An evolutionary stable strategy (ESS) is a strategy that the majority of the population utilizes. It is the only strategy that persists and cannot be invaded by deviant individuals. An example of an ESS is Fisher’s sex ratio of 50:50 males to females. If a population begins to shift its preference on one sex over the other, that sex will on average have lower mating successes, and therefore have a reduced offspring-production. As time progresses, selection will seek to shift/favor genes that bias towards the opposite sex, until the 50:50 ratio is achieved.
Autosomal chromosomes, being equal in number and having homologs in both males and females, will likely favor the 50:50 sex ratio. Sex chromosomes however have a bias 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. Cytoplasmic elements, usually coming from the female, can also distort the 50:50 sex ratios. The majority of sex distorters are cytoplasmic and they favor the sex through which they originate from
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