I'll have to do a little digging and don't have time now but I think there are mathematical predictions made about the spread of alleles and the balance they get to under certain conditions. If I'm remembering correctly the percentages are born out when examined. But I just have the most fuzzy idea of this.
Is there a geneticist in the house?
This one seems to hint at something like you want:
http://ocw.mit.edu/...407D-A8E4-D2FAC1A6F521/0/fa01lec26.pdf
[This message has been edited by NosyNed, 12-01-2003]