BEHAVIORAL REPRODUCTIVE ISOLATION AMONG POPULATIONS OF THE ROTIFER BRACHIONUS PLICATILIS.

Several theories have been proposed to explain how reproductive isolation develops between populations (see Mayr, 1970; Bush, 1975; Carson, 1975; Templeton, 1980, 1981). Different mechanisms have been suggested for allopatric, sympatric, and parapatric populations, and there is some consensus about the importance of geographical isolation. Considerable disagreement persists, however, concerning the role of selection and genetic drift in the formation of reproductive isolation, as well as whether pre-mating or post-mating barriers develop first. Muller (1942) was the first to argue that reproductive barriers appear as by-products of adaptations to different environments. Populations adapting to different environments accumulate genetic differences that eventually lead to reproductive isolation. Dobzhansky (1940) placed more emphasis on the role of natural selection in the formation of barriers to gene flow. He supposed that slight post-mating barriers appear in allopatric populations, but that pre-mating barriers arise when the two populations come into contact once again. At this time selection against hybrids produces the preponderance of reproductive isolation between the incipient species. Carson (1968, 1971) has emphasized the role of genetic drift through founder effects in the formation of reproductive isolation between populations with his population flush principle. While there is still disagreement about the evolutionary dynamics of how reproductive isolation develops, the existence of reproductive isolation in many groups is well documented. Numerous examples of both preand post-mating reproductive isolating mechanisms can be found in any modern textbook on evolutionary biology. Furthermore, it is generally agreed that behavioral reproductive isolation plays a premier role in animal speciation (Dobzhansky, 1970). In certain groups, the type of reproductive isolating mechanisms that operate and how they develop is still largely unknown. This is true of zooplankton in general and rotifers in particular. For rotifers, ecologists and systematists have provided some initial insights into how pre-mating mechanisms might contribute to reproductive isolation. Allopatry probably plays a major role, at least in fresh water rotifers, because of the patchy nature of lake environments. This discrete type of environment is likely to pose substantial barriers to gene flow between populations inhabiting different lakes. Temporal isolation among populations also appears to be important. In the cyclical parthenogenetic breeding system of rotifers (Birky and Gilbert, 1971; King and Snell, 1977), sexual reproduction is an ephemeral event. Observations in natural populations suggest that rotifer species which co-occur in a habitat, often do not overlap in their periods of sexual reproduction (WesenbergLund, 1930; Carlin, 1943; reviewed by King, 1980). This lack of temporal correspondence in sexual periods could be an effective reproductive isolating mechanism. Only a single work (Gilbert, 1963) has provided any detail about the operation of behavioral reproductive isolating mechanisms in rotifers. In reciprocal tests using the freshwater rotifers Brachionus calycifiorus and Brachionus angularis, Gilbert found the males of both of these species completely species-specific in their mating reactions. When B. calycifiorus males were tested with females of three additional ro-