The overall frequency of the alleles of the individual genes does not change in any way when a large population is divided into several smaller populations. However, small populations are endangered not only by the inbreeding effect, but some of its alleles are fixed much more rapidly through genetic drift. One of the two alleles is fixed at random in each of the smaller populations so that the frequency of the individual alleles in the overall population once again does not change. Then the Hardy-Weinberg law does not hold for the whole population as the frequency of homozygotes in the overall population will be higher and that of heterozygotes will be lower than would correspond to the frequencies of the individual alleles, i.e. the Wahlund effect is active here (Wahlund 1928). This is caused by the fact that fixation of a particular allele will occur in the individual subpopulations so that heterozygotes will not be formed in these subpopulations at all (Fig. V.4). In the extreme case, after a certain time, a situation could occur where one of the alleles is fixed in each subpopulation so that heterozygote individuals would not occur in the overall population. In practice, such a situation would require that gene flow would not occur at all between the individual subpopulations, i.e., migration of individuals from one population to another would not occur or any migrants in the population would not be able to cross with members of the domestic population. This situation will probably not be common in nature, but could occur, e.g., when an originally continuous water reservoir would be divided through a reduction in the water level in a lake.
The absence of heterozygotes in a real population is, however, frequently a consequence not of spatial fragmentation of the population, but rather the unrecognized presence of two or more cryptic species, i.e. species whose members have extremely similar or even indistinguishable phenotypes but cannot reproduce together. In a great many cases encountered in nature, the absence or a reduction in heterozygotes is caused by other mechanisms that are not related to the action of genetic drift, for example disruptive selection, parthenogenetic reproduction, etc.