Evolution is all about reproduction, right? It’s like…the whole point. So try to explain this: Male elephant seals are pretty unlucky in love. Most never get a chance to reproduce. Just four percent of males can account for eighty-eight percent of mating. The other 96% of male elephant seals just use up resources and don’t contribute anything to the next generation. Yet even though it would be a lot more efficient to have just a few males, and a bunch of females, roughly equal numbers of each sex are born every season. We see this in almost every animal species on Earth. Why do most animals, from eagles to armadillos to blue whales to us, make equal numbers of males and females?
INTRO The equal ratio of sexes is so common in nature, most people never wonder why it is that way. And it’s worth pausing here for a second to make clear we’re talking about biological sex, not gender. You probably learned that when male (XY) and female (XX) chromosomes shuffle and combine, there’s a 50% probability of either sex. But this just tells us how it IS, not why it is this way rather than some other way, and in biology that’s really what we want to know. If getting your genes from one generation to the next is the whole point of evolution, then you’d think the best strategy would be to put a lot of eggs into the female basket. Sperm are really cheap to produce, so males make…a lot of them, and animal males typically don’t do most of the childcare. So you get a situation in which you only need a few males to keep a population churning along.
This seems like the ideal situation not just for elephant seals, but for many animal species. Yet elephant seals produce both sexes in equal number, and the result is a whole lot of mooching mateless males. As far as evolutionary strategies go, this one seems like a loser. It’s not survival of the moochiest! It’s survival of the fittest. Fittest, in biology, means makes-the-most-offspring, and 96% of elephant seal males get a big zero for fitness! That’s totally bonkers! So how can we explain it? Let’s start with a hypothetical population of purple people-eaters, in which out of every 10 purple-people-eating babies born, 9 are female and 1 is a male.
When it comes to making adorable people-eating babies, every male will get to mate about nine times as often as every female. If every mating results in two new purple people-eaters, then in the next generation, each male’s genes are carried on by 18 offspring, but each female–on average–gets their genes into just 2. When we count up the “fitness” score, every male in this population has 9 times the fitness of every female. If, through the random chance of evolution, one muncher manages to change in a way that it produces say, 9 male kids for every female kid, those offspring would have a huge advantage. WIth this genetic change, one of these new males would produce roughly four and a half times more grandchildren than a male without the change.
This new mutation will spread like gangbusters and in a few generations, you’d have more males than females in the whole purple people eater population. But at this point, the mutation causing 90% male offspring would no longer be an advantage. The environment has changed, and now females are the rarer sex. Now, any parent with a mutation that results in extra FEMALE offspring would be favored. Then THAT mutation would spread until the sex ratio moves BACK towards more females. Repeat this whole process and we move back towards more males…then more females… then more males… The population would be in a constant see-saw towards one sex or the other until, finally, a mutation arises that results in an equal ratio of offspring.
That magic 50:50 sex ratio is the only one that is evolutionarily stable… and that, right there, is why we see it again and again and again… Of course, the 50:50 ratio isn’t just about numbers of offspring. It’s really about how much time and resources parents invest in offspring. We see 50:50 sex ratios whenever parents invest the equally in their male and female children. If investment is unequal, like, if females require twice the parental care of males, you’d expect to see twice as many males born than females. Sure, here each male has less chance to pass on their genes, but they cost half as much as females, so it evens out. Australian brushtail possum moms, for example, invest more resources in their daughters, so when these resources are scarce, they have more male offspring.
Like everything in biology, there are lots of weird exceptions to this, but it’s amazing how universal the 50:50 rule actually is. You see it in species where parents care for their babies and where they don’t, when they have many partners, or just one. You even see it in species where one sex dies a lot more than the other. It shows us the incredible power of natural selection to shape the characteristics of a population.