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Effect of Inbreeding on the Evolution of Altruistic Behavior by Kin Selection

Michael J. Wade and Felix Breden
Evolution
Vol. 35, No. 5 (Sep., 1981), pp. 844-858
DOI: 10.2307/2407855
Stable URL: http://www.jstor.org/stable/2407855
Page Count: 15
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Effect of Inbreeding on the Evolution of Altruistic Behavior by Kin Selection
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Abstract

We present a model for the evolution of altruism by kin selection in a family-structured population and relate the findings of this model to the empirical studies of kin selection by Wade (1980a) using laboratory populations of flour beetles. We use the model to test the implicit hypothesis of kin selection theory that inbreeding facilitates the evolution of altruistic behaviors. In this study, a proportion of the population, f, mates within families, and the remainder of the population (1 - f), mates at random. Therefore, we may control the level of inbreeding in the population and use computer simulation to test the effects of different levels of inbreeding on the evolutionary dynamics of an altruistic allele. The total change, Δ q, in the frequency of a, the altruistic allele, is partitioned into two components: (1) a component caused by differences in mean family fitnesses, or between-group selection; and (2) a component caused by selection acting on genotype frequencies within families, or within-group selection. The simulations show that increasing the level of inbreeding in the population greatly affects the frequencies of the six different family types that constitute the population. In particular, the frequencies of those families formed by matings between homozygous parents are increased. Changes in the relative frequencies of these families change the strengths of the two opposing components of group and individual selection. For the additive fitness model and range of fitnesses explored here, our main conclusions regarding the effect of inbreeding on the evolution of social behaviors are: (1) For conditions that would eliminate the altruistic allele in a randomly mating population, increased levels of within-family mating can cause the altruistic allele to increase to fixation. (2) Increasing the level of inbreeding can greatly increase the rate of change of gene frequency of the altruistic allele. (3) The effects of inbreeding on the evolution of altruism by kin selection can be explained by considering kin selection to be a balance between the opposing forces of group and individual selection. Inbreeding alone greatly changes the relative family frequencies, which, in turn, determine the relative strengths of individual and group selection. (4) These results support the conclusions of Wade (1980a) made on the basis of empirical studies contrasting random (f = 0) and within-group (f = 1) mating.

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