Why are there equal number of males and female people?

Wiki tells us that

As of 2012, the global sex ratio is approximately 1.01 males to 1 female– the greater number of men is possibly due to the significant gender imbalances evident in the Indian and Chinese populations.[62][63]

Reading a current thread on total human annihilation, I was turned on to the Toba catastrophe theory,, regarding a volcanic event reducing human breeding pairs (just like my wife and me) to 10,000 or even 1000 couples, 69,000 to 77,000 years ago: a so called bottleneck in evolution.

Which led me to mitochondrialEve and YchromosomeAdam (I have no overview of them due to iPad hiccups). Whenever they were (sorry), there were apparently six Evelets for every Adamlet. I sort-of understand the reasoning in a slice of time way. But I got lost reading the population history of their descendants.

Which leads me to my subject header.:slight_smile:

I’m not sure I follow. You seem to be saying that at some point, there were more women than men due to a catastrophe, so why are the sexes now present in equal numbers? I think that’s the question.

The answer is that women, taken as a whole across the population, tend to have roughly equal numbers of male and female children. So in a hypothetical situation where you take one man and a thousand women and put them on a new planet, if you assume the man is up to the task, within one generation you’d be back to (roughly) equal numbers.

I don’t. Where are you getting the claim that at one point there were six women for every man?

I saw what you did there.


Yes, barring some unknown anomaly, the imbalance would correct itself in one generation. I’d be interested in knowing whether there are any very long genealogies with a known propensity for one sex or the other.

That conflicts with this wikipedia page, which asserts that the sex ratio at birth is 107 boys for every 100 girls.

Edit: sex ratios here broken down by age group. Interesting.

See also Fisher’s Principle:

I think that might possibly explain your subject line.

The human sex ratio at birth is not the same as the sex ratio of everybody currently living.

It’s a genetic arms race and the score is kept by who has the most grandchildren.

Every child has a female parent and a male parent, each of whom contribute half the child’s genes.

Let’s say a species of elephants has a genetic propensity towards female births. We’ll use nine females to one male as a hypothetical ratio and say there are a million elephants in each generation - 900,000 females and 100,000 males. And we’ll assume the total population stays at a constant size.

So you have a million elephants in the next generation as well. Now do the math. On average, every male in the previous generation passed his genes on to ten offspring while every female passed her genes on to 1.1 offspring. So male offspring are significantly better at passing genes on to future generations.

But genetics aren’t stable. New genes get introduced by mutation. A new gene M arises that produces more male offspring - elephants with the M gene have 90% male offspring and 10% female offspring.

This is a huge advantage. Male offspring already produce more heirs and now elephants with the M gene produced more male offspring. The first elephant with the M gene will have ten offspring - nine males and one female. Those ten offspring will have ninety-one offspring of their own - eighty-two males and nine females. Those ninety-one offspring will have 830 offspring - 747 males and 83 females. Those 830 offspring will have 7562 offspring - 6806 males and 756 females. And so on. As you can see the M gene explodes into the population.

But it doesn’t have its effect in isolation. The old ratio of 900,000 female births to 100,000 male births is no longer accurate. In just four generations, the M gene has shifted the ratio to 893,950 female births to 106,050 male births.

Now project this trend ahead a few hundred generations. Eventually you’re going to have a population where all the elephants have the M gene and that means future generations of elephants will be 100,000 females and 900,000 males. So it’s now female offspring that have an advantage in passing on genes. The average female elephant will pass her genes on to ten offspring while the average male only passes his genes on to 1.1 offspring. The M gene is no longer an advantage.

Now if we hypothesize a new mutation - an F gene that causes elephants to once again have ten female offspring to every one male offspring, we could watch the whole trend work in reverse. But you can see that neither gene gives a permanent advantage. The only stable long-term situation is where males and females are equal in number and have an equal chance of passing on their genes. No gene can have a long-term advantage based on it being held by the minority because the success of the gene will turn its possessors into the majority.

There is just one mitochondrial Eve, and just one Y chromosome Adam. That’s their definition. They didn’t exist at the same time.

There is born an equal number of male and female children due to the previously mentioned Fisher’s principle.

A.W.F. Edwards has remarked that Fisher’s principle is “probably the most celebrated argument in evolutionary biology”…
Well toast my onions.

Thanks people. But upthread we have one generation doing the trick. Is Little Nemo being more general, or is there a difference of opinion?

They are talking about completely different things.

In a species with an equitable sex ratio, such as humans, any imbalance in sexes will equalise as soon as the original generation dies. In the case of humans that’s going to take just one or two generations. That’s just simple mathematics. If 50% of the offspring born are of each sex, then it doesn’t matter if you start with a billion females and one male. As soon as the billion females die, their offspring will consist of 50% males and 50% females, so the imbalance is corrected in just one generation.

What** Little Nemo** is talking about is the reason why we *have *species with an equitable sex ratio in the first place. That’s a completely separate issue. At first glance it seems as though most organisms should produce far more males than females. Sperm is much cheaper to produce than eggs, and as a successful female has an absolute limit in the number of offspring she can produce, while a successful male can produce thousands of times more offspring. So it would seem that the sex ratio in all species should have evolved to favour males. What Little Nemo was discussing is one of the reasons why this doesn’t occur. It isn’t the primary reason however. The primary reason is simply that as soon as an organism starts starts successfully cheating, it reduces the success of its own descendants by reducing the number of organisms they have to mate with.

What you are talking about here is known in evolutionary biology as Fisher’s Principle, which explains why the sex ratio is 1:1 in most species.

IIRC there is a slighter higher rate of child mortality for boys than girls, which is why the ration isn’t 1:1.

If you’re talking about the different mitochondrial families, they’re due to mutations at different points, not to a 1:6 ratio at one point.

Fisher’s principle totally makes sense, but even if it’s true, it doesn’t change the fact that your typical human has significantly more female ancestors than male ancestors – perhaps twice as many. This observation defies superficial common sense, but when you think about it, it makes a lot of sense. It also explains a lot of the observed differences between human males and human females.

How did you arrive at 2:1?

How did who arrive at 2:1 what?? Nobody said anything about any 2:1 ratio, as far as I can see.


Here’s a video in which a government scientist plans for a similar eventuality. I’ve set it to begin where sexual misbalance is discussed, though the entire lecture is interesting.

I think I read it in this article..