In article <Pine.SUN.3.91.951002095006.22944B-100000 at chuma> karl at CHUMA.CAS.USF.EDU ("Stephen A. Karl") writes:
>Subject: Re: man-ape hybrid
>From: karl at CHUMA.CAS.USF.EDU ("Stephen A. Karl")
>Date: 2 Oct 1995 12:25:21 -0700
>4) species status can (and should?) be assigned based upon a variety of
> characteristics. How about this -- is two organisms are
> reproductively isolated then they are two species. If not, and there are
> other characteristics (ecological, behavioral, etc.) that clearly delineate
> the organisms then they are two species.
I think it depends on what you want a species definition for...
If you want a species definition that allows you to catalogue things, then you
can use ecological / behavioural / morphological characters and define
whatever you like as a species.
If you want something which suggests an independent evolutionary unit (and
leaving out asexual lineages and reticulate evolution for the time being...)
then the presence of a barrier to genetic exchange (i.e. a
reproductive barrier) is a prerequisite. 'Biological species' are generally
defined as groups of individuals which interbreed with one another but not
with members of other such groups which occur in the same place (see various
papers and books by Ernst Mayr for (IMO) a pretty good justification).
Personally I find this the most useful definition to date.
Problems arising from occasional hybridisation are unlikely to be important in
large populations (though the situation in plants and some inverts. eg.
Anthozoa may be more complicated) as the initial frequency of introduced
alleles will be low and chances are they'll be lost in the first couple of
generations (unless there's a very strong selective pressure for the new
allele....) Introgression is more likely in small populations, so keep an eye
on those endangered turtle species (pity about their long generation times...).
What's my point? I think occasional hybrids are usually unimportant (in
animals) and a biological species concept is the one to run with...