Morphological Evolution of the Amphipod Gammarus minus in Caves: Quantitative Genetic Analysis

Abstract

-Cave-dwelling populations of the amphipod Gammarus minus have reduced eyes and elongated antennae relative to surface spring-dwelling populations. The hypothesis that the reduction in eye size in cave populations results as a negatively pleiotropic conse- quence of selection for increased antenna size in the aphotic environment is examined. The genetic patterns of variation and covariation underlying eye size traits and antenna size traits were obtained from two spring and two cave populations and an intermediate pop- ulation located in two subterranean drainage basins. Heritability estimates indicated sub- stantial heritable variation in eye size and antenna size in all populations. The median of genetic correlations between eye and antenna traits is near zero for each of the two spring populations, is negative for the intermediate population, and is positive for the two cave populations. The relative difference between the spring and cave populations was consistent between drainage basins. It is argued that the observed patterns agree with the hypothesis of indirect selection with negative pleiotropy. It is also argued that these patterns are not in accord with the hypothesis that the reduction in eye size in cave populations results from accumulation of neutral mutations alone.