Jane Westervelt wrote:

> > This reminds me of being back in California and down in the Monterey
> > area and seeing Heermann's Gulls with a large white patch in their
> > wings.
> snip
> > We never saw these birds north of Monterey, only south.

> I have seen such birds in Humboldt bay, 400 miles or so north of
> Monterey. The explanation that I got is that these white patches are
> a naturally occuring genetic expression that shows up in about 10%
> of immature Heermann's Gulls

> In the cases we have been discussing, the causes are very likely
> genetically based. Albinism is not a new trait, and, as with
> anything genetic, expression varies over different populations and is
> usually dependent on local (or regional) pressures on the organisms.
> An often cited example of this is the high rate of sickle-cell anemia
> in people of African descent because the heterogenous condition gave
> these individuals an advantage in resisting malaria.

Two points: First, let's be sure to get the terminology right--
"heterozygous," not "heterogeneous." Second, differential geographic
expression of such a trait may be related to environmental factors, but
there may be more strictly genetic explanations. The mutant allele might
be selectively neutral but simply might be diffusing in the observed
pattern from its point of origin; it might also have differential
penetrance (that is, frequency of expression) in certain genetic
backgrounds that change geographically for other reasons. I only point
these considerations out to remind folks that populations have complicated
genetic structures, so we have to be cautious in speculating about the
causes of variations.

Burt Guttman guttmanb at elwha.evergreen.edu
The Evergreen State College Voice: 360-866-6000, x. 6755
Olympia, WA 98505 FAX: 360-866-6794

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