The adaptive significance of dorsal spine variation in the fourspine stickleback, Apeltes quadracus. II. Phenotype–environment correlations

Abstract

The fourspine stickleback (Apeltes quadracus) is polymorphic for dorsal spine number, the variation is heritable, and morph frequencies are highly differentiated geographically. Our purpose here is to determine how spines vary in relation to environment. Dorsal spine number is significantly correlated (P < 0.001) with environmental differences among sites; the environmental correlates include predators, potential competitors, vegetation, physical environment, habitat, and geographical position. Based on these correlations we hypothesize that selection by predators favours the higher spined morphs and that selection by competitors favours the lower spined morphs. The correlations with other environmental variables probably reflect interactions with predation and (or) competition, but they may be concomitants of independent and unidentified selective agents. The observed patterns of geographic variation in spine number are in agreement with those expected if predators and competitors are selective agents. We conclude that selection acts on this polymorphism; the geographic differentiation in spine number is adaptive.