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Abstract

A central question in evolutionary biology concerns the ecological and genetic processes by which species hybridize. We analyze the evolutionary consequences of hybridization, biogeographic patterns and fitness comparisons in the crustacean Daphnia (Branchiopoda) within the conceptual framework of theories on interspecific hybridization. In contrast to most cases of hybridization among animals, Daphnia species and hybrids do not form interpopulational transition zones (hybrid zones), but rather patchy distributions of hybrids and parentals have been found. In addition, due to ameiotic parthenogenetic reproduction, hybrid breakdown can be avoided and hybrids can reach higher abundances than parental species. Hybrids within the D. galeata complex occur across broad geographic ranges, and lakes vary significantly in species and hybrid composition over time. Species differ in traits related to predator avoidance, such as size at maturity, intrinsic rate of increase (r), behaviour, and induction of helmets and spines. Hybrids tend to exhibit a combination of characters which enables them to persist in ecological niches which arise seasonally due to changes in predation and food regimes. Hybrids seem to be of recent and multiple origin and seem to form backcrosses occasionally, but no mitochondrial and only low levels of nuclear introgression have been detected. Interspecific matings are non-random, which leads to directionality of hybridization and introgression. Environmental settings seem to faciliate hybridization (temporal hybrid superiority model), whereas evolutionary consequences may arise from repeated backcrossing, which in some cases results in paterns of reticulate evolution.