Character Release and Displacement in Fishes: A Neglected Literature

Biologists concerned with the evolutionary consequences of competition seem unaware of a large literature on freshwater fishes that bears directly on the debates over ecological character displacement, character release, and the concept of "empty niches." Only five potential instances of character displacement in teleosts are cited in the conceptually oriented literature, whereas data exist for 97 species comprising 52 genera and 17 families. We review these studies, focusing on lacustrine species, and draw the following conclusions. First, competition is frequently a diversifying force that creates differences between species (character displacement) and differences within species (character release) when other closely related species are absent. Second, differentiation in lakes occurs along predictable pathways and almost always includes pelagic and benthic forms. The lake environment can therefore be viewed as a set of non-Hutchinsonian or "environmental" niches that exist apart from the species that occupy them. Third, a traditional criterion for demonstrating character displacement has been that phenotypic differences between populations in sympatry and allopatry should have a genetic basis. Although this criterion is frequently met in freshwater fish, it is not necessary because phenotypic plasticity can itself be an evolutionary consequence of competition. Finally, in the conceptually oriented literature, the evolutionary consequences of predation are well accepted, but the evolutionary consequences of competition are regarded more skeptically. This skepticism is unwarranted for freshwater fish, whose evolution is clearly influenced by both predators and competitors.