I've found some interesting papers about sex ratio and parental control over it (I've not read the whole of them though): Natural Selection of Parental Ability to Vary the Sex Ratio of Offspring http://www.sciencemag.org/content/179/4068/90.short Evolutionary conflict over the control of offspring sex ratio. http://www.ncbi.nlm.nih.gov/pubmed/3657204 Biased Sex Ratios in Plants: Theory and Trends http://www.academia.edu/1789755/Biased_Sex_Ratios_in_Plants_Theory_and_Trends Parent-Offspring Conflict Over the Sex Ratio in a Diploid Population with Different Investment in Male and in Female Offspring http://www.jstor.org/stable/2462443 --- --- This one is about evolution convergence: Evolution’s Random Paths Lead to One Place http://www.simonsfoundation.org/quanta/20140911-evolutions-random-paths-all-lead-to-the-same-place/ Até mais! Leonardo 2014-09-12 3:38 GMT+02:00 Herman Miller <[log in to unmask]>: > Alex Fink wrote: > > I assume that's some sort of largest eigenvector of some Markov >> process. But they say (not in a good place to look up a better >> citation now, sorry) that the reason mammalian births are half male >> and half female is because that's the game-theoretic optimum -- at >> least assuming the cost of bearing and raising a male and a female is >> equal. Indeed, if the distribution were otherwise, say more than >> half of births were male, then a mutation which brought about more >> female births would be advantageous, as your children would be more >> likely to have a larger pool of potential mates and thus spread their >> genes broaderly; then in the long term this mutation would be >> selected for and tip the balance back towards parity. >> > > That's one thing I've wondered about. In previous versions of my world > I've had species with uneven sex ratios. But I couldn't figure out how the > genetics of that would work. And if it's determined by other factors > besides selecting an X or a Y chromosome (like temperature with crocodile > eggs), it does seem to make the most sense that it would end up with a > system that produces roughly the same numbers of males and females. I've heard some saying that Y spermatozoids are lighter and faster than X spermatozoids, but the X ones are more resistant, so that a more acid or more alkaline medium would favor X (females) and a neutral one would favor Y (males). I've no idea of how true is this statement.