Intralocus sexual conflict

Intralocus sexual conflict is one of two genetic forms of sexual conflict (the other being interlocus sexual conflict). It occurs when selection on a shared allelic trait in one sex displaces the other sex from its phenotypic optimum. It arises because many shared traits have a common genetic basis but undergo contrasting selection because they have opposite effects on the Darwinian fitness of both sexes. A classic example is the human hip, where females need larger hips for childbirth as opposed to smaller hips (optimal for walking) for males.^[1] The genes that affect hip size must reach a compromise that is at neither the male optimum nor the female optimum. In some cases, this conflict may be resolved by the restriction of the gene’s expression to only the sex that it benefits, resulting in sexual dimorphism.^[2] Furthermore, evidence points to these sexually antagonistic genes being common in both plants and animals.^[2] Thus, intralocus sexual conflict is a widespread phenomenon.

Development

Bonduriansky and Chenoweth (2009), proposed a 4 phase model for the development of intralocus sexual conflict, in which the first phase is stabilizing selection on a trait in both sexes. Intralocus conflict then originates in the second phase when a change in physical or social conditions causes intense selection on that trait in males and/or females, and both sexes are displaced from their optimum. In the third phase, diverging selection continues on both sexes, but is attenuated. In the fourth phase, intralocus conflict is fully resolved and sexual dimorphism has occurred.^[3]

Examples of this include sexually antagonistic selection on body size in collared flycatchers, leading to small males and large females,^[4] and sexually antagonistic selection on the beak color between male and female zebra finches.^[5] This dimorphism is the result of differential expression of the sexually antagonistic gene. Thus, intralocus sexual conflict often leads to and is resolved by dimorphism.

Effects

Intralocus conflict impacts the fitness of the offspring in a negative manner. The benefits of sexual selection are completely reversed because high-fitness males produce low-fitness daughters and high-fitness females produce low-fitness sons.^[2] Additionally, a sexually-shared trait selected for in males can have negative impacts on the Darwinian fitness of females, and vice versa.

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