From: Chris Taylor (email@example.com)
Date: Fri 28 Oct 2005 - 12:07:36 GMT
Hmm. I get what you're saying but this comes down to definitions
of species (and by extension, speciation) really. The inability
to interbreed at all where that occurs is often a product of
drift/rearrangements/founder, and of course directional
_selection_, but it is the 'no role in most cases' thing I take issue with; consider the plants that form (almost continuous) sequences of whatever-you-want-to-call-them; allochronic flowering or divergence of flower structure is absolutely selected for and fairly rarely is there complete isolation. Also consider parapatric in animals rather than sympatric -- at the boundary there is definitely reinforcement under selection by call/smell/shape of bits or whatever to prevent gene flow
(driven obviously by the drift/rearrangements/dir.sel. in the bulk of the population), without which no true speciation could be said to have occurred.
Full-on allopatric is fine, but even then when the species come
back into contact reinforcement shows its head to avoid
fruitless matings (under selection), and reinforcement is pure
selection. _Drift_ in flowering time (for example) could have
the same effect though of course.
Anyway it was not the substance I took issue with, rather the
degree (which I have to admit is hard to characterise).
Hair-splitters Inc. :\
John Wilkins wrote:
> Most speciation is thought to occur through geographical isolation and
> subsequent evolution, mostly by drift, founder effect sampling of the
> original gene pool and selection for local adaptation. But the
> selection here is not speciating selection most of the time. Speciation
> is a by-product of evolution of the isolate population that results in
> reproductive isolation when back in sympatry.
> The type of speciation in which selection plays a role *in causing
> speciation* is sympatric speciation. In this case variants within a
> local population adapt to divergent fitness peaks, and so results in
> divergent selection, leading to lowered fitness of hybrids. But most of
> the time this is caused more by sexual selection than ecological
> adaptation. And it requires quite rare circumstances.
> Darwin thought that most speciation was caused by divergent selection
> but it seems not, at least in sexual organisms, to be a major factor.
> Selection causes adaptation, but adaptation doesn't drive most
> speciation events.
> On 28/10/2005, at 7:28 PM, Chris Taylor wrote:
>>> But of course most speciation now is in fact thought to occur
>>> through random variation and random fixation rather than by
>>> selection as Darwin thought. There's good reason to think that some
>>> speciation is due to selection, but not much. I worry that we think
>>> only that Darwinian evolution is about selection (natural or
>>> sexual), when in fact another really deep aspect of his view is
>>> common descent, and this is not tied now to selection.
>> Selection has _no role_ in the generation of species the majority of
>> the time? Are you just purely talking about permanent absolute
>> allopatry / completely discrete allochrony or whatever equivalent you
>> care to pick?
>> Elephants and fleas will never successfully mate (having diverged
>> somewhat); but where this matters (i.e. in recent speciation events,
>> where those species ranges [or whatever] overlap) selection is key in
>> ensuring that hybrids are (1) demonstrably crap and that (2) parents
>> who find a way to avoid sinking their genes into such crappy hybrids
>> propagate more of those genes forwards to subsequent generations..?
>> Random variation and fixation is _not good_ at producing adaptation
>> without selection. Have I misunderstood you?
>> Cheers, Chris.
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