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Central to the study of speciation is the identification of the isolating mechanisms that reduce gene flow. Of special interest are those isolating mechanisms that are the earliest to arise. Sperm fate and function play a major role in two distinct types of reproductive isolation in the genus Drosophila. First, postcopulatory-prezygotic (PCPZ) incompatibilities between the male ejaculate and a heterospecific female reproductive tract can result in a reduction in sperm fertilisation success. Although PCPZ incompatibilities have, until recently, received little attention, overwhelming diversity in reproductive morphology and biochemistry indicates that they may play an important role in speciation in this genus. The second type of isolating mechanism that affects sperm occurs in the testes of hybrid males produced by heterospecific matings. These individuals often suffer from dysfunction in spermatogenesis, resulting in the production of aberrant sperm. Hybrid male sterility and hybrid inviability are examples of postzygotic (PZ) reproductive isolation. The observation that hybrid sterility is pervasive among males of all taxonomic groups is known as Haldane's rule. Here we discuss both the evolutionary origins, and functional causes of both PCPZ incompatibilities, which affects sperm fate and function in females, and one type of PZ incompatibility, hybrid male sterility, or sperm dysfunction in hybrid males. Although these two mechanisms of isolation are quite distinct, they are similarly caused by breakdowns in epistatic interactions which occur in the encounter between two divergent genomes. Molecular, cytological, and empirical data are discussed, as is relevant evolutionary theory.

Populations of the North American cactophilic fruitfly Drosophila mojavensis and its sibling species D. arizonae exist both in sympatry and in allopatry. Females of D. arizonae, regardless of their population of origin, are effectively completely isolated behaviorally from D. mojavensis males. On the other hand, females of D. mojavensis from the sympatric populations in Sonora, Mexico exhibit significantly stronger premating isolation from D. arizonae males than do D. mojavensis females from allopatric populations from the Baja California peninsula. Earlier studies interpreted these limited observations as support for reinforcement. Since the time of those studies, additional allopatric populations of D. mojavensis have been collected from southern California and from Santa Catalina Island, off the coast of southern California. Here, we tested the prediction that if sympatry is in fact associated with increased isolation in D. mojavensis, these additional allopatric populations also should show, relative to the sympatric ones, less isolation from D. arizonae. Our results are consistent with this prediction and suggest that isolation is in fact stronger in sympatry.