At present mankind is experiencing a formidable demographic revolution. World population is increasing fast, mortality before and during reproductive age is being reduced, and traditional patterns of marriage are disappearing. One aspect of this demographic revolution of particular interest in population genetics is the break-up of isolates.

For millions of years, our predecessors lived in small population groups, often consisting of fewer than one hundred individuals. Each of these groups was to some extent isolated from others and usually marriage occurred within the group, sometimes between subgroups. As a consequence, their genetic composition changed relatively quickly, depending on chance fluctuations of gene frequencies, and to a certain degree under the influence of changing patterns of natural selection.

Effects of isolation

Whereas in an infinitely large, randomly mating population the frequency (q) of a given allele depends on its mutation rate and selective advantage or disadvantage, its frequency in a small population is greatly affected by chance - provided only that its selective disadvantage is not too strong. Figure 5.1 shows the distribution of the frequency of an allele (initial mean q = 0.5) at a diallelic locus in groups of populations depending on their effective sizes largely determined by the number of couples breeding. Mutation rates and back mutation rates are assumed to be identical. With small population size (a), many populations will be homozygous for one of the two alleles (q = 0 or 1) whereas, for very large populations, q clusters around 0.50. An allele may become homozygous (= fixed) in a small population by chance fluctuation in frequency even against a certain selective disadvantage (Figures 5.2 and 5.3).