Divergence in male mating tactics between two populations of the soapberry bug: II. Genetic change and the evolution of a plastic reaction norm in a variable social environment

Many social behaviors arc conditional, but behavioral comparisons between populations do not normally distinguish genetic and environmental causation. As a result, the opportunity to test predictions about the evolution of strategic conditionality (genotype x environment interaction) is lost. We apply these concepts in an examination of how interpopulation differences in mean and variance of sex ratio have led to genetic differences in the allocation of male effort to mate guarding versus nonguarding between genetically isolated populations of the soapberry bug in Oklahoma and Florida. We observed the mating behavior of males from the two populations at a series of experimental sex ratios, and modeled their mating decisions as first-order Markov chains of independent mating states. Likelihood ratio tests of these behavioral sequences showed that the populations differed significantly in their response to sex ratio, and that only males from the variable environment (Oklahoma) altered their behavior in response to differences in female availability among the treatments. The flexible strategy of this population may be adaptive and probably has evolved in response to sex ratio variability.