Excessive inbreeding can be deleterious to a population, resulting in inbreeding depression. Gene flow through metapopulations can be amplified by hybrid vigor, if the hybrid offspring of immigrants and residents have a competitive advantage. In the January 18 Science, Ebert et al. describe experiments that test this theory (Science 2002, 295:485-488).

They studied the colonization and extinction dynamics of local populations of the water flea Daphnia magna in rock pools on islands along the Scandinavian Baltic Sea coast. Ebert et al. examined the significance of hybrid vigor by introducing immigrant genotypes into 22 Daphnia populations and monitoring the success of hybrid offspring throughout the summer. They cleaned out pools and refilled them with 200 individuals from the original populations (residents) and added immigrant clones, and then followed populations using several alloenzyme marker loci.

Their genotyping results clearly showed that outbred offspring displayed the greatest fitness. Ebert et...

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