Monte Carlo Simulations of Sexual Reproduction

Modifying the Redfield model of sexual reproduction and the Penna model of biological aging, we compare reproduction with and without recombination in age-structured populations. In contrast to Redfield and in agreement with Bernardes we find sexual reproduction to be preferred to asexual one. In particular, the presence of old but still reproducing males helps the survival of younger females beyond their reproductive age. today's higher species prefer sexual over asexual reproduction, and even asexual species often employ some form of genetic recombination. There are many theories but no consen-sus[1] why sex is preferred. For example, sex may spread advantageous mutations better through the population, sex increases genetic variety which may help species to adjust after a drastic change in the environment, or sex may protect against parasites. In a computer model with only deleterious mutations, a constant environment, and no parasites, none of these advantages hold. Indeed, in such a simulation Redfield[2] found only seldomly a clear advantage of sex to justify its cost to feed the males (which were also stated [2] to be unreliable in transmitting the correct genes). Then, what are males useful for ? Charlesworth et al [3] have claimed that large sexual, as opposed to large asexual, populations are protected against the mutational meltdown due to the accumulation of deleterious inheritable mutations[4]. However, this claim was refuted [5] as being due to insufficient observation times: In their model the decay time increases exponentially with population size. Different models of Bernardes [5] with and without age structure gave clear advantages for sex: The population can survive with sex better than without. More recently, however, Bernardes [6] emphasized the advantages of asexual reproduction with genetic recombination within the same individual (" meiotic parthenogenesis ") observed in many asexual species. The present paper ignores these intermediate forms and compares sexual with asexual reproduction, assuming (as did Redfield[2]) genetic recombination only in the sexual case. In contrast to Redfield we follow Bernardes[5,6] in assuming that only a fraction h of the genetic mutations are dominant, the others being recessive. We want to see if now sex is more helpful in the Redfield model, using both her techniques (Chapter II) as well as Monte Carlo simulations based on a bit-string model of the genome (Chapter III). Then we use in Chapter IV the Penna bit-string model [7,5] for age-structured populations to find