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ally fail to demonstrate an excess of heterozygotes. On the other hand, most general surveys do not distinguish among age or size classes, and the opportunity of observing differential survival or fitness is obviously de- creased when such classes are pooled. In addition, the small sample sizes of many studies preclude the observation of statistically significant de- partures from Hardy-Weinberg (expected genotypic) frequencies. The other approach to the relationship between genetic variance and fitness in natural populations is to compare populations known to differ in mean heterozygosity. Such studies exploit the differences in genetic variation that often occur among populations of species with poor powers of dispersal. For example, nonflying vertebrates tend to have more ge- netic variation near the center of their range or where the species is most dense (Soule, 1973; 1976) and some predominantly inbreeding plants have single, peaked clines in allozyme diversity (Clegg and Allard, 1972; Rick et al., 1977; Nevo et al., in press). The last two items in Table I refer to investigations of this type. Gar- ten (1976) observed that body size and success in aggressive encounters in old field mice (Peromyscus polionotus) were correlated with the mean heterozygosity in the population from which the mice were collected. Soule (1979) observed in island populations of the side-blotched lizard (Uta stansburiana) that fluctuating asymmetry, a putative inverse mea- sure of developmental homeostasis, is negatively correlated with mean heterozygosity among the populations. (One disadvantage of such studies is the hazard of spurious correlation, but this danger exists in within pop- ulation work as well.) Surveys of the genetic structure of predominantly inbreeding plants (for example, Brown, 1978) yield a vast range of allelic polymorphism. Even within a single species one may observe some populations with no observable polymorphism (evidence that heterozygosity is not a neces- sary condition for survival), while other populations of the same species have levels approaching those of outbreeding species. Apparently, some kind of heterozygote advantage is a factor contributing to survival. in some polymorphic plant populations, since these plants maintain twice the expected level of heterozygosity in the face of considerable inbreeding (Clegg and Allard, 1972; Rick et al., 1977). In summary, heterozygote advantage is a reasonable interpretation of much of the data from natural populations. On the other hand, the data do not allow us to determine what kind of selection model (over- dominance, frequency dependent, marginal overdominance) is most ap- propriate. In any case, the prudent strategy is to preserve genetic varia- bility and to avoid, as far as possible, unnecessary decreases in population size, since the rate of genetic erosion depends on population size as dis- cussed in Chapters 8 and 12. 156