Text 1965, 79 rader
Skriven 2006-06-05 11:32:04 av DAVE SACERDOTE (1:123/140)
     Kommentar till en text av DALE SHIPP
Ärende: Re: squirrels
=====================
DSh> And there is an area in North East Ohio (e.g. Kent State University
DSh> campus) where the only squirrels that you see are black.

JW> Those black squirrels are the same species as the grays. They are
JW> quite rare except in a very few places.

Note to JW:  Black squirrels are a lot more common than you may
think. 

DSh>   We were told that the melanistic trait was actually 
DSh>   due to a recessive gene, but if that were true then 
DSh>   I wonder how they could have become the dominant 
DSh>   variety on campus.

Longmeadow MA also has a population of black squirrels and they
are beginning to replace the common greys.  This is actually
quite possible given the way recessive characteristics work:

Let's say that the grey coloration is carried by gene X (dominant), 
and the black coloration is carried by gene x (recessive.)  Each
squirrel must carry 2 coloration genes, one passed to it by
"dad" and one by "mom".  Two genes make up one chromosome, which
would color the squirrel as follows:

XX = grey
Xx = grey, carrying black
xx = black
xX = grey, carrying black

Now, follow the gene distribution in a table showing every possible
combination of the above genes:

XX + XX = XX  XX  XX  XX (four "true" grey)
XX + Xx = XX  Xx  XX  Xx (two "true" grey, 2 grey carrying black)
Xx + Xx = XX  Xx  xX  xx (one "true" grey, 2 grey carrying black, 1 black)
Xx + xX = Xx  XX  xx  xX (one "true" grey, 2 grey carrying black, 1 black)
xX + xX = xx  xX  Xx  XX (one "true" grey, 2 grey carrying black, 1 black)
xX + Xx = xX  xx  XX  Xx (one "true" grey, 2 grey carrying black, 1 black)
xx + xx = xx  xx  xx  xx (four black)

So, out of 28 offspring you would have:

10 "true" grey
10 grey carrying the black gene
 8 "true" black

If the black coloration is a trait unto itself, the black squirrel
population will always be stable or growing slightly since 
carriers + blacks always outnumber "true" greys.  But if the 
black pigmentation gene is associated with another trait favorable
to survival (intelligence, perhaps, or greater aggression) it 
would be well within probability to see the black squirrel
population grow as more squirrels begin to carry 2 recessive genes.
The greys would likely never die out, but would indeed become a
minority.

[Warning: anecdotal evidence below - this is not tested data]
In Longmeadow, in the area near Baypath College and St. Mary's
RC church, the population of black squirrels which was small 
when Lynn was a kid at St. Mary's School is now the dominant
squirrel population.  In the past 15 years or so, I have only
seen 1 black squirrel carcass on the side of the road, but many
dead greys. That could be an indicator that the blackies are
more savvy of automobile traffic, or that they are more
territorial and don't stray from their turf so much.

Human intervention could be a factor as well.  Black squirrels
are unusual to most people, and many people find the sable
coats attractive.  People seem more willing to feed blackies,
who then adapt, lose some of their fear of humans, endear 
themselves further to their patrons, get fed more, and so
on (again, giving them a survival advantage.)



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