Predator-mediated genotypic shifts in a prey population: experimental evidence.

We demonstrate the effect of fish predation on genotype frequencies in a laboratory population composed of two Daphnia magna clones, with historically contrasting exposures to fish predation. The two clones differed in their responsiveness to predation via differential avoidance/escape behavior. The clone which coexists with fish in nature is more responsive to the presence of a fish predator, while the clone not exposed to fish predation does not exhibit the defensive reaction. Fish caused a rapid (within 18 h) and significant shift in Daphnia clonal composition, from 1:1 to 8:1, in favor of the responsive clone. Genotype-specific defensive abilities (modus defendi) can contribute greatly to the phenomenon of genotype replacement under selective predation.