Normally we understand the (near) 50-50 male/female population sex ratio with this simple model: if there were more males than females then individuals who are genetically disposed to have female children will have more grandchildren because their female children will find more mates. Thus females will increase in proportion, restoring the balance.
But here’s the interesting thing. That model doesn’t work for humans (and many other species) and in fact 50-50 is highly unstable, with potentially catastrophic consequences.
Suppose that a male has a mutation on his Y chromosome which causes him to produce Y-chormosome sperm that swim faster than his X-chromosome sperm. Then he will have only boys. And his boys will have the same gene and the same super Y-chromosome sperm.
Now suppose that his male children have an equal chance of mating as all other males in the population. Then our original mutant will have more male grandchildren than other males of his generation. Thus, the proportion of this super-Y gene increases in the population, and this trend continues generation after generation.
The balance is not being restored anymore. In fact eventually the super-Y’s dominate the male population. And that means that all offspring in all matings are boys. That means very little reproduction can happen because there are so few females. And the species goes exctinct.
I learned this from a paper by W. D. Hamilton called Extraordinary Sex Ratios.
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July 20, 2011 at 12:54 am
Danny Lynch
The tricky part is in this statement “Now suppose that his male children have an equal chance of mating as all other males in the population.”
In this proposed society, men who have previously fathered daughters would be highly desirable mates, and their sons would be as well.
July 20, 2011 at 9:27 am
twicker
Not to mention, we don’t know what other traits would go along with the super-Y. Being really fast would be only one of the traits passed on by a Y-chromosome; the other traits may, or may not, help them survive (e.g., super-fast Y sperm may also lead to, say, low testosterone counts or premature prostate problems – or any of a host of other complications).
Further, evidence suggests that the super-fast sperm die quickly (see http://www.sciencedaily.com/releases/2003/10/031014071905.htm ) and would, in fact, pave the way for the slower X-chromosome sperm ( http://www.nytimes.com/2007/06/12/science/12angi.html ). Which is the problem with biology articles from 1967; they can’t incorporate anything we’ve learned since.
Obviously, that doesn’t reduce the main thrust of the argument (just reverse X and Y, so that super-fast X-chromo sperm produces more boys). However, it does mean that, if significantly more boys are in the gene pool, then the females can attract multiple mates. Now, lets say that Femme has sex with Aman, and Aman has super-fast X-chromo sperm. Femme (being female) can have multiple partners, and one of the new entries is Anotherman – who also has super-fast X-chromo sperm (remember, we’re hypothesizing that the environment is selecting for super-fast X-chromo sperm). Anotherman’s sperm take advantage of the portals opened by Aman’s sperm and impregnate Femme – with a girl. Balance returns, since these are not dyadic games (and grow less dyadic the more the balance is disturbed).
In other words, the species is likely to survive. Especially with humans at this point in time, since, if the ratio became too tipped, people would figure out what was happening and would intentionally try to have girls.
July 20, 2011 at 9:40 am
David Gonzales
I have read some interesting stuff about sex ratios and evolution in a couple of blogs recently. In the same vein this might be interesting and relevant (an all female population):
“Stick insects survive one million years without sex”
URL:
http://www.bbc.co.uk/nature/14122050
July 27, 2011 at 3:50 pm
rjd100
I guess stick insects get married.
July 29, 2011 at 11:19 pm
Return to equilibrium ā Evolving Economics
[…] post on Cheap Talk reminded me about an old paper of Bill Hamilton’s on the potential for extraordinary sex […]
August 14, 2011 at 10:36 pm
Anonymous
This also ignores the effects of culture and social programming. I would love to get your take on that.