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Sex is always well worth its two-fold cost

Sex is considered as an evolutionary paradox, since its evolutionary advantage does not necessarily overcome the two fold cost of sharing half of one's offspring's genome with another member of the population. Here we demonstrate that sexual reproduction can be evolutionary stable even when its Darwinian fitness is twice as low when compared to the fitness of asexual mutants. We also show that more than two sexes are always evolutionary unstable. Our approach generalizes the evolutionary game theory to analyze species whose members are able to sense the sexual state of their conspecifics and to switch sexes consequently. The widespread emergence and maintenance of sex follows therefore from its co-evolution with even more widespread environmental sensing abilities.