On the Theory of Parent-Offspring Conflict: A Two-Locus Genetic Model

Trivers (1974) suggested that although natural selection should operate to increase the ability of parents to care for their young, parent-offspring conflict may arise and be explained on the basis of Hamilton's inclusive fitness rule (Hamilton 1964). Thus, while the relatedness of a parent to each offspring, or of one offspring to its sib is one-half, the relatedness of an offspring to itself is one. (For a detailed study of the appropriate definition of relatedness, see Uyenoyama and Feldman [1981] and Uyenoyama et al. [1981].) An event which increases an offspring's viability by -q, say, and decreases that of its sibs by less than 2-q will increase the former' s inclusive fitness and therefore be desired by him. However, a decrease in the parents' total reproductive success may occur and the result is the parentoffspring conflict in fitness. Alexander (1974) challenged this argument, claiming that such "selfish" alleles (in the offspring) would not spread because they would reduce the fitness of their carriers when the latter became parents and had selfish offspring. Alexander further suggested (see also West Eberhard 1975) that such parent-offspring conflict would be reduced by the asymmetry between the parties to the conflict; parents can force offspring to behave in a way which would increase brood survival thereby increasing the parent's fitness. However, Zahavi (1977) pointed out that such punishment by parents of their selfish offspring, either active or by withdrawal of resources, would be difficult o accomplish without a reduction of the parents' reproductive success. Standard models of sib-to-sib altruism which usually involve brood selection, can be equivalently rephrased in terms of offspring-to-parent altruism. Such altruism may be considered outside of the usual context that originated with consideration of the social Hymenoptera. Thus, for example, increased efficiency in using a parent's limited resources at the expense of higher viability or reproductive success in the future might be regarded as altruism. Conversely, the delayed attainment of maturity at the expense of present or future sibs may be regarded as selfish. In this paper the terms "handicapped" and "selfish" are used interchangeably although it is realized that, in a behavioral sense, a selfish individual may not be handicapped. It has been suggested by Zahavi (1977) that parents of such handicapped offspring may allocate a disproportionate share of their resources to them, to reduce the mortality of the brood containing such progeny. The selfish phenotype in the offspring and the modified parental behavior in the parent would be complex traits for which models developed in terms of only a few genes may not apply. Nevertheless, some aspects of the conflict between the selection of genes which act in offspring (but are carried and passed by parents) and those which act in parents (but are also carried by offspring) are worth investigation, as long as the disparity between the simplicity of these models and