Huh. I like you spelling out the puzzle. I had the seed of a partial answer but hadn’t noticed how incomplete it was before.
My guess had been that male expendability was a major evolutionary advantage. The fact that males can still have offspring after dying (because they already impregnated a female) means the evolutionary costs of deadly risk-taking drop by quite a bit. It also removes a lot of politics when it comes time to decide who has to risk death to protect the community from some threat. (If everyone’s a hermaphrodite, it becomes a status thing. But if you have males, you obviously bias heavily toward risking losing males. Hence “Women and children first!”)
I hadn’t thought about it before, but it does seem to explain the animal/plant difference. This risk-taking thing isn’t an advantage for plants. But it’s super important for things that move and explore.
But I don’t know how this would have first arisen.
I’m remembering some tickle of a claim that supposedly the human Y chromosome is a mutated X. That being male is a mutation and the default is female. I don’t actually know what that would mean since human females would also have to have mutated (since they can’t impregnate each other). But if there’s a sensible interpretation, that strikes me as probably relevant to this question.
I think the X and Y chromosomes are a bit of a red herring. It’s true that the Y is a degraded X, but in birds, males are ZZ and females are ZW. In both cases, the Y and W chromosomes slowly degrade due to their lack of a partner to do sexual recombination with. Eventually, the Y (and W) chromosome is (by default at least) expected to eventually disappear, but this does not mean males would disappear, instead the XY sex-determination system would evolve to replace the Y (and maybe the X too) with something else.
Sounds like this is a combination of #1 and #5, depending on whether the risks come from the environment or from other males. But I don’t think that “removing the politics” can be a general answer—maybe it’s relevant for humans, but there are plenty of species which don’t live in groups, or don’t sacrifice very much for their groups.
My guess had been that male expendability was a major evolutionary advantage. The fact that males can still have offspring after dying (because they already impregnated a female) means the evolutionary costs of deadly risk-taking drop by quite a bit. It also removes a lot of politics when it comes time to decide who has to risk death to protect the community from some threat. (If everyone’s a hermaphrodite, it becomes a status thing. But if you have males, you obviously bias heavily toward risking losing males. Hence “Women and children first!”)
This explanation seems tailored to a particular ecological niche occupied by humans and very few other animals. It’s interesting and I expect does have some effect on the margin, but for it to work as a general explanation for the existence of sexes:
there have to be interventions where males can save their offspring in particular, not be generically helpful. This requires either a fair amount of physical isolation, or tracking which kids out of many are theirs and only intervening when predators attack those kids in particular, or having an ongoing relationship with the mother that almost certainly implies more paternal care. You can’t have group level protection without strict gatekeeping, or males will freeride off of other males’ protective efforts. I think e.g. chimps could theoretically pull this off via general social intelligence, and elephant seals because they’re a harem species with a lot of physical isolation, but the conditions are not so common as to be an explanation for the general phenomenon of sexual specialization.
They also can’t provide so much parental care that their death renders their offspring nonviable. In birds males can be equal parents post-egg-laying, but species typically do so only when offspring demand so much care a female cannot do it themselves. There are intermediate versions of this- maybe paternal death leads to half their offspring dying from lack of food, but not all.
Huh. I like you spelling out the puzzle. I had the seed of a partial answer but hadn’t noticed how incomplete it was before.
My guess had been that male expendability was a major evolutionary advantage. The fact that males can still have offspring after dying (because they already impregnated a female) means the evolutionary costs of deadly risk-taking drop by quite a bit. It also removes a lot of politics when it comes time to decide who has to risk death to protect the community from some threat. (If everyone’s a hermaphrodite, it becomes a status thing. But if you have males, you obviously bias heavily toward risking losing males. Hence “Women and children first!”)
I hadn’t thought about it before, but it does seem to explain the animal/plant difference. This risk-taking thing isn’t an advantage for plants. But it’s super important for things that move and explore.
But I don’t know how this would have first arisen.
I’m remembering some tickle of a claim that supposedly the human Y chromosome is a mutated X. That being male is a mutation and the default is female. I don’t actually know what that would mean since human females would also have to have mutated (since they can’t impregnate each other). But if there’s a sensible interpretation, that strikes me as probably relevant to this question.
I think the X and Y chromosomes are a bit of a red herring. It’s true that the Y is a degraded X, but in birds, males are ZZ and females are ZW. In both cases, the Y and W chromosomes slowly degrade due to their lack of a partner to do sexual recombination with. Eventually, the Y (and W) chromosome is (by default at least) expected to eventually disappear, but this does not mean males would disappear, instead the XY sex-determination system would evolve to replace the Y (and maybe the X too) with something else.
Right!
IIRC this was also be related/causes to the fact that men suffer more genetic diseases (hemophilia? red-green colorblindness?)
Sounds like this is a combination of #1 and #5, depending on whether the risks come from the environment or from other males. But I don’t think that “removing the politics” can be a general answer—maybe it’s relevant for humans, but there are plenty of species which don’t live in groups, or don’t sacrifice very much for their groups.
This explanation seems tailored to a particular ecological niche occupied by humans and very few other animals. It’s interesting and I expect does have some effect on the margin, but for it to work as a general explanation for the existence of sexes:
there have to be interventions where males can save their offspring in particular, not be generically helpful. This requires either a fair amount of physical isolation, or tracking which kids out of many are theirs and only intervening when predators attack those kids in particular, or having an ongoing relationship with the mother that almost certainly implies more paternal care. You can’t have group level protection without strict gatekeeping, or males will freeride off of other males’ protective efforts. I think e.g. chimps could theoretically pull this off via general social intelligence, and elephant seals because they’re a harem species with a lot of physical isolation, but the conditions are not so common as to be an explanation for the general phenomenon of sexual specialization.
They also can’t provide so much parental care that their death renders their offspring nonviable. In birds males can be equal parents post-egg-laying, but species typically do so only when offspring demand so much care a female cannot do it themselves. There are intermediate versions of this- maybe paternal death leads to half their offspring dying from lack of food, but not all.