Evolutionary Ecology Research, 2000, 2: 3–14 © 2000 David G. Lloyd The selection of social actions in families: I. A collective fitness approach David G. Lloyd Department of Plant and Microbial Sciences, University of Canterbury, Christchurch, New Zealand ABSTRACT Here I analyse the basic properties of relatedness, inclusive fitness and kin selection, and derive alternative ways of formulating the selection of social behaviours. In particular, a ‘collective fitness’ rule is sought, which determines the direction of selection on a gene in terms of the effect an act has on the fitness of various individuals in a population, and the number of copies of the ‘action’ allele in various individuals in the population. The use of collective fitness allows for all the simplifications in kin-selection explanations of social behaviour that use inclusive fitness. In addition, it covers analyses of social behaviours in which factors other than kinship influence the distribution of genes in interacting individuals, without invoking an open-ended expansion of the concept of ‘relatedness’. The treatment of social actions ends with a dis- cussion of how far the concepts of relatedness, inclusive fitness and kin selection should be extended when factors other than kinship contribute to the distribution of genes to actors and recipients. Keywords: collective fitness, inclusive fitness, kin selection, relatedness, social behaviour. INTRODUCTION Until 30 years ago, evolutionists thought of selection only as a process that maximizes the number of direct descendants that individuals or their genes leave – their lineal fitness. The scope of natural selection was broadened dramatically by the introduction of the concept of inclusive fitness by W.D. Hamilton (1963, 1964). Following earlier undeveloped insights by Fisher (1930) and Haldane (1932, 1955), Hamilton recognized that the behaviour of an individual can affect not only its own fitness, but also that of other conspecific individuals. If the other affected individuals are relatives, a proportion of them will inherit the same gene from their common ancestor by virtue of their kinship. Hamilton postulated that the selection of social behaviours takes these non-lineal fitness effects into account and maxi- mizes the ‘inclusive fitness’ of individuals. Inclusive fitness comprises the base fitness of individuals in the absence of a social behaviour plus the effect on the fitness of the ‘actor’ and effects on the fitness of ‘recipients’ weighted by the genetic relatedness of the actor to the recipient. In its simplest and most common formulation as identity by descent (IBD), the relatedness, r, measures the probability that a gene in the actor will also be present in a recipient by descent from a common ancestor possessing the gene.