Evolution__3rd_Edition

130 PART 2 / Evolutionary Genetics
The Wahlund effect concerns the
frequency of homozygotes in
subdivided populations
The spatial movement of genes is
called gene flow
populations are fused together. The gene frequencies of A and a in the combined population
are (0.3 + 0.7)/2 = 0.5, and the Hardy–Weinberg genotype frequencies are:
Genotype AA Aa aa
Frequency 0.25 0.5 0.25
In the large, fused population there are fewer homozygotes than in the average for the
set of subdivided populations. This is a general, and mathematically automatic, result.
The increased frequency of homozygotes in subdivided populations is called the
Wahlund effect.
The Wahlund effect has a number of important consequences. One is that we have to
know about the structure of a population when applying the Hardy–Weinberg principle
to it. Suppose, for example, we had not known that populations 1 and 2 were
independent. We might have sampled from both, pooled the samples indiscriminately,
and then measured the genotype frequencies. We should find the frequency distribution
for the average of the two populations (0.29, 0.42, 0.29); but the gene frequency
would apparently be 0.5. There would seem to be more homozygotes than expected
from the Hardy–Weinberg principle. We might suspect that selection, or some other
factor, was favoring homozygotes. In fact both subpopulations are in perfectly good
Hardy–Weinberg equilibrium and the deviation is due to the unwitting pooling of
the separate populations. We need to look out for population subdivision when interpreting
deviations from Hardy–Weinberg ratios.
Second, when a number of previously subdivided populations merge together, the
frequency of homozygotes will decrease. In humans, this can lead to a decrease in the
incidence of rare recessive genetic diseases when a previously isolated population
comes into contact with a larger population. The recessive disease is only expressed
in the homozygous condition, and when the two populations start to interbreed, the
frequency of those homozygotes goes down.
5.14.2 Migration acts to unify gene frequencies between
populations
When an individual migrates from one population to another, it carries genes that
are representative of its own ancestral population into the recipient population. If it
successfully establishes itself and breeds it will transmit those genes between the populations.
The transfer of genes is called gene flow. If the two populations originally had
different gene frequencies and if selection is not operating, migration (or, to be exact,
gene flow) alone will rapidly cause the gene frequencies of the different populations to
converge. We can see how rapidly in a simple model.
Consider again the case of two populations and one locus with two alleles (A and a).
Suppose this time that one of the populations is much larger than the other, say population
2 is much larger than population 1 (2 might be a continent and 1 a small island
off it); then practically all the migration is from population 2 to population 1. The
frequency of allele a in population 1 in generation t is written q 1(t) ; we can suppose that
the frequency of a in the large population 2 is not changing between generations and
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