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From: dmark@acsu.Buffalo.EDU (David Mark)
Newsgroups: sci.bio
Subject: Re: What distinguishes a species?
Message-ID: <16447@eerie.acsu.Buffalo.EDU>
Date: 29 Jan 90 21:51:12 GMT
References: <33962@ucbvax.BERKELEY.EDU>
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In article <33962@ucbvax.BERKELEY.EDU> bwood@janus.Berkeley.EDU.UUCP (Blake Philip Wood) writes:
>I'm aware that the inability to interbreed is the standard definition
>of two populations being of differing species, but more precisely, 
>is it:
>
>a)  can't physically mate due to differing genitals.
>b)  could, but don't mate due to differing sexual lures, rituals, etc.
>c)  can mate but produce no offspring.
>d)  can mate but produce sterile offspring, ie. horse+donkey=mule.
>
>or is it several of the above?
>
It is several of the above, and more.  It also could be:

 e)  can mate, and produce fertile offspring, but those offspring have
     strong selective disadvantages, and don't succeed in producing as
     many offspring of their own as a non-hybrid would.
 f)  can mate, and produce healthy, vigorous, fertile offspring, but
     the next generation after that has selective disadvantages.

The current "biological species" model popularized by Ernst Mayr, just
is concerned about whether forms *DO* interbreeed freely in the wild,
not whether they can.  A commonly used example is the ducks, Mallard
(Anas platyrhinchus) and Pintail (Anas acuta).  Hybrids are very rare
in the wild, and they overlap through most of the Pintail's breeding
range.  In captivity, in the absence of same-species, opposite-sex,
they will freely interbreed.  The young, and subsequent generations
apparently are fully fertile and viable.  They are separated normally in the
wild by different courtship rituals, etc.  In the Mayrian view, two
forms should not be lumped if both parental types occur throughout the
overlap zone.

For other groups, especially plants, the Mayrian definition is not
really applicable.  Systematic botanists tend to use a morphological
species definition more often.

David Mark

>                 Blake P. Wood - bwood@janus.Berkeley.EDU
>                 Plasmas and Non-Linear Dynamics, U.C. Berkeley, EECS