Re: Basic evolutionary theory question.


<dog1998@xxxxxxxxx> wrote in message news:ecsq81$1pvu$1@xxxxxxxxxxxxxxxxxxxxxx
A group of a species that is isolated from other members can adapt over
time, but is there any evidence about why the ancestors of this "new
species" are not able to breed with the ancestors of those other
members millions of years (or whatever) later?

You have confused me with this talk about 'ancestors'.
Do you mean descendents?

Assuming you do, there are lots of possible reasons. If
enough million years have passed, it is possible that the
two populations have diverged enough in physique that
the parts no longer fit together.

In the case of species which have diverged less, one
possibility is chromosome re-arrangements. Suppose there
are three possible ways of ordering the genes in a particular
chromosome - call these three orders A, B, and C. Also
assume that there is no problem for individuals with
two A chromosomes, two Bs, or two Cs. Also no problem
with AB or BC. But individuals with one A chromosome and
one C chromosome are inviable or infertile. Now assume
that the original population has everyone as a BB. Split
the population in two. In one of the two separated
populations, there is a mutation in one individual changing
a B to and A. So now this population will have a mix
of BB and AB individuals, and a few generations later
also some AAs. Over time, this population could drift to
100% AA. Meanwhile, over in the other population, a
mutation from B to C occurs, and that population drifts
to 100% CC. At this point the two populations can no
longer interbreed because any hybrid offspring would be
AC and that is infertile or inviable.

There are many other possibilities for why the two species
might not be able to interbreed, but most of them can
be thought of as a triangle A-B-C with the AB and BC sides
being OK, but the AC side being fatal.

You mentioned 'evidence'. If you are unsatisfied with this
handwaving, and want to know the real facts, consult the
book "Speciation" by Coyne and Orr. I admit I haven't
read it.


.

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