The X chromosome favors males under sexually antagonistic selection

The X chromosome is found twice as often in females as males. This has led to an intuition that X‐linked genes for traits experiencing sexually antagonistic selection should tend to evolve toward the female optimum. However, this intuition has never been formally examined. In this paper, I present a simple mathematical model and ask whether the X chromosome is indeed biased toward effecting female‐optimal phenotypes. Counter to the intuition, I find that the exact opposite bias exists; the X chromosome is revealed to be a welcome spot for mutations that benefit males at the expense of females. Not only do male‐beneficial alleles have an easier time of invading and spreading through a population, but they also achieve higher equilibrium frequencies than comparable female‐beneficial alleles. The X chromosome is therefore expected over evolutionary time to nudge phenotypes closer to the male optimum. Consequently, the X chromosome should find itself engaged in perpetual intragenomic conflicts with the autosomes and the mitochondria over developmental outcomes. The X chromosome's male bias and the intragenomic conflicts that ensue bear on the evolution of gene regulation, speciation, and our concept of organismality.