Men, am I right? They're everywhere. But why are there so many of them?
Because strictly in terms of reproductive quantty, we don't need as many as we have. Even early naturalists realised there were way too many men in the human race — far more men than the species needs for maximum efficiency. Not sure why? Take a look at the reproductive capacity of a population of 10 people. Five men and five women can have, approximately, five children per year. Four men and six women can have six children per year. One man and nine women can have nine children per year. When it comes to reproduction, mammalian-style, males are a waste.
So the abundance of men made no sense at all — until Ronald Fisher cam along. Fisher was a mathematician who published multiple papers that translated mathematics into everyday situations, from tea drinking to biology. Fisher noted that evolution doesn't just shape populations, it shapes individuals within populations. Thus if a population is mostly female, with just a few males, it will reproduce faster than a population that has an equal number of males and females.
Within that population, though, each individual is more likely to pass down his or her genes if they produce a male than if they produce a female. The fewer males in a group, the more likely these males are to pass on their genes with a wide variety of female partners.
By equaling out the sexes, humanity has achieved the best combination of reproductive speed and genetic differentiation. Which is why the overall population has a roughly equal number of males and females.
[Source: Know It All, edited by Mick O'Hare]