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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

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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.