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Darwin-L Message Log 7:56 (March 1994)

Academic Discussion on the History and Theory of the Historical Sciences

This is one message from the Archives of Darwin-L (1993–1997), a professional discussion group on the history and theory of the historical sciences.

Note: Additional publications on evolution and the historical sciences by the Darwin-L list owner are available on SSRN.


<7:56>From kent@darwin.eeb.uconn.edu  Thu Mar 17 07:02:56 1994

Date: Thu, 17 Mar 94 08:04:15 EST
From: kent@darwin.eeb.uconn.edu (Kent Holsinger)
To: darwin-l@ukanaix.cc.ukans.edu
Subject: Re: Humanoid fossils in Time

Bayla Singer writes:
>We all know that mutations can arise in several independent
>places; the case of hemophilia (in all its types) is an example of this.
>If the genetic difference between -erectus- and -sapiens- is small, it
>seems plausible that in the course of geologic time -e- could have made
>the transition to -s- in more than one place, by the mutation of a few
>labile sites.

*Individual* mutations may, of course, arise repeatedly.  The question is
whether a species can have multiple origins.  To use the jargon of evolutionary
biolgy, can a species be polyphyletic?

The only examples for polyphyletic origin of species that I know of involve
multiple origins of polyploids.  The best example I know of is in the genus
_Tragopogon_, where the polyploid, _T. mirus_ arose independently at least
three times from different ancestral populations of the diploids _T. dubius_
and _T. porrifolius_.  I know of know examples where multiple origins of a
species at the diploid level has been postulated.  That, of course, is what the
multiregional hypothesis proposes.

Why don't species have multiple origins?  Well, for a new species to originate
a very special set of circumstances must apply.  Most populations spread
geographically and found new populations remain part of the same species.  If
they didn't, most populations of animals and plants would be part of different
species.  Given that speciation is a rare event, and that once it occurs the
new species evolve independently from one another, it is very unlikely that
species B will arise from species A both at place X and at place Y.  Why?
Because the genetic composition of a population of species A will be different
in place X and place Y.  Whatever the processes involved in the origin of a new
species and even if they are the same at place X and place Y, which they may
not be, the results of that process are likely to be different in the two
places.  In short, if speciation occurs at place Y, it is likely to give rise
to species C.

That's why we call evolutionary biology a _historical science_.  The contingent
facts of history, the place you start from, has an enormous impact on where you
end up.  It is not impossible for the multiregional hypothesis to be correct.
It would be the *only* example I know of where multiple independent origins of
a diploid species applies.

-- Kent

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