SEXUAL CONFLICT AND SPECIATION

We review the significance of two forms of sexual conflict (different evolutionary interests of the two sexes) for genetic differentiation o f populations and the evolution of reproductive isolation. Conflicting selection on the alleles at a single locus can occur in males and fem ales if the sexes have different optima for a trait, and there are ple iotropic genetic correlations between the sexes for it. There will the n be selection for sex limitation and hence sexual dimorphism. This se x limitation could break down in hybrids and reduce their fitness. Ple iotropic genetic correlations between the sexes could also affect the likelihood of mating in interpopulation encounters. Conflict can also occur between (sex-limited) loci that determine behaviour in males and those that determine behaviour in females. Reproductive isolation may occur by rapid coevolution of male trait and female mating preference . This would tend to generate assortative mating on secondary contact, hence promoting speciation. Sexual conflict resulting from sensory ex ploitation, polyspermy and the cost of mating could result in high lev els of interpopulation mating. If females evolve resistance to male pr e-and postmating manipulation, males from one population could be more successful with females from the other, because females would have ev olved resistance to their own (but not to the allopatric) males. Betwe en-locus sexual conflict could also occur as a result of conflict betw een males and females of different populations over the production of unfit hybrids. We develop models which show that females are in genera l selected to resist such matings and males to persist, and this could have a bearing on both the initial level of interpopulation matings a nd the likelihood that reinforcement will occur. In effect, selection on males usually acts to promote gene flow and to restrict premating i solation, whereas selection on females usually acts in the reverse dir ection. We review theoretical models relevant to resolution of this co nflict. The winning role depends on a balance between the 'value of wi nning' and 'power' (relating to contest or armament costs): the winnin g role is likely to correlate with high value of winning and low costs . Sperm-ovum (or sperm-female tract) conflicts (and their plant parall els) are likely to obey the same principles. Males may typically have higher values of winning, but it is difficult to quantify 'power', and females may often be able to resist mating more cheaply than males ca n force it. We tentatively predict that sexual conflict will typically result in a higher rate of speciation in 'female-win' clades, that fe males will be responsible for premating isolation through reinforcemen t, and that 'female-win' populations will be less genetically diverse.