The Evolutionary Dynamics of Selfish Replicators: a Two-level Selection Model

The aim of the present paper is to study the evolutionary dynamics of selfish replicators in a constant genetic background. Selfish replicators are viewed at a single locus, having a pleiotropic effect. Infinitely many alleles are possible; they act on individual fitness and have various levels of ability to distort segregation. This results in a two-level process of selection, including inter-individual selection (effect on individual fitness) and intra-individual selection (ability to distort segregation). The model takes other parameters into account, such as dominance, inbreeding and inbreeding depression. The system can have two different behaviours. (1) In some cases, evolutionary 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 selection, corresponding to small-effect mutations. (2) For other values of the parameters, a synthetic fitness can be defined: this absolute allelic fitness is estimated as a function of oneʹs fitness due to both inter-individual and intra-individual selection. During the course of evolution, the synthetic fitness increases. The optimisation of a synthetic fitness is the most general process. The optimised value is essentially homologous to the value optimised for resource allocation to male and female function in hermaphrodites (female function being homologous to the effect on individual fitness, and male function being homologous to distortion ability). The relative importance of both behaviours is discussed. It is argued that repeated sequences causing some human degenerative hereditary diseases may follow a two-step evolutionary process: a progressive increase in number of sequences accompanied by a decrease of the individual fitness would be followed by massive elimination of such sequences. But in general the optimisation of the synthetic fitness seems to be more likely.