Re: No Such Thing As "Macro" Evolution

r norman wrote:

> On Thu, 24 Nov 2005 09:32:24 -0700, dkomo <dkomo871@xxxxxxxxxxx>
> wrote:
>
>
>>I think what really produces a new species is that a segment of a
>>population migrates to new location where it becomes geographically
>>isolated from the main population. Given enough time, it then undergoes
>>enough genetic change to be classified as a new species. Reproductive
>>isolation is a mere side effect of geographic isolation, not the main
>>cause of speciation. Even if there is some gene migration between the
>>two populations, if there is enough selection pressure, the populations
>>will evolve away from each other anyway, so reproductive isolation is
>>irrelevant.
>>
>>That's my speciation theory de jour. The lesson may be, "be careful
>>what you label as ontology." One man's ontology may be another's chimera.
>
>
> Unfortunately your speciation theory is somewhat lacking. A short
> while back I indicated that this theory would get only one/third
> credit on my essay question in intro biology to describe a scenario
> that starts with one population and ends in two species. First, you
> do not describe any mechanism to produces genetic change.

Primarily natural selection, with some mutation. A change of geographic
location can drastically alter the environment, which can change the
selection pressures on the migrating population. This environmental
change is by no means confined only to climate -- rainfall, temperature,
length of growing season, and so forth. The environment also includes
the resources used by the species -- the plants and animals it eats, and
the predators and parasites that feed upon it. When a species enters a
new ecological niche, it can evolve quite rapidly to exploit that niche.

> Second, you
> are most distinctly wrong about reproductive isolation being a mere
> side effect. Without that, if you remove the geographic barrier then
> interbreeding between the isolated population and the original
> population will quickly merge the two gene pools and wipe out any
> differences.

This is a good problem in population genetics. It depends upon the rate
of gene migration versus strength of selection pressure. You seem to
imply that gene migration is strong enough to counter selection in most
cases. I'd have to see a lot more evidence and hear some good arguments
before I believe that.

You include a separate factor -- differential selection
> between the two populations. That is seen in every clinic variation
> where a species is distributed across a wide geographical area with
> different environments. Still gene flow diminishes the differences
> until the difference is so great as to include a reproductive
> isolating mechanism.
>

Certainly reproductive isolating mechanisms would increase the *rate* of
speciation, but these mechanisms could be secondary. Don't forget that
the new species had to evolve enough differences in the first place
before these mechanisms could have come into play.

So here's what you seem to be saying. Geographic isolation was the
initial cause of speciation, but *after* that isolation occurred,
reproductive isolation had to be there in order to *maintain* the
effects of that geographic isolation. So in any case, the primary
*cause* was the initial migration and geographic isolation, which is
what I basically said to begin with. Worst case, I'd only have to
concede that reproductive isolation was somewhat more important than I
indicated previously.


--dkomo@xxxxxxxx

.

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