Re: Why is there a balance between the male and female population?


"Steven J." <steven_j@xxxxxxxxxxxxx> wrote in message
news:1194765924.481116.202590@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
On Nov 11, 12:06 am, "Steve O" <hoo...@xxxxxxxxxxxxxxxxx> wrote:
Please excuse a question from a layman, but I'm interested in knowing why
the current human population is split between males and females almost
50/50.
In the human species, wouldn't it be an evolutionary advantage to have
lots
of females and a few men, similar to a pride of lions?

Natural selection doesn't care about the good of the species. Okay,
strictly speaking, natural selection is not the sort of thing that can
care about anything, any more than gravity or density is, but the
point is, if a trait better enables the individual who bears it to
pass on his (or her) genes, that trait will become more common,
regardless of its effects on the species as a whole.

Eventually, I realized that,(dumb, I know) which is why I then started
thinking that if it is easier for a man to pass on his genes than it is for
a woman, then evolutionarily speaking, why aren't there more men?
Which THEN led me to thinking that if the situation favoured more men and
less women, then male reproduction would be further restricted by the lack
of availability of women, which would in turn favour women, then my poor
brain started to frazzle simply thinking about the conundrum. ;-(

Maybe I should do less thinking.heh.


Suppose we start out with a situation where, say, four girls are born
for every boy. For this analysis, we will ignore complications such
as childhood deaths, differential death rates for males and females,
adultery, lifelong celibacy, infertility, and such, and assume
everyone marries and has children. In such a situation, the average
man, assuming replacement level reproduction, will have five children,
while each of his four wives has an average of 1.25 children.

Why would each wife have less than 2 children?

Therefore, a son will, on average, give you four times as many
grandchildren as a daughter. So any mutations that make you more
likely to produce male offspring than female will have a very high
fitness, assuming they have no other, antagonistic effects. As long
as females outnumber males, male offspring will be more valuable *from
the standpoint of the individual's genes*. Obviously, this will
reverse as soon as males start to outnumber females. So no matter
what is good for the species as a whole, natural selection will favor
a rough balance of the sexes.

Mmmmmmokay.
But could you explain that as if you were talking to Homer Simpson?
;-)
I still don't see why women would be limited to 1.25 children.


Note that the above makes a number of tacit assumptions, which don't
hold for all species. Boys and girls are about equally expensive to
raise. If one sex is much more expensive to raise than the other,
this will factor into which sex it pays to have more of. Humans tend
not to mate with very close kin, as do many other species. In species
where offspring of the same parents mate with each other, then
daughters do outnumber sons, since there is no advantage to producing
more sons if they're going to end up competing for the same very
limited pool of mates (and since the winner will mate with all of them
anyway).

Aren't females more valuable, evolutionarily speaking?
If so, shouldn't there be more of them? (Maybe I wish there was!)

-- Steven J.



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