Battle of the Sexes

According to Dawkins, an evolutionary stable strategy (ESS) is a strategy that the majority of the population employs. In a population of individuals maximizing their own successes, the only strategy that persists will be the one that cannot be bettered by any deviant individual. An example of an ESS is Fisher’s sex ratio of 50:50 males to females. If a population suddenly began to shift its preference on one sex over the other, in succeeding generations, that sex will on average have lower mating successes, and thus, offspring-production is reduced. In time, selection will favor genes that biases towards the opposite sex, until the 50:50 ratio is reached. The strategy of a 50:50 sex ratio is an ESS in the sense that any deviation from it leads to a net loss (Dawkins, 145).

Autosomal chromosomes, being equal in number and having homologs in both males and females, will likely favor the 50:50 sex ratio. Sex chromosomes will be biased towards one sex over the other since the sexes differ on a single chromosome- the Y chromosome. It is probable that the X chromosome can select repressors on the Y-chromosome, and thus favor the production of females (Werren, 1998). Cytoplasmic elements, usually obtained from the female, can also distort the 50:50 sex ratio. Indeed , the majority of sex distorters are cytoplasmic and they favor the sex through which they will be transmitted (Hardy, 2002).

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