Theoretical and empirical studies will be carried out in three interrelated problem areas of evolutionary genetics--selection in multi-locus systems (the unit of selection), speciation and ecologically realistic fitness models. In my theoretical work, I will generalize and extend previous models of the evolution of homeostasis in heterogeneous environments to multi-locus systems, and I will elaborate my theory on speciation via the founder effect. My empirical studies will use Drosophila mercatorum as the experimental organism. Previous work has shown that D. mercatorum females from natural, sexual populations have a low level capacity for parthenogenetic reproduction by pronuclear duplication which results in totally homozygous offspring. This transition to parthenogenesis involves alterations in morphology, life-history parameters, development, behavior, and physiology - some of which are related to juvenile hormone. I intend to investigate the genetic, physiological and developmental basis of the adaptive significance of these alterations and relate them to models of speciation and multi-locus selection. Moreover, some of the genetic systems revealed in previous work will be utilized to test theories on the evolution of life history parameters, density-dependent selection, and the validity of Levene-models for evolution in heterogeneous environments.