Stupid question from somebody who’s completely ignorant about biology, but how is it that the gender ratio can stay roughly equal in so many species, especially ones that tend to only produce one or two offspring per mating session? Is there some degree of randomness built in to the process?
If it was skewed one way or the other, wouldn’t they evolve themselves into extinction?
The theory of sex allocation is a major topic in evolutionary biology. Many species have a nearly equal sex ratio; in others it deviates greatly in favor of one sex or the other. There are many mechanisms that may account for these different ratios.
Sorry I don’t have time to get into this in more detail now, but I’ll try to get some links later on. Try searching on “sex allocation theory.”
No, it depends on their mating behavior. and 90% male would be very different than 90% female.
Of course. Each child has a 50-50 chance (or very close to it) of being a boy or a girl. I thought everyone knew that?
If you’re asking why the chance is 50-50 instead of some other ratio, consider what it would happen if it were otherwise. Suppose, for instance, you had a species where two females were born for every one male. That means that each male will, on average, produce twice as many offspring as each female. In such a species, it’s good to be a male. Now, further suppose that some individual in that species has a mutation that causes most of its offspring to be male, instead of mostly female. Now, that individual’s offspring will be predominantly of the variety which is reproductively favored. This is an evolutionary advantage, so that individual will have a lot of descendants, and for so long as the gender ratio is more female, the more-male gene will spread. Eventually, you get back to the situation where the genders are balanced, and you reach an equilibrium.
My guess:
Those species which are unable to produce a male/female ratio that is conducive to species survival go “bye-bye”. They “manage” themselves out of the equation.
Since you’re “Ignorant of biology”, I’ll explain the human side. BTW, you’re asking about sex ratio, not gender ratio.
In humans, women’s sex chromosomes are XX (one X from each of her parents). Males are XY. When the gametes (sperm and eggs) are produced, one cell splits into 4. First, it undergoes mitosis (straight-up cloning), then each of the resulting 2 splits via meosis. The process is the same, except meosis doesn’t double the DNA, so each of the resulting 2 cells (4 total) has half the DNA of the organism. Since the starting cell of the man has an XY, it doubles to XY and XY, then splits up to X, X, Y, Y. Each of these 4 sperm cells has a chance at fertilizing the egg. In women, they start with XX, so they end up with X,X,X, and X. 3 of these shrivel up and die, giving all their nutrients to the fourth one, which is the egg. So the woman contributes an X to her kid, while the man gives either an X or a Y. That’s where the 50/50 comes in, that’s why men determine sex.
Anecdotally, it’s a fallacy that it “only takes one sperm” to impregnate. It takes hundreds of sperm to eat through the egg’s jelly coat, so one can finally slip in there. Saying it takes one is like saying it takes one player to score a touchdown…you still need the rest of the boys!
Not true for crocidile babies.
eta some more examples
It’s kind of like the efficient market hypothesis. If a stock is underpriced, people will buy it in order to make money, which will drive the price up to a price that is roughly fair.
Analogously, if there is a large imbalance in the sex ratio, then it may be an advantage to produce offspring of the rarer sex. So any genetic predisposition to have offspring of that sex will spread around until the imbalance is corrected.
Ah… thanks for the replies. The question I was really trying to ask was how nature managed to maintain the element of uncertainty through generation after generation when so much else seems to be deterministic (e.g. eye color).
If I understand IntelSoldier’s post right, it’s because 50% of sperm cells are X, 50% are Y, and the baby’s sex depends on whichever one happens to fertilize the egg first?
Yes. Boy sperm are a tad faster so babies are slightly more than 50% likely to be male (IIRC it’s 50.1% but don’t quote me). But males are more likely to die young than females (fast cars, wars, risk taking in general) so by the time you get to retirement age there are more females alive than males.
I thought this thread was going to be about how nature “knows” to produce an excess of male babies after a lot of men die, like in post WWII USSR for instance. But I don’t know how well established either of those “facts” are.
There are certainly species that do it differently - there’s a fish where the whole school is female with one male. When the male dies, one of the females becomes a male to take his place.
In mammals, the way that sperm is manufactured tends to favour a 50/50 distribution, but there’s no particular reason why that always has to translate to a 50/50 distribution in the final population - suppose, for example, a mutation arose that made the Y-chromosome sperm, say, on average, 20% less efficient at swimming - that might translate to a comparable shift in sex distribution at birth - and if that happened to be favourable to the overall survival of the species, the mutation would tend to be preserved.
Not all organisms even determine sex in that way anyway - with reptiles, it’s often dependent upon the temperature at which the eggs are incubated - with some other organisms, it’s determined by environment or other factors - including probably the distribution of sex in the existing population - for example slipper limpets cling together in clusters that look like peeled tangerines - the bottom one is always female (and because the larvae tend to attach themselves to the shells of existing slipper limpets, she’s the oldest in the cluster too). All the others in the cluster are male, but when the female dies, the bottom male turns into a female.
(although this might just be a change of function - as gastropods are often hermaphrodites, physiologically)