Fitness and sexual response to population density in Daphnia pulex

1. The switch between asexual and sexual reproduction is an important fitne ss component in cyclically parthenogenetic populations as it is the key to persistence in unstable habitats and because it influences population genet ic characteristics such as linkage disequilibrium and population genetic st ructure. 2. Genetic variation for sexual and asexual reproductive rate (R-C) was exa mined, under varying population density, in Daphnia pulex sampled from a ro ckpool system and two other distinct European localities. 3. Density affected the switch to sexual behaviour, as found in other studi es and reduced R-c. Production of males was not correlated with sexual repr oduction, promoting outcrossing. 4. Genetic variation was found for the response to density, both within and between populations. There was substantial variation within the rockpools despite their demographic instability, indicating rapid recovery of life-hi story variation or maintenance during bottlenecks.