Evolution__3rd_Edition

132 PART 2 / Evolutionary Genetics
The American population illustrates
the model ...
. . . with a “migration” rate of
about 3.5% per generation
5.14.3 Convergence of gene frequencies by gene flow is illustrated by the
human population of the USA
The MN blood group is controlled by one locus with two alleles (Section 5.4).
Frequencies of the M and N alleles have been measured, for example in European
and African Americans in Claxton, Georgia, and among West Africans (whom we can
assume to be representative of the ancestral gene frequency of the African American
population of Claxton). The M allele frequency is 0.474 in West Africans, 0.484 in
African Americans in Claxton, and 0.507 in the European Americans of Claxton. (The
frequency of the N allele is equal to 1 minus the frequency of the M allele.) The gene
frequency among African Americans is intermediate between the frequencies for
European Americans and for the West African sample. Individuals of mixed parentage
are usually categorized as African American and, if we ignore the possibility of selection
favoring the M allele in the USA, we can treat the change in gene frequency in the
African American population as due to “migration” of genes from the European
American population. The measurements can then be used to estimate the rate of
gene migration. In equation 5.13, q m = gene frequency in the European American
population (the source of the “migrant” genes), q 0 = 0.474 (the original frequency
in the African American population), and q t = 0.484. As an approximate figure, we
can suppose that the black population has been in the USA for 200–300 years, or about
10 generations. Then:
0.484 = 0.507 + (0.474 − 0.507)(1 − m) 10
This can be solved to find m = 0.035. That is, for every generation on average about
3.5% of the genes at the MN locus have migrated from the white population to the
black population of Claxton. (Other estimates by the same method but using different
gene loci suggest slightly different figures, more like 1%. The important point here is
not the particular result; it is to illustrate how the population genetics of gene flow
can be analyzed.) Notice again the rapid rate of genetic unification by migration: in
only 10 generations, one-third of the gene frequency difference has been removed
(after 10 generations the difference is 0.484 − 0.474, against the original difference of
0.507 − 0.474).
5.14.4 A balance of selection and migration can maintain genetic
differences between subpopulations
If selection is working against an allele within one subpopulation, but the allele is continually
being introduced by migration from other populations, it can be maintained
by a balance of the two processes. We can analyze the balance between the two processes
by much the same arguments as we used above for selection–mutation balance
and heterozygous advantage. The simplest case is again for one locus with two alleles.
Imagine selection in one subpopulation is working against a dominant A allele. The
fitnesses of the genotypes are:
..