Are parallel morphologies of cave organisms the result of similar selection pressures?

Abstract

The amphipod Gammarus minus is present in both caves and springs, with cave populations showing elaborated (size and antennae) and reduced (eye) characters relative to spring populations. Cave populations have resulted from independent invasions of hydrologically isolated subterranean drainages and there is genetic variation for both elaborated and reduced characters. The authors used variation in mating success and fecundity to test for the presence of directional selection on eye, antennal, and body size characters in a set of cave and spring populations during a series of seasonal cross-sectional samplings. There were significant directional selection for smaller eyes in caves and larger eyes in springs, which supports the hypothesis that selection is responsible for reduced eye size in cave populations. There was also selection for larger body and antennal size in cave populations, consistent with the hypothesis that parallel patterns of selection in caves are responsible for the elaboration of body and antennal size, but selection for larger body and antennal size in spring populations is not consistent with the observed divergence of spring and cave populations. Unmeasured components of viability selection could be more important in springs than in caves and may act against the selection for larger size found in spring populations. -from Authors