Ecological genetics of the cyclic parthenogens, Daphnia longispina and Daphnia pulex

A population of Daphnia longispina was found to possess a relatively stable genetic structure, while a population of D. pulex appeared genetically very unstable. Unlike the population of D. magna inhabiting single-species pools in the same area and which maintained approximately the same level of genetic variation throughout seasons, the D. pulex population was characterized by decreasing genetic diversity toward autumn and across years. A clue to the surprisingly different behaviour of the D. pulex population could be different biotic selective pressures. The population of D. longispina and D. pulex studied here had to compete with another closely related species, D. magna. D. longispina appeared to outcompete D. magna completely, while D. pulex was exposed to more continuous competition. Selection favouring a few well competing clones is a likely explanation for the extensive decrease in genetic diversity observed in the population of D. pulex.