FIGURE 3.1 Inter-locus antagonistic coevolution within and between the sexes can promote the rapid evolution of reproductive traits that are expected to contribute to reproductive isolation (prezygotic mating isolation and postzygotic hybrid infertility) among allopatric populations.

expressed in females are expected to evolve to reduce the harmful effects of interacting with males. If the counteradaptations by female-expressed genes reduce the efficacy of male–male competition to fertilize eggs, then an inter-locus intersexual arms race is expected to ensue between genes that promote female resistance to male-induced harm and the genes that code for male offense and defense (Parker and Partridge, 1998; Rice, 1996, 1998; Rice and Holland, 1997).

The arms races between male offense and defense, and both of these processes with female resistance, would be expected to occur independently in allopatric populations and drive rapid genetic divergence among them (Fig. 3.1). Because, in principle, these arms races can be expected to cause changes in phenotypes influencing reproductive traits (mating behavior and reproductive physiology and anatomy), the genetic divergence produce by inter-locus antagonistic coevolution is expected to cause reproductive isolation by means of pleiotropy in the context of prezygotic reproductive isolation and hybrid infertility, and be an important engine of speciation (Fig. 3.1).

There are two major lines of evidence supporting the hypothesis that inter-locus arms races within and between the sexes drive rapid genetic divergence among allopatric populations: studies of molecular evolution (Swanson and Vacquier, 2002) and studies of experimental evolution (Holland and Rice, 1999; Hosken et al., 2001; Martin and Hosken, 2003; Rice, 1996; Wigby and Chapman, 2004). Molecular studies have demonstrated