Homeostasis,enzymatic heterozygosity and inbreeding depression in natural populations of Drosophila melanogaster

Brother-sister mating effects on offspring viability in natural populations of Drosophila melanogaster were analysed in relation to developmental homeostasis and enzymatic heterozygosity of the individuals constituting the sib pairs. High developmental homeostasis and high degree of heterozygosity are associated with a low inbreeding depression in offspring and, therefore, with a low frequency of major deleterious factors. These results are discussed in relation to maintenance of polymorphism in natural populations and the role of genetic load and founder events in speciation and adaptation. We conclude that an unusual environmental condition followed by a strong reduction in effective population size due to random elimination of individuals is a better candidate to speciation than an environmental shift leading to selection for homeostatic, highly heterozygous lethal-free individuals.