Re: [asa] Speciation

From: Keith Miller <>
Date: Mon Oct 05 2009 - 15:40:25 EDT

Randy asked:

 Isn't it correct to say that every act of reproduction leads to offspring
> that are members of the same species as the parents? And yet, speciation
> does occur. The significance is that there is seldom, if ever, a "first" of
> any species, not just humans. So it seems that there is never a first
> organism, in the sense of a first of a species. As always, the exceptions
> prove the rule. Any time a single mutation event is sufficient to generate
> differentiation for a new species, the rule would be broken. Perhaps the
> biologists on this list can tell us how often that occurs.

Single generation speciation can and does occur. It is especially common in
plants in which chromosomal duplications can occur.

However, probably one of the most common modes of speciation is through
reproductive isolation from a parent population. This process results in
the splitting of lineages. This can occur in a variety of ways: isolation
as peripheral isolates; colonization of "islands;" or isolation by spatial
separation within the same environment, food choice, etc. In these cases,
the isolated population diverges such that development of other reproductive
isolating mechanisms prohibit interbreeding when the isolate makes contact
with its parent population. Although the new species evolved gradually (at
least in ecological time) it is an objectively distinct species.

In the case of phyletic evolution, a whole population evolves as it tracks
environmental change. In this case there is no splitting of lineages.
After some period of time, the population may have changed sufficiently
anatomically to be classified as a new species. However, the point of
distinction between the species is arbitrary -- there was no point at which
the derived species was reproductively isolated from its parent. There is
considerable debate about how often this actually occurs. Some think that
this is rare.


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Received on Mon Oct 5 15:41:15 2009

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