The geometry of natural selection in cave and spring populations of the amphipod Gammarus minus Say (Crustacea: Amphipoda)

The shape of the fitness function was investigated in a spring population (Davis Spring) and a cave population (Organ Cave) of the amphipod Gammarus minus in southern West Virginia. The first three principal components of a series of size, antennal and eye measurements and two fitness components (amplexus and number of eggs) were investigated. Smoothing splines using generalized cross-validation scores were found as were least-squares quadratic regression equations. Confidence intervals on shape, as determined by a smoothing parameter, were calculated using a bootstrap approach. Out of 18 cases, there were six cases of no selection, six cases of directional (monotonic) selection, one case of stabilizing selection, one case of disruptive selection, and four cases of selection with multiple internal maxima and minima. Selection on the cave population was frequently directional (5), resulting from strong directional selection for increased size, larger antennae, and smaller eyes. Selection on the spring population, the ancestral habitat for the species, was frequently non-monotonic (five cases) and rarely directional (one case). In both populations, stabilizing selection was rare and no more common than disruptive selection.