Sex Ratio Redux...

The idea of sex ratio being an ESS is broken down in an easy to follow manner in W.D. Hamilton's work published in his 1967 paper: "Extraordinary Sex Ratios"...follow this link http://www.sciencemag.org/cgi/content/citation/156/3774/477

This is based on the assumption the condition that males and females cost equal amounts to produce:
1. Suppose male births are less common than female.
2. A newborn male then has better mating prospects than a newborn female, and therefore
can expect to have more offspring.
3. Therefore parents genetically disposed to produce males tend to have more than average
numbers of grandchildren born to them.
4. Therefore the genes for male-producing tendencies spread, and male births become
more common.

As the 1:1 sex ratio is approached, the advantage associated with producing males dies away.
The same reasoning holds if females are substituted for males through-out. Therefore 1:1 is the equilibrium ratio.

Fisher seems to have been looking at this from a parental expenditure standpoint.

Fisher's principle is an ESS because once it has been established in a population, it benefits those that "play by that rule" and punishes those who deviate from the ESS.

The sex chromosome assymetry is interesting, due to the fact that xx are homogametic and xy are heterogametic. Y should benefit, because XX never passes along the y chromosome, but x should benefit because the homogametic sex passing it along (but with only half the efficiency). The mitochondria (as well as some other non-nuclear genetics) can bias because they are only carried on through the cytoplasm of the egg. Other non-nuclear items causing bias are bacteria that may be "male-killers".