Eco-evolutionary theory in the wild: Can range expansion theory predict evolution of a range expanding biocontrol agent?

Eco-evolutionary theory predicts that species undergoing range expansions will experience different
evolutionary forces at the core and edge of expansion, and thus may evolve differences in traits such
as reproductive output or dispersal ability. Evolution during range expansions can influence the fitness
of populations at the expanding edge and the speed of spread. We apply range expansion theory to
biocontrol agents spreading from original release sites. Reproductive output may increase or decrease
at the edge of range expansions depending on the relative strength of selection and genetic drift at low
density edges. Dispersal may increase at the edge if good dispersers accumulate there and mate with
each other, but may not change if environmental gradients slow spread so that both good and poor
dispersers are found at the edge. Dispersal decisions also depend on local population density and mate
availability. We evaluate evolution of reproduction and dispersal according to theory across the
naturally replicated range expansion of the biocontrol agent the northern tamarisk beetle (Diorhabda
carinulata). We found that edge populations are larger, more fecund, and reproduce earlier than
populations from the core of the range, indicating that selection at low density overwhelms drift.
Dispersal generally increased at the edge compared to the core, particularly in unmated individuals and
at low population density. Evolution of reproduction and dispersal are likely improving efficacy of
control and accelerating the range expansion D. carinulata. Evolution of biocontrol agents during post-
release range expansion may be common, rapid, and even occur over environment gradients.