Re: [asa] donkey and horse

On Mon, 9 Jul 2007 11:33:53 -0500 "David Campbell" <pleuronaia@gmail.com>
writes:
> Such examples do reflect the great range in degree of limitations on
> hybridization and corresponding difficulties in deciding exactly
> what
> constitutes separate species. There are also more philosophical
> issues relating to how one defines species, the question of how one
> applies definitions to asexually reproducing forms, etc. The fact
> that there is a continuum rather than sharp lines matches much
> better
> with the expectations of evolution than of separate creation.
>
> Examples: Frogs in which you can place a female from one population
> and a male from another in a bucket of water and get tadpoles, but
> the
> female would never come find the male herself because she expects a
> different croak than the one he makes. Baltimore and Bullock's
> orioles, lumped for a time when fertile hybrids were found and then
> split again when the hybrids were found to be at a disadvantage
> relative to either parent. In fact, they do not appear to even be
> each other's closest relative. It may be a natural example of
> coming
> into contact after separation.
>
> Over time, barriers of some sort to breeding are expected to
> develop.
> If hybridization is frequent and counterproductive, such barriers
> are
> expected to develop rapidly; if not, such barriers may be very slow
> to
> develop but are expected eventually on the basis of random changes
> in
> DNA and in traits associated with reproductive compatibility.
>
I'm going back at least half a century to recall reading Goldschmidt, who
noted a circumpolar species in which adjacent populations interbred all
around the ring. But individuals from the extremes brought together
either could not breed or produced sterile offspring. As with the frogs,
it takes human messing to test the limits.
Dave (ASA)

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Received on Mon Jul 9 14:40:58 2007