EFFECT OF INBREEDING ON THE EVOLUTION OF ALTRUISTIC BEHAVIOR BY KIN SELECTION

Kin selection is defined as that evolutionary process which occurs when individuals interact with one another in a nonrandom way with respect to kinship and these interactions affect fitness. Kin selection is believed to have exerted a significant influence on the evolution of social behaviors (Hamilton, 1964a, 1964b; WestEberhard, 1975; E. 0. Wilson, 1975; Grant, 1978). Maynard Smith (1964) defined kin selection in contrast to and as an alternative for group selection (Williams, 1966; Lewontin, 1970; Wade, 1977, 1978a). The key distinction between these two evolutionary processes was the presence or absence of discontinuities in the population breeding structure. Kin selection did "not require any discontinuities in population breeding structure" while discontinuities in population breeding structure were "an essential condition for group selection." This distinction was made in response to the suggestions of Wynne-Edwards (1962) that group selection was responsible for the evolution of a variety of social behaviors including territoriality and dominance hierarchies. It was later suggested (Maynard Smith, 1976) that group selection be restricted to those cases in which the group was the unit of selection; that is, to those cases in which gene frequency changes occurred by the differential extinction and proliferation of populations. However, the similarities between kin and group selection are pronounced. Kin selection, like group selection, depends upon the population being structured into groups of related individuals, whether these kin groups exist as actual physical entities or as a result of the facultative expression of social behaviors. For this reason, several authors have classified kin selection as group selection (D. S. Wilson, 1975, 1979) or as a form of group selection intermediate between individual and interpopulational selection (Lewontin, 1970; E. 0. Wilson, 1975; Wade, 1978a, 1979; see Grant, 1978, for a review). However, this suggestion has been strongly opposed (Maynard Smith, 1976), and Dawkins (1979) listed the idea that "kin selection is a form of group selection" as one of the ''common errors" of misunderstanding in discussions of sociobiology. In experimental studies of kin selection, using laboratory populations of the flour beetle, Tribolium confusum, Wade (1980a) investigated the effects of discontinuities in population breeding structure on the predictions of kin selection theory. The experiment consisted of synthesizing a strain of T. confusum with genetic variability for the tendency of larvae to cannibalize eggs. Beetles randomly chosen from this strain were then placed in six different treatments, each representing a different population structure, and the evolution of the cannibalism behavior was followed for several generations. No artificial selection for or against cannibalism was imposed in these studies; the larval behavior by itself generated selective pressures and the different population structures determined how the selective forces were distributed. The population structure was varied by varying both the degree of genetic relatedness between the larval cannibals and their egg victims and the degree of random mating. Depending on the treatment, the genetic relationship between the larvae and eggs was .50 (full-sibs), .25 (half-sibs), or .00 (no relationship). These "interaction treatments" were factorially combined with two different breeding structures,