Shouldn't there be more males than females? What am I forgetting about evolution? (cf breeders)

Factual Questions
1

Chain of thought: Kentucky Derby horse sires are a jackpot that keeps on giving, Kentucky Derby dams not so much.

Males have the DNA goods almost always ready to go for most of their lives. Enough for bazillions of matings. Females have that whole time and space limit for their fecundacity (can’t think of the word).

I understand the genetics/phenotype would get seriously debilitated with such a skew. I get that, and our ethology does just fine, thank you, this way, and although I wouldn’t bet on it, the proof is in the pudding of all animals that do the 50%-50% m/f thing.

So God/evolution doesn’t/didn’t operate they way the Kentucky Derby horse breeder does. Where did I miss a step?

2

The essential factor is that in any bi-gender species, you need one male and one female to pass on a set of genes. So if you had a species where males significantly outnumbered females, it would be difficult for males to pass on their genes and easy for females to pass on their genes. This would create a genetic advantage for individuals who develop genes that lead to female births; their genes would get passed on more readily. But as the female birth genes became more common, they would lose their advantage. If the female birth genes became too common, females would eventually outnumber males and it would reverse the original gender advantage. This would create an opportunity where male birth genes would become a genetic advantage. In the long run, a balance is reached where the two genders are approximately equal.

3

Nature often does work that way. Look at all the animals that practice polygyny. All those alpha males that manage to have a harem are your Kentucky Derby sires. The other males are filtered out. And that filtering out constitutes the bulk of the males, just as in the races.

4

Follow the argument to its conclusion.

Imagine that there were 9 males born for every 1 female. While it’s true that each male can reload quickly, and therefore in principle can impregnate lots and lots of females… there’s shitload of other males out there who would like to do the same. It’s not possible for every single male to impregnate the entire female population. One male might manage it, but that must necessarily mean that the other males do not have mates. Including that male’s numerous brothers. Success for the first male means failure for every other male.

Their DNA is not passed on.

So this species is, what? Putting out 9 males for every 1 female. The one female is pretty much guaranteed to breed, but the male babies are, statistically speaking, very likely to be a complete fucking waste. Too much competition from other males. Now mutation enters the picture. Imagine that a female starts giving birth to more females, and fewer males. Closer to the 50/50 ratio, rather than 9-to-1. Those females will breed. The grandmother who gives birth closer to the even ratio will have many, many, many more grandchildren than the other individuals in the species. And in that way, the 50-50 ratio will quickly spread through the population.

As long as the biological cost of creating male progeny and female progeny is roughly the same, the children of the species are going to be roughly a 50-50 ratio. The species is not going to tilt heavily female over male, either, for an exactly symmetrical reason. (Species like bees have different breeding habits, which is why they can sustain different sex ratios of their offspring. But the argument above holds for the sorts of animals close to us on the evolutionary tree.)

The parallel question was more often asked in evolutionary history. Why are there not a huge number of females, and only a few males? The species as a whole would be more likely to multiply in that case. Females can’t reload as quickly, so if the species had a lot more females, that species could increase in population much more quickly. But the answer to that is exactly the same: if males have next to zero competition, and can copulate on average with 10 or 20 females without any danger, than those males will have many, many offspring. Rather than sustaining a 9-1 ratio in favor of females, any mutation that levels out the sex ratio is going to produce more males, and those males are going to succeed wildly in breeding, and the more even sex ratio is going to burn quickly through the population until it’s much closer to 50-50.

Evolution is not about the survival of the group. It’s about the survival of the individual, sometimes even at the expense of the group.

5

Wild horse sex ratio is something like 43/57 in favor of females, according to a bit of Googling. If you are thinking that males should be overwhelmingly favored just because they can reproduce anytime, consider that about half of the father’s sperm actually codes for girl horses. (The mother always contributes an X chromosome, the father has roughly even chances of contributing an X or Y chromosome).

As to why the racetrack population seems skewed, it’s nothing to do with the birth rate, it’s just the fact that the faster horses tend to be male, so breeders send more males to the track.

6

You can play the evolution game.

Get a bunch of index cards and a twenty sided dice.

Take a hundred cards. Mark ten of them as female and ninety of them as male. Now write down the “genes” numbered 1 to 200. Write two genes on each card. Make a list of the twenty genes that are on the ten female cards. Separate the cards into one deck of female cards and one deck of male cards.

Now create a new generation. Get a bunch of blank cards. Shuffle the male deck and get a father. Shuffle the female deck and get a mother. Take a blank card and write down one of the numbers from each card on the blank card (these are the genes each parent passed on to the child). Now roll the dice; if it’s a 1 or 2, make the new card a female. Otherwise make it a male. Shuffle the father and mother cards back into their decks. Put the new son and daughter cards in their own decks. Repeat this a hundred times to create a new generation.

When you’ve completed the new generation of a hundred offspring, discard the original generation. Repeat the procedure with the second generation cards creating a third generation. Do this for a few generations.

