Re: [asa] Question on speciation

  Bill asked:

> A biological species is a population that is reproductively isolated
> from other species -- while interbreeding may be possible, such unions
> don't produce fertile offspring.
>
> I have heard it said that speciation usually takes place in small,
> isolated populations.
>
> Here's my question: Suppose one individual carries a mutation that
> makes him/her reproductively isolated from the members of his (former)
> species. Then in order for this new species to propagate, a member of
> the opposite sex in the same population must carry the same mutation.
> And of course the two must mate. Whatever the probability of the
> mutation in question is, say p, the probability of two individuals
> carrying the same mutation is p^2, where I have used ^ to signify
> exponentiation. Isn't this an incredibly small probability?

Reproductive isolation does not develop (except very rarely) through
some single mutation. Rather a particular population of individuals
becomes reproductively isolated from the rest of that species. In that
isolated condition it continues to evolve - to accumulate mutations -
independently of the parent population. After some period of time,
that isolated population may become sufficiently divergent that it does
not interbreed with the rest of the species when that population comes
back into contact again.

There are a number of ways in which a population may become isolated.
The easiest to understand is that of geographic separation such as
often exists on island populations. Populations can also become
separated due to partitioning of resources or environments -- for
example insects being isolated on particular hosts or food sources.

The types of evolutionary changes that can result in reproductive
isolation once the population are reunited are also highly varied.
Some examples include: mating behavior changes, changes in features
associated with sexual recognition and selection, changes in breeding
times, changes in resource specialization, genetic barriers to
interfertility, anatomical barriers to interfertility, etc.

There is also the possibility of producing reproductive isolation
through a single mutation on a single individual. However, this will
only work in a an organism that can reproduce asexually or clonally.
This is actually quite common in plants. I can also occur in animals,
and even some vertebrates.

I should also state that speciation is a reality having been observed
in many examples of plants and animals in the wild.

Keith

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Received on Fri Feb 1 13:51:44 2008