Modelling of sexual reproduction in a world of diverse and limited resources

Evolution is based on reproduction and survival of offspring. Reproduction in most organisms is sexual, i.e., a gamete sexually produced by a female thereby fuses with a gamete produced by a male to form progeny of the next generation. The evolution of sex has drawn attention since C. Darwin (1859), but remains enigmatic until today. Sexual reproduction suffers an inevitable disadvantage of a factor of two in comparison with asexual reproduction (Williams, 1975). In a sexual population, only one of the two sexes is capable of bearing young. Over the century, several hypotheses and models have been proposed to explain the maintenance of sexual reproduction. However, none of them has been commonly approved. Recently, Scheu and Drossel (2007) introduced a structured resource model that is based on limited and structured resources combined with stochastic effects. The advantage of sexual individuals in this model is the ability to produce offspring which can exploit new and underutilized resources. In this model asexuality wins over sexuality only when mortality is high, resource diversity is low, resources regrow fast, or many different genotypes are allowed to coexist at the same place. By adding a spatial structure into this model, we obtain a pattern resembling geographic parthenogenesis, i.e., sexuals prevail in central regions of low mortality or high resource diversity, while asexuals prevail at the boundary of species’ range, where mortality is high or resource diversity is low. In order to apply the structured resource model to long-lived organisms, we construct a mathematical model for a long-lived consumer species and its resources. The model takes into account the allometric scaling of consumption, metabolism, and mortality with consumer body mass. Mortality is assumed to be density dependent, and the dynamics of resources are explicitly modelled. We explore thereby the consequences of metabolic theory on life histories and life history evolution. We find that populations that have more or faster growing resources have a shorter life span and a higher mortality. Moreover, populations with a larger adult body mass have a larger number of offspring per female and a larger biomass density in this model. When we allow the adult body mass to evolve, it increases with time without limits. When we allow the offspring body mass to evolve, it becomes smaller. Both trends result from the allometric scaling of mortality and are kept in limits by other trade-offs than those included in our model. By combining the two ecological models we find sexual long-lived organisms prevailing over asexual long-lived organisms in regions of low mortality, high resource diversity, or low resource growth rate. The advantage of sexual reproduction is larger in long-lived organisms compared to the advantage of sexual reproduction in annual organisms. In populations of long-lived organisms the offspring generation directly competes with the parent generation for resources, while there is only direct competition among siblings in populations of annual species. Therefore, asexual clones and parthenogenetically produced offspring suffer from more severe intraspecific competition in long-lived organisms. This is consistent with the dominance of sexual reproduction in large long-lived organisms and may provide an ecological explanation for the absence of asexual reproduction in birds and mammals. It might well be that in the evolutionary past of animals, such as vertebrates, the need to parthenogenetically produce offspring has almost completely vanished.