr. Watson and Dr. Pauling thread.
Schraf posted an extract...
Let's look at the major claim: that humans will subspeciate. I can't think of anything less likely among a species that has major gene flow between all its populations on a scale of thousands of generations. Species aren't formed by selection for differing adaptive traits within a population, but by the interruption of the gene flow that is caused by migration or invasion between populations. Take a look at human gene flow over the past 10,000 years - massive amounts of interbreeding and invasive gene flow. Not a hint of the sorts of isolation required for a mammalian species to speciate in sight. Not even the Tasmanian aborigines, who were isolated for about 10,000 years. Not even the San or M'buti. Nada.
And Hoot said ...
Hoot writes:
Just a question of clarification: Do you mean to say that you agree with the sentence I've highlighed in orange? I'd say the author has demolished only his own credibility.
Why not try and make a reasoned argument Hoot.
No one conversant with current evolutionary theory would consider this to demolish the authors credibility, they might disagree with it, or with some of its particulars, but it is not beyond the pale of current evolutionary theory. to some extent it depends on how you read the statement.
Taking the most extreme interpretation that adaptive selection played no role, there is still some currency for the idea that non-adaptive neutral drift can play a part in speciation, even though it may not be accepted and its importance is often considered minor.
A review by Douglas Futuyma of two books on speciation mentions points both for and against a role for drift.
Futuyma writes:
Very different kinds of data, ranging from DNA sequences to correspondence between RI and ecological divergence, support natural selection, but there is hardly enough evidence, in my opinion, to support Coyne and Orr's strong conclusion that “at least one important debate has been settled: selection plays a much larger role in speciation than does drift”
Coyne and Orr appear to adopt selection as the null hypothesis for speciation, whereas drift is generally taken as the null hypothesis in much of evolutionary genetics, for the simple reason that drift operates at all loci in all finite (i.e., real) populations, whereas selection need not. The burden of demonstrating that selection is not responsible for an evolutionary event (i.e., demonstrating a negative) is, of course, far heavier than the burden of demonstrating selection; indeed, Coyne and Orr do not address the difficult question of what would constitute evidence for drift. Having, perhaps, stacked the deck, Coyne and Orr find almost no evidence that drift has contributed to speciation in nature, but conclude that there is “considerable evidence” that selection has done so
Assuming that experiments with laboratory populations can be validly extrapolated to natural speciation processes, founder-effect speciation may indeed be a moribund hypothesis, but I do not believe long-term genetic drift can yet be ruled out, and cannot agree that this “important debate has been settled”
This article also bring up another good example which may be considered a suitable basis for considering 'selection for differing adaptive traits' not to be the foundation of speciation, sexual selection. It is debatable whether the products of sexual selection coud be considered adaptive in the usual sense. For myself I
would consider them so, as adaptations to that element of the environment consisting of other members of the same species, but others might take a differing view that sexual selection is a distinct process .
That aside I think his argument falls down on as the mere fact that human have not speciated and show some signs of differing traits associated with more isolated geographical populations in no way makes a coherent argument with the proposition that species
cannot be formed by 'selection for differing adaptive traits', merely that this has not occurred in the case of
H. sapiens considered as one extended population.
And lets not forget he is talking about
within a population, or sympatric speciation.
At its best interpretation all the author is claiming is that reproductive isolation is required for speciation, the most acceptable evolutionary claim imaginable, although exceptions may be posited. Even in a sympatric case speciation is considered to require reproductive isolation, the inital basis of the RI may be due to adaptive selection on variation in the population but the speciation itself need not be driven by this.
So what exactly is it you object to, how do you understand the statement and what about it makes you think it demolishes the authors credibility?
It seems to me that such offhand glib unsupported statements argue more against your own credibility.
TTFN,
WK
™ Version 4.2
