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for the near absence of heterozygosity in a few of these populations; ge- netic drift is almost certainly a contributing factor. The reason for presenting these data is simply to illustrate that the depletion of varia- tion can a.nd does happen in nature, given small enough populations. Incidentally, several workers have pointed out that the number of founders in a colony, so long as it is greater than about five individuals, is not nearly as important as the long-tenn maintenance size of the colony (Chapter 12; Nei et al., 1975; Denniston, 1978). The reason is that a single bottleneck event has much less impact on heterozygosity than does ge- netic drift in a perennially small population. Over how many generations can a small population persist before it has a high probability of going extinct? A possibly useful empirical rule is suggested by the experience of animal breeders. Animal breeders have no- ticed that there is an obvious effect on fecundity in small populations as the inbreeding coefficient approaches 0.5 or 0.6. Using these numbers as arbitrary thresholds, we can estimate the number of generations it will take at various values of ~ before the group becomes susceptible to ex- tinction from genetic causes. Setting l1f equal to 0.6 (or 0.5) in the fonnula l1f = 1 - (1 _ ~) t 2Ne and solving for t, the number of generations, we obtain the rough approxi- mation t::::: 1.5 Ne, or in other words, the expected number of generations to the extinction threShold is about 1.5 times the effective population size. Effective Population Size and the Basic Rule The effective application of the basic rule requires more than merely maintaining a stock of 50 or more breeding adults. The reason is that the actual (census) number of adults may not be even close to the effective number. Franklin (Chapter 8) discusses this distinction in some detail, but some further comments and examples will be given here. Breeding structure is absolutely critical. In species with harems, leks or other polygynous systems, the effect of a relatively small number of breeding males is a significant increase in the minimum census number, N, required to reach a desired effective size. It may be quite common for the ratio N / ~ to be 3 or 4, meaning that the minimum size of the colony is 150 to 200 individuals. In species where social inhibition or other factors prevent many adults from breeding, the ratio is even higher. In canids, for example, it is typical for only the dominant pair in the pack to breed (Chapter 14). This applies to the cape hunting dog, Lycaon pictus. In the Serengeti, one of the largest national parks in Mrica, the population of hunting dogs num- bers 30 or so adults. Assuming that these animals are divided among five 162