Sander van Doorn (Groningen Institute for Evolutionary Life Sciences)
Complex trait dynamics in a model of co-evolving male and female mating strategies
Evolutionary biologists have long recognized that conflicts of interest between the sexes can induce arms races in which males evolve traits that are detrimental to the fitness of their female partners, and vice versa. This interlocus sexual conflict (IRSC) has been proposed as a cause of perpetual intersexual antagonistic coevolution with wide-ranging evolutionary consequences. However, theory suggests that the scope for perpetual coevolution is limited, if traits involved in IRSC are also expressed in other contexts. Here, we consider a biologically plausible case of this scenario and arrive at drastically different conclusions. Our analysis is based on a quantitative genetic model of sexual conflict, in which genes controlling IRSC traits have side effects in the other sex. As a result, the genes are exposed to intralocus sexual conflict (IASC), a tug-of-war between opposing male- and female-specific selection pressures. We find that the interaction between the two forms of sexual conflict has contrasting effects on antagonistic coevolution: IASC stabilizes the dynamics of arms races if the mating traits are close to evolutionary equilibrium but can prevent populations from ever reaching such an equilibrium state. When this occurs, the sexes are drawn into a continuous cycle of arms races, causing the buildup of IASC, alternated by phases of IASC resolution that trigger the next arms race. A bifurcation analysis clarifies how this alternative evolutionary attractor emerges and reveals and unexpectedly rich bifurcation structure in a relatively simple model of coevolution. These results encourage an integrative perspective on the biology of sexual conflict and generally caution against relying exclusively on equilibrium stability analysis.