Re: A Further Note

Dwight W. Read (dread@ANTHRO.UCLA.EDU)
Thu, 1 Aug 1996 13:47:07 -0700

Snower replies to Kephart:
>I think I am right that mutations would make no difference. They have no
>place to go. They would not change the relative gene frequencies in the
>gene pool as long as the mating was absolutely random beeause of the
>Hardy-Weinberg law. Ask a population geneticist--if you can find one. And
>let me know.

there is basic misunderstanding here.
Let me briefly review what I have just finished teaching (I go through the
mathematical derivation of the Hardy-Weinberg law):

The Hardy-Weinberg law states that under the following assumptions: (1) no
mutation, (2) no selection, (3) no migration, (3) random mating and (4)
large populations (so there is no drift), then the allele frequencies in the
offspring generation will be the same as in the parental generation and the
genotype frequencies in the offspring generation will be give by p^2, 2pq
and q^2. (By the way, this is a mathematical, not an empirical, law.) The
latter set of genotype values are referred to as Hardy-Weinberg equilibrium
values (since the breeding population will remain with this genotype
frequencies so long as the conditions hold true.

Violation of random mating in the form of either phenotypic assortative
mating or inbreeding leads to changes in genotype frequencies towards more
homozygosity but does not change allele frequencies, so does not lead to
evolution. (Actually, so long as all individuals in the breeding population
are involved in mating--which is one of the implications of no
selection--then regardless of the mating system there will be no change in
allele frequencies). Mutation causes evolution (by definition) as Kephart
notes since mutation changes allele frequencies. One way recurrent mutation
will, over time , drive out the allele against which mutation is acting
(regardless of the mating system). Migration between breeding populations
will change allele frequencies when the respecive populations have different
allele frequencies, and the effect is to make the breeding populations more
alike, genetically speaking. Migration is sort of the coverse of
selection--selection can introduce diversity between breeding populations
(in terms of allele frequencies) and migration reduces diversity (hence the
importance of geographic isolation in Darwin's arguement for speciation via
natural selection).

The effect of selection does not depend upon the mating system (though the
mating system can affect the intensity of selection--e.g., inbreeding
increases homozygosity, hence making recessive traits occur more frequently,
hence giving selection greater opportunity to act on the alleles responsible
for the recessive trait).

I hope this clarifies the matter.

D. Read