The limits to life history evolution in Daphnia.

The components of phenotypic variance and covariance for fitness traits were periodically determined for an intermittent population of Daphnia pulex and applied to phenotypic selection models to determine the limits to the response to selection. Levels of expressed genetic variance were extremely high early in the year as a consequence of the release of hidden genetic variance via sex in the previous year. However, <25 generations of continuous clonal selection were required to depress the expressed genetic variance to levels expected under obligate parthogenesis. A minimum of 10-20% selective mortality per generation is required to account for this erosion in genetic variance. Since D. pulex populations generally consist of several closely related clonal groups that are distinct with respect to both phenotypic means and genetic covariance structure, selection between groups supplements clonal selection within groups as a mechanism for phenotypic evolution. Of particular interest in the study population is the co-existencec of an obligately unisexual race with 2 cylindrically parthogenetic clonal groups. Rates of phenotypic evolution in excess of approx 25% of the mean phenotype/generation are unlikely to occur in most cladoceran populations, especially those that have forgone sex for more than a few generations, without a substantial reduction in population size.-from Author