Species are groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups.
Recent studies by the students of animal behavior, as well as the revised interpretation of many earlier observations, indicate that behavior differences are among animals the most important factor in restricting random mating between closely related forms.
E. Mayr (1942)
One of the major challenges facing modern biology is to under stand the genetic mechanisms causing speciation. Because sexual isolating mechanisms that act before fertilization [“ethological” isolating mechanisms (Mayr, 1942)] are thought to precede the evolution of postzygotic isolating mechanisms (inviability and sterility), we need to understand the genetic basis of sexual isolation if we are to gain insight about the early stages of species formation. However, mating behaviors are complex traits, with variation attributable to multiple interacting loci with individually small effects, whose expression depends on the environment. Thus, understanding the genetic architecture of sexual isolation requires that we overcome the twin obstacles of mapping genes causing differences between organisms that, by definition, do not interbreed [Orr, “The Genetic Basis of Reproductive Isolation: Insights from Drosophila” (Chapter 2, this volume)] and solving the problem of genetically dissecting complex behavioral traits (Anholt and Mackay, 2004).
Drosophila species present an ideal model system in which to investigate the genetic basis of sexual isolation. Several species pairs are only partially reproductively isolated, producing fertile hybrids that can be backcrossed to one of the parental species to generate segregating backcross mapping populations. Furthermore, Drosophila melanogaster is a model organism with excellent genetic and genomic resources that are ideal for genetically dissecting complex traits, including the ability to clone chromosomes, replicate genotypes, and rear large numbers of individuals under uniform environmental conditions; publicly available mutations and deficiency stocks useful for mapping; abundant segregating variation in natural populations that can readily be selected in the laboratory to produce divergent phenotypes a complete well annotated genome sequence; and several platforms for whole-genome transcriptional profiling. Courtship behavior of Drosophila is composed of sequential actions that exchange auditory, visual, and chemosensory signals between males and females, allowing for individual components of the behavior to be