Theory suggest that sexual conflict can promote rapid evolutionary diversification and speciation. Although some empirical studies support this idea, other studies have failed to demonstrate such a pronounced role of sexual conflict as a strong driver of population differentiation. Alternatively, it has been suggested that sexual conflict might halt population divergence and instead promote the formation and coexistence of sympatric male and female genetic clusters within populations. However, this remains to be investigated in natural populations. Combining theories of evolution by sexual conflicts with the love of diving beetles, we have documented how a female polymorphic trait that has evolved in response to male mating harassment (granulated and smooth elytra) is dependent on male mating morphology (front tarsal suction cups). We have conducted a comprehensive investigation of sexual conflict traits within the diving beetle species Graphoderus zonatus across 29 wild populations. We have directly quantified the presence of male polymorphism based on morphological trait measures and correlated these to the presence of female morph types based on observed assortative mating, spatial links between the proportion of male and female clusters, and population dynamics across 20 years.
Results/Conclusions
We show, how sexual conflict plays a conservative role in maintaining male and female polymorphisms locally, rather than promoting population divergence. Variation in male front tarsal suction cup morphology was allocated into two distinct clusters, reflecting the expected adaptation to female elytra structure. Furthermore, mating pairs did show that the two male clusters were connected, via assortative mating, to the two female morph types (granulated or smooth elytra surface) independently. Changes in male and female sexual traits led to a predictable change in the other sex across spatial scales and within population dynamics. These findings are consistent with a scenario where sexual conflict leads to assortative mating between male offence and female defense traits, and this process in turn maintains such sexual polymorphisms and thereby stopping evolutionary diversification. Our results challenge the current view of sexual conflict as an engine of rapid speciation via exaggerated co-evolution between sexes. Instead we show that emerging sexual traits are maintained in a polymorphic state creating a system in which sympatric speciation could occur as a result of sexual conflicts.