  The famous "two-fold cost of sex" is really the cost of anisogamy -- why
should females mate with males who do not contribute resources to offspring,
rather than isogamous partners who contribute equally? In typical anisogamous
populations, a single very fit male can have an enormous number of offspring,
far larger than is possible for any female or isogamous individual. If the
sexual selection on males aligns with the natural selection on females,
anisogamy thus allows much more rapid adaptation via super-successful males. We
show via simulations that this effect can be sufficient to overcome the
two-fold cost and maintain anisogamy against isogamy in populations adapting to
environmental change. The key quantity is the variance in male fitness -- if
this exceeds what is possible in an isogamous population, anisogamous
populations can win out in direct competition by adapting faster.
