Recombination in a Simulated Predator-Prey Interaction

The influence of a co-evolved predator-prey interaction on theoutcome of competition between sexual and asexual prey was examinedby computer simulation. Intraspecific competition in both predatorand prey populations is governed by relative success in theinterspecific interaction. Predators key on a certain variableprey character under the control often independently assorting,diploid loci. Individual predators vary genetically in theirability to handle prey of the various states of this character.Predator fitness is proportional to the prey available to them.Prey fitness is inversely proportional to the frequency of predatorsto which they are susceptible. In addition, sexual prey of agiven phenotype are assigned half the fitness of the correspondingasexual form, representing a "cost of meiosis." Mating is randomand there is no facultative response allowed to changes in thephenotypic distribution of the interacting species. Three parametersare varied: (1) the maximum contribution of a phenotype to thenext generation (fecundity limit), (2) population size, and(3) clonal diversity of the asexual prey form. Most realisticcombinations of these variables favor sexual reproduction. Accordingto the model, it is particularly unlikely that an asexual mutantcould spread in an existing sexual population. However, theconditions found to favor asexuality are consistent with observationsfrom nature where reduced recombination is relatively common.The model predicts asexual forms should persist in very large,low fecund populations, as in the prokaryotes, and where theirphenotypic diversity is high, as near zones of recurrent hybridization.It is also consistent with the occurrence of parthenoforms inregions of reduced interspecific interaction where the assumptionsof this model, and therefore the advantages to sex, are lesslikely to be realized.