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Interdemic Selection and the Evolution of Altruism: A Computer Simulation Study

Bruce R. Levin and William L. Kilmer
Evolution
Vol. 28, No. 4 (Dec., 1974), pp. 527-545
DOI: 10.2307/2407279
Stable URL: http://www.jstor.org/stable/2407279
Page Count: 19
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Interdemic Selection and the Evolution of Altruism: A Computer Simulation Study
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Abstract

The role of interdemic selection in the evolution of altruistic characters has been the subject of considerable controversy for the past eleven years. Much of this controversy has taken the form of verbal arguments about the restrictive nature of the conditions necessary for this mode of selection to operate. During most of this period, however, little quantitative work was done on this problem. As a result, the precise nature of these necessary conditions remained almost entirely a matter of conjecture. In the past few years, a number of investigators have begun to treat interdemic selection and the evolution of altruistic characters in a formalized and mathematical manner. In this investigation we used a Monte Carlo simulation to examine the demographic and genetic conditions necessary for the evolution of altruistic characters by the process of interdemic selection. The computer simulated population consisted of an array of demes which were connected by migration and subject to periodic extinction and recolonization. The probability of a deme surviving between successive generations was a direct function of the frequency of alleles responsible for the altruistic phenotype in the deme. Consideration was given to a single-locus two-allele mode of inheritance in a monoecious diploid population with non-overlapping generations. The results of our simulation study are consistent with the qualitative conclusions drawn from the mathematical studies of this process involving theoretical populations with an analogous structure. Interdemic selection favoring an allele was able to override the effects of Mendelian selection operating against it and led to the maintenance of this allele in either fixed or stable polymorphic states. However, with potentially realistic deme survival functions and significant levels of Mendelian selection, restrictive conditions were necessary for this result to be obtained. In our simulated population, genetically effective deme sizes of less than 25 and usually closer to 10 were required, and the rate of gene exchange, through flow migration, could not be too much greater than 5% per generation. The restrictive nature of these demographic conditions clearly limits the generality of their occurrence in natural populations. At this juncture, however, information about the structure of natural populations and extinction rates is too sparse to justify the conclusion that these conditions will not be met. In fact, the results of some recent population genetic and biogeographic studies suggest that the necessary demographic conditions for the evolution of altruistic characters by interdemic selection may well exist in natural populations. Whether the genetic conditions for the evolution of altruistic characters by interdemic selection will also be met in real populations remains solely a matter of conjecture. In our simulated population, a character determined by a single locus had to considerably enhance the survival probability of its deme. Although we do not anticipate an abundance of such loci, we are able to offer plausibility arguments for their existence. We also discuss the distinction between interdemic and kin selection and the joint action of these two processes.

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