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From: alanm@hpindda.cup.hp.com (Alan McGowen)
Newsgroups: sci.bio
Subject: Re: Re: What is the origin of domestic dogs?
Message-ID: <6290012@hpindda.cup.hp.com>
Date: 11 Jan 91 01:37:54 GMT
References: <20853@crg5.UUCP>
Organization: HP Information Networks, Cupertino, CA
Lines: 38

>Is the definition of species "genetic inability to breed" or "practical
>inability to breed"?  

Mayr defines the "biological species concept" as follows: _Species are groups
of interbreeding natural populations that are reproductively isolated from 
other such groups._ (Ernest Mayr, _Populations, Species and Evolution_, Harvard
1970)

Closely related species may be interfertile to some degree. The key point is
that they do not in fact interbreed in the wild -- no gene flow occurs between
the groups -- regardless of what might be accomplished by way of laboratory
crosses.

If a population is reproductively isolated for a long enough time, enough
genetic differences will accumulate for it to be considered at least a separate
subspecies. This is so even if there are no obvious morphological differences
(as is the case e.g. in the Mount Graham red squirrel).  If after the 
reproductive barrier is removed the group no longer interbreeds with the 
parent group you have a new species. Usually the cause of the reproductive
barrier is a geographical separation from the parent group. Speciation by 
this route is called _allopatric speciation_.  _Sympatric speciation_, in
which the new species develops in the same geographic area as the parent
group, is more rare.

In practice species and subspecies designations are often based on
morphology rather than on genetics. This is why it can happen that two
designated species (designated on morphological grounds) are discovered
to interbreed in nature, so that in fact there is one polymorphic species.
Conversely, geographically separated subspecies may be elevated to species 
status if evidence accumulates that the genetic difference between them is 
large enough to warrant it.

>Nick Szabo			szabo@sequent.com

----
Alan McGowen

alanm@hpindbu.cup.hp.com