Heritability of body size matches trait evolution in the range expansion of a biological control agent

Adaptive evolution requires both natural selection and genetic variation. In introduced species, the selective dynamics of range expansion are predicted by theory to lead to differences between the core and the leading edge, with edge individuals evolving to be more fecund (under r-selection) and have greater dispersal ability than core individuals. In arthropods, both fecundity and dispersal ability are often positively correlated with body size. Here, we quantify genetic variation available for evolution of body size in a beetle (Diorhabda carinulata) introduced into North America as a biological control agent. Previously, we found that females at the edge of the range expansion have evolved to be larger than those at the core as predicted by theory, while male body size has not clearly changed, despite the evolution of increased dispersal capacity. Using a half-sib mating design, we measure genetic variation in mass at eclosion and thorax width of female and male beetles from a single introduced population at the core of the range expansion. We find significant heritable genetic variation in females in both traits, but not in males. Thus, lack of genetic variation in body size may preclude evolution of size in males along this expansion front.