theory wanted

mel turner mturner at
Wed Nov 16 16:34:38 EST 1994

In article <3aaom4$79f at> frist at (Brian Fristensky) writes:
(much I don't disagree with deleted)

 >allopolyploidy - addition of one or more chromosome complements from a
>  different species. This would be the result of a rare intercross 
>  between two closely-related species. The important point here is
>  that the two 'species' must be interfertile. What this really means
>  is that these species are recently diverged in the first place,
>  and there hasn't been enough time interfertility to be lost.
>The point is that, by the strictest definition, species A & B
>are really still one species, because they are still interfertile.
>If fact, when we identify separate plant species, what we are
>really doing is identifying separate populations that have been
>reproductively isolated long enough to have diverged with
>respect to morphological characteristics. It can often (but not
>always) take a long time before interfertility is lost.

No, in many cases of classic allopolyploidy that have been studied the 
parental species involved are not even one another's closest relatives.   They 
may belong to different sections of a genus or to different related genera.   
They only  need to be "interfertile" enough to be able to make a sterile 
diploid hybrid.  The populations of the two parent species thus remain 
reproductively isolated, even if the sterile hybrids  later give rise to a new,
fertile, polyploid species.

I agree that same-ploidy-level  _introgressive_ hybridization  can be 
construed as evidence the forms involved are members of one species, according 
to strict definitions of biological species. 

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