THE EVOLUTIONARY DYNAMICS OF SELFISH REPLICATORS - A 2-LEVEL SELECTION MODEL

The aim of the present paper is to study the evolutionary dynamics of selfish replicators in a constant genetic background. Selfish replicat ors are viewed as alleles at a single locus, having a pleiotropic effe ct. Infinitely many alleles are possible; they act on individual fitne ss and have various levels of ability to distort segregation. This res ults in a two-level process of selection, including inter-individual s election (effect on individual fitness) and intra-individual selection (ability to distort segregation). The model takes other parameters in to account, such as dominance, inbreeding and inbreeding depression. T he system can have two different behaviours. (1) In some cases, evolut ionary cycles are possible. The cycles correspond to an alternation of phases with predominant inter-individual selection, corresponding to major-effect mutations, and phases with predominant intra-individual s election, corresponding to small-effect mutations. (2) For other value s of the parameters, a synthetic fitness can be defined: this absolute allelic fitness is estimated as a function of one's fitness due to bo th inter-individual and intra-individual selection. During the course of evolution, the synthetic fitness increases. The optimisation of a s ynthetic fitness is the most general process. The optimised value is e ssentially homologous to the value optimised for resource allocation t o male and female function in hermaphrodites (female function being ho mologous to the effect on individual fitness, and male function being homologous to distortion ability). The relative importance of both beh aviours is discussed. It is argued that repeated sequences causing som e human degenerative hereditary diseases may follow a two-step evoluti onary process: a progressive increase in number of sequences accompani ed by a decrease of the individual fitness would be followed by massiv e elimination of such sequences. But in general the optimisation of th e synthetic fitness seems to be more likely. (C) 1997 Academic Press L imited.