locus increases the lag-load at a second locus by generating selection for a new optimal allele, and the resulting adaptive allelic replacement at the second locus increases the lag-load at the first locus, thereby stimulating a new round of the antagonistic cycle of adaptation and counteradaptation. The genetic conflict that drives inter-locus antagonistic coevolution is termed “intragenomic” when conflict occurs between genes that reside in a single individual [for example, genetic conflict associated with genomic imprinting (Haig, 2002), meiotic drive (Jaenike, 2001), and/or cytonuclear conflict (Werren, 1997)] and “intergenomic” when gene products from different loci mediate conflicts of interest between different individuals of the same species (Rice and Holland, 1997).

Inter-locus antagonistic coevolution (intragenomic or intergenomic) can potentially drive rapid genetic divergence among allopatric populations because the antagonistic cycle of adaptation and counteradaptation maintains a persistent lag-load at each interacting locus and thereby drives perpetual evolutionary change. In this article, we focus on intergenomic conflict because two of its forms are predicted to contribute to reproductive isolation by causing genetic divergence among allopatric populations (Parker and Partridge, 1998; Rice, 1996, 1998; Rice and Holland, 1997). Intergenomic conflict can occur both within and between the sexes. An example of intrasexual conflict occurs between gene loci that mediate the male “offense” and “defense” phenotypes in the context of male–male competition to fertilize eggs. A male that mates a virgin female is selected for a defense phenotype, that is, to prevent the female from mating with other males and to prevent his stored sperm from being displaced by a secondary male if the female remates. A male encountering a previously mated female is selected for an offense phenotype, that is, to induce the female to mate with him even if she has ample stored sperm from another male, and then to replace the stored sperm from the previous male with his own. To the extent that the male offense and defense phenotypes are controlled, at least in part, by gene products derived from different gene loci, these interacting genes can antagonistically coevolve in a perpetual cycle of adaptation and counteradaptation.

Persistent male courtship, insemination, and seminal fluid proteins are known to be harmful to females in many species because they lower the female’s survival, fecundity, and short-term fertility (Arnqvist, 1989; Burpee and Sakaluk, 1993; Chapman et al., 1993, 1995; Cohet and David, 1976; Crudgington and Siva-Jothy, 2000; Dean, 1981; Fowler and Partridge, 1989; Friberg and Arnqvist, 2003; Kasule, 1986; McKinney et al., 1983; Moore et al., 2001; Partridge and Fowler, 1990; Pitnick and Garcia-Gonzalez, 2002; Prout and Clark, 2000). Available evidence indicates that the harm to females is an incidental byproduct of adaptations that increase male fertilization success in promiscuous species (Hosken et al., 2001; Morrow et al., 2003). Genes