Darwin-L Message Log 7:63 (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.

<7:63>From kent@darwin.eeb.uconn.edu  Fri Mar 18 08:49:30 1994

Date: Fri, 18 Mar 94 08:08:07 EST
From: kent@darwin.eeb.uconn.edu (Kent Holsinger)
To: darwin-l@ukanaix.cc.ukans.edu
Subject: Re: Time article

Just a minor point, but worth mentioning.

Patricia Princehouse writes:
>	1) one migrant per generation is enough to prevent speciation in
>	   animals generally

That's not quite right.  Wright's classic result is that the exchange of
one migrant per generation is sufficient to _limit_ genetic divergence
between populations (not to prevent it completely) _only_ if the only process
producing divergence is genetic drift.  Both points are important.

1) Migration between semi-isolated populations reduces divergence through
   genetic drift, but cannot eliminate it unless the product of effective
   population size and migration rate is infinite.  Since the migration rate
   is intrinsically between 0 and 1, that means the population size must be
   infinite for migration to prevent eliminate genetic differences among
   semi-isolated populations, i.e., there must be no genetic drift.

   Wright's result refers to whether the distribution of allele frequencies
   among semi-isolated populations is unimodal or bimodal.  If there is more
   than one migrant per generation (and drift is the only process producing
   divergence), then the distribution will be unimodal.  If there is fewer
   than one migrant per generation, then the distribution will be bimodal.

2) Wright's result also assumes that divergence among populations is occurrring
   only as a result of genetic drift.  If populations are subject to different
   selection pressures, genetic divergence may occur even in the face of
   *much* more gene flow.  To the extent that differences among species
   reflect adaptive differentiation, it is conceptually possible at least
   that divergence happened in the face of substantially more gene flow than
   one individual per generation.  Whether that has actually occurred, of
   course, is another question entirely.

-- Kent

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