Hybrid Zones and Homogamy in Australian Frogs

The origin and maintenance of mechanisms of homogamy (positive assortative mating, selective fertilization, or premating reproductive isolation), which characterize closely related sympatric, biparental species, represent a controversial and poorly understood area of contemporary evolutionary thought (26). The significance of reproductive character displacement (reinforcement) has been repeatedly questioned (17, 45, 46), and sexual selection now is assuming a greater theoretical role as a mechanism that may explain the origin of homogamy (6, 7, 18, 19, 57). Even so, an incidental, noninteractive, allopatric evolution of selective mating systems remains as the most favored process (10, 43). In spite of much theoretical deliberation and conjecture (see 16, and references therein), thoroughly worked natural examples, specifically directed at evolutionary aspects of homogamic mechanisms, remain inadequate. Parapatric, or contiguously allopatric, distributions of closely related taxa, whose contacts are marked by zones of hybridization, may represent critical stages in the dynamics of the speciation process, wherein the levels of divergence are directly and naturally tested by ecological and reproductive interactions between parental individuals and/or recombination products (44). The outcomes of such interactions may be introgression, accelerated divergence (character displacement), or stalemate, depending on the extent of contact between individuals of each parental taxon, and levels of genetic input from neighboring allopatric populations (23, 38).