EVOLUTION BY KIN SELECTION: A QUANTITATIVE GENETIC MODEL ILLUSTRATED BY MATERNAL PERFORMANCE IN MICE

Models of evolution by kin selection have been developed primarily at the single locus level (Hamilton, 1964; Wade, 1978, 1980; Michod and Abugov, 1980; D. S. Wilson, 1980), even though most characters of evolutionary interest are polygenic. Recently, several quantitative genetic models have been derived for the evolution of altruism by kin or group selection (Yokoyama and Felsenstein, 1978; Aoki, 1982a, 1982b; Crow and Aoki, 1982; Engels, 1983). All of these models, both single locus and polygenic, have essentially confirmed the reliability of Hamilton's rule, that altruism will evolve when the additive genetic regression of the recipient on the altruist is greater than the ratio of selective costs to benefits. Alternatively, the evolution of altruism through kin selection can be approached as a special case of the joint evolution of correlated characters. In such a quantitative genetic model, there are two traits, the target phenotype and kin performance for this phenotype. The target phenotype can be any trait which is affected by interaction with kin. In most previous models, the target phenotype has been fitness, but in the model presented here this is generalized so that it can be any trait of interest to the researcher, such as weight. Kin performance subsumes all aspects of an individual's phenotype which have an effect on the target phenotype in its relative. Thus kin performance is the source of kin effects on the target phenotype. These kin effects are the measured phenotypic effects of kin performance on the target phenotype. Kin performance is, by definition, only measurable in terms of its effects on the target phenotype and therefore only exists in reference to a particular aspect of a relative's phenotype. Kin performance can be considered altruistic under certain selection regimes. This view of kin selection as a special case of the joint evolution of correlated characters highlights previously unrecognized or underrated features of the models cited above. First, the evolution of altruism through kin selection is a special case of a more general model of the evolution of traits which are affected by and affect the phenotypes of relatives. This has been especially appreciated by those using "trait group" models (Wade, 1978, 1980; Yokoyama and Felsenstein, 1978; D. S. Wilson, 1980; Crow and Aoki, 1982). Second, the only genetic effects on the target phenotype which have been considered are kin effects, which are the phenotypic manifestations of kin performance. The effects of genes carried by the target on its own phenotype, usually referred to as the direct effects of genes on the target phenotype, have not been included in the models. This is because the models have concentrated on the evolution of the potentially altruistic kin performance trait to the exclusion of the evolution of the target phenotype itself. This second feature of the previous models has also led to an unstated assumption that there is no net pleiotropic effect of genes directly affecting the target phenotype and kin performance. In the model presented below, direct effects of genes on the target phenotype and pleiotropic effects on the target phenotype and kin performance will be taken into account. The influence of kin performance on polygenic phenotypes has long been a