In message <Pine.A32.3.91j.960205084720.144234E-100000 at homer01.u.washington.edu>
writes:
> Dan,
>
> You say "populations" are "difficult to define" but also precisely
> defined." Is there a paradox here? How can one precisely define a
> population that extends throughout an extensive habitat, e.g., Song
> Sparrows around here, with no clear habitat divisions to isolate breeding
> populations? Do populations then vary in size from a few pairs that nest
> in an isolated patch of riparian woodland to nearly continent wide
> extensions of species such as robins? Is there an arbitrary statistical
> watershed with respect to rates of gene exchange?
>
> Gene Hunn.
>
Gene, A better way to say it would have been: Populations are difficult to
*delineate* especially in those species with homogeneous geographical
distributions. Population geneticists concentrate on demes (local populations,
or mendelian populations), and look for clumpy distribution to find them. Your
Song Sparrows could be defined as a population in the broad sense. I would look
for aggregations, sight fidelity, etc.to identify local populations. The
continum of geographical clumpyness (there must be a better word) from rails and
seabirds to Song Sparrows and Robins (along with birds great mobility) certainly
make delineating their populations challenging. As far as a statistical
watershed with respect to gene exchange, do you mean some threshold level at
which gene flow is essentially nill - thus defining a population?

Dan Stephens (509) 662-7443
Dept. of Biology fax: (509) 664-2538
Wenatchee Valley College e-mail: dstephen at ctc.edu
1300 Fifth Street
Wenatchee, WA 98801