AN EXPERIMENTAL STUDY OF KIN SELECTION

In this paper, I report the results of an experimental study of kin selection and its influence on the tendency of larvae to cannibalize eggs in the flour beetle, Tribolium confusum. Kin selection has been defined by Maynard Smith (1964, 1976) as that evolutionary process which occurs when individuals within a randomly mating population interact with one another in a non-random way with respect to kinship. This process is believed to exert a significant influence on the evolution of social behaviors, that is, on the ways in which individuals interact (Hamilton, 1963, 1964a, 1964b; E. 0. Wilson, 1971, 1975; Dawkins, 1976; Trivers and Hare, 1976; Barash, 1977). Kin selection was originally defined in contrast to and as an alternative to group selection (Maynard Smith, 1964). The key distinction between the 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" (Maynard Smith, 1964, p. 1145) and partial reproductive isolation of breeding groups was "not essential." On the other hand, group selection required "groups of relatives . . . wholely or partially isolated from other members of the species" (ibid. p. 1147) and the existence of partially isolated breeding groups was "an essential condition for group selection" (ibid. p. 1145). In 1976, Maynard Smith elaborated on this distinction and added that the differential "extinction of some groups and the 'reproduction' of others are essential features of evolution by group selection," (p. 279) and that the "terms group selection should be confined to cases in which the group (deme or species) is the unit of selection" (p. 282). This elaboration is in accord with most uses of the term group selection (Wright, 1931; Lewontin, 1970; E. 0. Wilson, 1973; Levin and Kilmer, 1974; D. S. Wilson, 1975; Wade, 1977, 1978a). However, it is precisely this aspect of the definition of kin selection which has led certain authors (D. S. Wilson, 1975, 1977; E. 0. Wilson, 1975; Wade, 1978b, 1979a, and in press) to assert that kin selection is a type of group selection. That is, kin selection depends on a population structure whether kin groups exist as actual physical units or whether individuals modify their behaviors according to the degree of genetic relatedness between themselves and the individuals they encounter. And, it is the heritable variance between these groups which determines the rate of evolutionary response to kin selection (D. S. Wilson, 1975, 1977; Wade, 1978b, 1979a). The experimental study of kin selection reported here contributes to the elucidation of the role of population breeding structure in the evolution of social behaviors. I will adopt the definition of a social behavior as an interaction between conspecifics which changes the fitness of the interactants. Social behaviors are one example from a class of traits which are uniquely influenced in their evolution by population structure. This occurs because social behaviors can affect not only genotypic fitnesses within a group but also the fitness of the group as a whole relative to other groups of different genotypic composition (Wade, in press). Templeton (1979) and Wilson (1979) discuss the evolution of several other traits where population structure is likely to be important. Kin selection has received considerable attention in regard to its possible influence on the evolution of altruistic social behavior, but as Hamilton (1964a, 1964b) has emphasized, kin selection can influence