Darwin-L Message Log 7:74 (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.

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<7:74>From princeh@husc.harvard.edu  Sun Mar 20 18:40:51 1994

Date: Sun, 20 Mar 1994 18:53:30 -0500 (EST)
From: Patricia Princehouse <princeh@husc.harvard.edu>
Subject: Re: Time & drift
To: darwin-l@ukanaix.cc.ukans.edu

On Fri, 18 Mar 1994, Kent Holsinger wrote:

> 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.

	Yes, they are certainly important points. I'm not saying that all
morphological and genetic change would be prevented but that it would be
limited enough to prevent speciation.  Obviously I can't prove it, and I'm
not knowledgeable about stochastic differential equations or good at
picturing things in 5 or 6 dimensional space as Wright was, but a lot of
people I know tend to think of 1 migrant per generation as generally (but
certainly not in every case) inhibiting speciation.

	As for drift, yes, I am assuming that drift would be the key
process in producing a new species. I'm using, with regard to erectus, the
idea that the only things which would have kept erectus from interbreeding
with sapiens would be:

a) if they never encountered each other but would have been able to
interbreed if they had (in which case erectus is a race of sapiens and
ergaster is probably sunk too) or

b) if they were genetically isolated by chromosome number or significant
inversions or similar type thing -things which are presumably acquired by
drift.

I picture speciation as always involving drift but not necessarily
involving different selection pressures from those acting on the parent
species (Different peaks can be created based on the same selection
pressures due to differences in the make up of the original population or
chance events along the way -1 vs 2 horned rhinos, etc). Some
multiregionalists claim that some uniform selection pressure acted on all
populations of humans (usually something to do with culture) to produce
the evolution of larger brains, shorter faces and other _H.s.s._ traits.
It seems more likely to me that this hypothetical uniform selection
pressure would have resulted in heterogeneity, not detailed convergence.

	Of course one can very reasonably take issue with any part or all
of what I have to say. But, that's my two cents' worth.

Patricia Princehouse
Princeh@harvard.edu

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