Naked Science Forum
Life Sciences => Plant Sciences, Zoology & Evolution => Topic started by: dentstudent on 09/02/2009 13:48:56
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Humans produce a roughly 50/50 split of boys to girls, yes? Does this remain true in other animals? My thoughts are that in humans, there is no evolutionary benefit to producing more of one sex than another, since there is no group heirarchy, ie a dominant sexual male. But in other social structures where there is a dominant male such as lions, gorillas, walrusses etc, there could be a concieved (ahaha) benefit to having more females born than males. Is this true? Does this actually happen? If so, how does the reproduction work that results in more females being born?
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IF there was a massive death toll in the world it would be more advantageous to have as many females as possible be birthing babies. So I'm not sure your premise that there is no evolutionary benefit to one sex over the other holds. However.. not everything has to be about evolutionary benefits.
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OK, but irrespective of the reasons why, in general there is about a 50/50 split in humans, at least at birth. Does this hold for other species?
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Apart form some local pressures, I would think that mammals at least have a similar pattern.
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I remember a particularly good movie clip by Dawkins in which he quoted the 50/50 ratio for nearly all animals and explained a very plausible reason for it. Anyone remember the link?
(I guess this doesn't apply to animals which can choose the sex of their offspring - like bees.)
It was a 'selfish gene' type of argument which said "What would a mother produce, if she could choose, to maximise the probablity that her genes would survive?"
All females would produce a low output. All males could mean that, if none of the males becomes a dominant male, then none of her genes would survive. The optimum is a mix of about half and half male and female. The sums show this, apparently.
I'm sure there are some creationists who would have their own ideas about it but, on what grounds?
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Imagine that some odd gene gives rise to offspring that are mainly male and is, for whatever reason, common. That means most individuals will be males.
If you had children that were mainly female they wouldn't have to compete in any way for mates, so your reproductive success would be higher. So your genes with the (more female than male) outcome would be favoured. Pretty soon your offspring would come to dominate the scene and most individuals would be female, but then imagine what happens if someone turns up with the original "mainly male" gene.
If there were a relative shortage of one sex or the other then it would be an evolutionary advantage to have children of that sex. This would restore the balance.
For most animals 50:50 is the optimum.
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If you had children that were mainly female they wouldn't have to compete in any way for mates
But that isn't the whole story. With only females, you can't expect many offspring to have your genes because the breeding cycle is so long (one or two per year for each female). You have such a limited impact on the population, albeit a reliable one. The other females in your pride will have more success with their genes than you will if they have some males.
You need a two way bet to optimise your chances by also taking the long shot which the male offspring represent.