Now look at the cards in the last generation. Make a list of the two hundred numbers (genes) on these cards. Compare them to the list of genes the original ten females had. You should be able to graphically see the effect of gender imbalance on gene transmission.

7

That’s hot.

8

Fisher’s Principle states the basic dynamics at work. W.D. Hamilton described it this way:

9

What happens when large numbers of female babies are killed off?

This happened in China starting in 1979 and continued until 2015.

What effect did 35 years of killing large numbers of female babies have on the present day ratio of male and females in china? How long will it take to “replace” these females?

10

A long way to go yet it seems.

11

And:

12

My 10th-grade biology teacher taught us that the slightly higher birthrate for male humans (something like 1.01-1.08-to-1, according to Wikipedia, when not affected by sex-selective abortion etc.) was due to X-sperm being heavier (hence slower) than Y-sperm. An X-chromosome does have a fourth “arm,” looking at the chromosomes’ shapes, so I guess it would be about 25% heavier, though I don’t know how much that would affect the entire sperm cell’s mass.

Googling around, this doesn’t seem to be anything like established science. Another lie my teacher told me ;). (Or she was ahead of her time.)

“Differential motility” is cited as one possible explanation in this article, which also finds it statistically unlikely that the sex ratio is due to different levels of production of X and Y-bearing sperm cells.

Anyway, as noted above, the Fisher Principle does explain why the sex ratio is close to 1:1, but the jury seems to be out on what causes the small but consistent pro-male bias in live births.

13

As is frequently the case, I find it hard to understand Leo Bloom’s post. Reading between the lines and placing heavy emphasis on the thread title, I think he’s questioning why there are approximately the same number of men and women, when men can have many offspring but women are more limited in the number of offspring they can have.

This argument seems a complete non sequitur. Each child born is (approximately) equally likely to be male vs. female. It’s not like, say, brown hair, where a person with brown hair is more likely to have a child with brown hair than some other color. Surely you’re not suggesting that men are more likely to have male children and women are more likely to have female children? That makes no sense. Every child has a male parent and a female parent.

14

The same thing, probably. Males are less likely to make it to adulthood than females, therefore slightly more males are born to make up the difference.

By the same token a male embryo is less likely to successfully make through pregnancy, therefore more males are* conceived* than females to even out the numbers.

15

I think the OP may be somewhat misled by his analogy with horse breeders making more money off winning stallions than winning mares. Yes, the reason for that is that stallions can be bred much more easily and often than mares. But why it’s profitable in the first place is that there’s an effectively infinite number of **non-**champion horses seeking to mate with each champion.

Consequently, the much greater fecundity of males is an advantage from the point of view of breeders who control and profit from the mating of a tiny elite subset of the horse population. But that doesn’t mean it would make sense for the horse population as a whole to have more males than females.

16

According to my Chinese friends it stills happens today.
There is still a great preference for baby boys in China.

Revealing the sex of the fetus is illegal is China, due to selective abortions.
But, a small bribe will get you the results of a sonogram.

17

I, too, don’t understand the source of the OP’s question. If we approach this completely naively, one would expect there to be many more females than males since one male can impregnate many females. Imagine the extreme case:

A population of 50 males and 1 female, and in which that ratio tends to equilibrate.

A population of 50 females and 1 male, and in which that ratio tends to equilibrate.
Which population is going to grow the fastest?

18

Ad hominem static aside, thank you for your posts. Little Nemo right off the bat gave what Colibri cited, and the Wiki process is exactly Nemo’s the other way around. Helestal Evolution is not about the survival of the group. It’s about the survival of the individual, sometimes even at the expense of the group." is profound.

19

Depends on what you mean by “replace.”

This artificial sex-selection is mostly not heritable. Each day the same number of male & female conceptions occur. And will continue to occur tomorrow and into the future. So the very day people stop sex-selective abortion/infanticide the birth ratio will snap to the normal (approximately) 50/50.
If by “replace” you mean “When will China’s total population be 50/50 (or at least the same skew as other advanced countries)?” the answer becomes roughly when the last of the people born in the sex-selecting era have died and every Chinese alive was born in the non-sex-selecting era.
If by “replace” you mean “When will China’s total population be what it would have been had sex-selection never happened?”, well that’s a tall order to guess. What else is changing over time? Sex-selection pre-dated the one-child policy, but was certainly exacerbated by it. So when is our baseline and what do we want to assume about the population trajectory every year thereafter? In any event it’s clear we’re talking multiple generations to make up for the losses.

During those multiple generations we’re creating not only more females but also more males. So does that constitute “replacing” the females? Or do we need to have a corresponding cull of males so the total number of, say, Chinese who lived to age 2 in the last 200 years is/was the same for males as it is/was for females?

20

There was a puzzle in one of Martin Gardner’s columns, I think, that proposed a society where a couple is required by law to stop having children after the first girl. They can have any number of boys, but as soon as they have a girl they must stop having children. The question is how does this affect the sex ratio of the society in the long run.

The answer, somewhat non-intuitively, is that it will have no effect. The sex ratio will remain at 1:1 despite the law, since every child born has an equal chance of being a boy or girl.