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Species concepts & interfertility

>As for George's objection... The postmating isolation mechanism which
>Art wrote about may represent an impenetrable barrier to gene flow
>now, but it need not have always been that way.  That is, fertility
>may (I should probably say "must") have been relatively low (but not
>zero!) between the lineage which originally contained the
>rearrangement and the parent population which did not.  To be more
>concrete, Chimpanzee and human chromosomes can't currently line up
>well enough to allow development to take place from the egg of a
>chimp-human cross, but the chromosomes still partially align
>themselves.  Before the lineages diverged to their current extent, the
>alignment would have been better, and must have allowed for
>interfertility.  Or we wouldn't be here.

For another case, brown bears (Ursus arctos) and polar bears (Ursus
maritimus) are capable of having offspring (some of which are even
viable!!).  Although most mammalogists agree that these are separate
species, they have not yet diverged enough to prohibit chromosomal alignment.

Thomas R. Holtz, Jr.
Vertebrate Paleontologist
Dept. of Geology
University of Maryland
College Park, MD  20742
Email:Thomas_R_HOLTZ@umail.umd.edu (th81)
Fax: 301-314-9661