Re: [asa] Question on speciation

Keith,

All your examples are populational and gradualist. Not that I dispute
them. But isn't it the case that if you compare major orders of
mammals (even human/chimp), for example, that there seems to be
signficant chromosome rearrangements that accounts for the
differences. A chromosome rearrangement would be more akin to a single
mutation, but one that might wreak havoc in producing viable offspring
in a mating with an individual without the rearrangement.

What kind of models are there for a chromosomal rearrangements turning
into speciation events. I suppose one could imagine a successful
interbreeding with a limited viability in the offspring, but as the
rearranged chromosome propagated in the population that the chances of
two individuals with the rearrangement increasing with time. A pairing
of two individuals with the rearrangement would may then produce a
more viable progeny and a form of reproductive isolation, especially
if there were any phenotypic consequences of the rearrangement. With
sibling interbreeding this might fix the rearrangement in a small
population fairly rapidly.

Alternatively, it could be suggested that chromosomal rearrangements
are the consequences of reproductive isolation and perhaps the end
result of a long process of speciation rather than the cause of it. Is
there any definitive opinion in the literature?

TG

On Feb 1, 2008, at 11:51 AM, Keith Miller wrote:

> 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
>

________________
Terry M. Gray, Ph.D.
Computer Support Scientist
Chemistry Department
Colorado State University
Fort Collins, CO 80523
(o) 970-491-7003 (f) 970-491-1801

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Received on Fri Feb 1 17:34:44 2008