rising@zoo.toronto.edu (Jim Rising) (06/18/91)
The formula for calculating the effective population size (Ne) is:
Ne = (4NfNm)/(Nf + Nm)
where Nf = the number of females breeding and Nm the number of males
breeding (note that this would not include sterile, or post- and pre-
breeders; thus Ne is always going to be smaller than N (total
population) in the real world). You can play around with numbers
to see what the effect of unequal breeding sex ratios is, but note
that if you have only 1 individual of 1 sex, the Ne can never be
as large as 4. Thus, if you has a population with 20 females and 1
male, Ne = 3.8; with 100000 females and 1 male, 3.99. Observed
sex ratios are often misleading because many species have "polyganous"
mating systems (if there are equal numbers of each sex of breeding
age, and 1/2 of the individuals have two mates, than the other 1/2
have zero, if one sex is monogamous).
The rate of random loss of rare alleles is very sensitive to
population size. In simulations it happens quickly in small
populations (Ne 10-20), but significantly more slowly in Ne > 100.
Just a little bit of gene flow from another population can greatly
reduce the probability of drift (random loss) in a population.
--
Name: Jim Rising
Mail: Dept. Zoology, Univ. Toronto, Toronto, Ontario, Canada M5S 1A1
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