Consequences of Small Population Size

The degeneration of function by random drift is of great concern for the conservation of endangered species. Is selection in small populations effective enough to allow their long-term survival? From the point of view of individual genes, the answer depends on whether selection coefficients maintaining function are stronger than 1/N_e, because any more weakly selected functions will eventually be lost. We can also look at this issue in terms of quantitative traits, rather than individual alleles. We have seen that mutation generates additive genetic variation, V_A, at a rate V_M per generation (p. 409), and that random drift erodes it by V_A/2N_e per generation (p. 418). Thus, in a balance between mutation and drift, we have a genetic variance of V_A = 2N_eV_M. Because the total rate of generation of genetic variance by mutation is typically at least 0.001V_E, a population of effective size of 1000 or more should be able to maintain high heritability (i.e., V_A ~ V_E) despite drift. Indeed, such moderate-sized populations do show high heritability (p. 397), and in artificial selection experiments, populations of this size show a continued response to selection that is based at least in part on new mutations (pp. 480–484). However, as we discussed before, one caveat is that many mutations may have deleterious side effects, and so may not be a good basis for adaptation in the long term.

In practice, these evolutionary considerations may be of secondary importance, because any population that is small enough to suffer a serious loss of genetic variation and hence be unable to remain adapted is also vulnerable to extinction for purely demographic reasons; for example, a chance run of bad years may lead to extinction of a population of even a few thousand individuals. In the short term, ecological considerations may be most important (see Lande 1988).