The Divergence of Adjacent Populations

Abstract

In Union Bay, a temperate embayment in Seattle, Washington, USA two different kinds of predators maintain a polymorphism of the cladoceran Bosmina longirostris. The boundary separating these two Bosmina populations is unusual because it is maintained in a freely mixing body of water, and important because its distinctness is a consequence of trophic dynamics, a result of the interactions of visual predators (fish) and grasping predators (copepods). Not only do the fish directly affect the morph composition by differentially removing the more conspicuous morphs, they also indirectly control the magnitude of counter—selection forces by regulating the abundance of predatory copepods. The joint action of both predators sets up a spatial situation in which adjacent populations of Bosmina actually diverge phenotypically and genetically during the summer.