Re: On the Origin of a Species

chris.linthompson@xxxxxxxxx wrote:

dkomo wrote:

John Wilkins wrote:


snip


In sexual species DNA relatedness can be very high because the reproductive
isolating mechanisms are what causes specieshood, not DNA similarity.


Reproductive isolating mechanisms are what *maintain* specieshood, not
what cause it. Species are *caused* by selection and genetic drift on
split populations, which will move the two sets of organisms to
different locations in genome space.


I think I have to jump in here. dkomo, I think this is not the case.
Selection and drift result in the evolution of isolating mechanisms.
The fastest way to generate a new sexual species is to evolve new
courtship rituals. Behavior is the most plastic aspect of reproduction
(as opposed to, say, mechanical fitting of genitalia). Courtship,
pheromones, plumage perhaps- all these can cause speciation by erecting
that reproductive barrier, and the genetic distance need not be all
that large.


Perhaps reproductive barriers play a causative role in some sympatric
speciation, which for animals is relatively rare compared to the
geographic isolation at work in allopatric speciation, where such
reproductive barriers are almost irrelevant as causative agents.

I checked a source on this:

"In allopatric speciation, a new species forms while geographically
isolated from its ancestor. As the isolated population's gene pool
evolves by genetic drift and natural selection, reproductive isolation
from the ancestral species may evolve as a by-product of the genetic
change."

--Campbell, _Biology_ 5th ed., p. 473


Note the term "by-product". The true causes of the speciation are the
geographical isolation, genetic drift and natural selection. If the
species never come back into contact (Australia, Galapagos Islands, and
other examples of island speciation), reproductive isolation is even
irrelevant to maintaining them as separate species. The only role
reproductive isolation has here, then, is as a "diagnostic" (to use
Wilkins' term) to identify them as distinct species according to the
BSC. Biologists take samples of two species and toss them together in
the lab and see if they mate.


--dkomo@xxxxxxxx

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