by This post is unimportant, so only read it if you really want to waste time.
Following is a hypothetical conversation, although it has roots in an actual conversation I had with a friend on 28 Sept 2012. Most of it is speculation that I have not confirmed with people who know what they're talking about, so don't take it too seriously.
A: The long pole in producing offspring is the 9 months of female pregnancy. During those 9 months, males sit around idle, except for providing resources. If a species as a whole could program its genome to maximize offspring, presumably it would have almost all females and just a few males.
B: Yes, but each organism is selfish. Females can only have a bounded number B of offspring -- say B = 20 or however many they can fit into their fertile lifetime. The best males can have almost unboundedly many offspring -- even hundreds or thousands in rare cases. So for any given animal, it might be better to have a male.
A: Okay, but then why aren't there mostly males? Even if there were only, say, f = 25% females in the population, and m = 75% males, parents that were really good at having males could still do better to have another male if it could have more than B kids.
B: Yes, but on average across the whole population, that won't be true. With only 25% females, on average, each male has only (25/75)*B kids. So there's pressure to have more females. In fact, (f/m)*B < B whenever f < m, so this pressure continues until f = m.
A: And then when f > m, each organism wants to cheat by having more males, because the males have an expected (f/m)*B kids, while the females have only B kids, so there's downward pressure until f = m once more.
B: Exactly.
A: Okay, but why are we only looking at averages? Maybe some parents are really good at having successful males -- e.g., because the parents are very masculine. Why won't they have more than 50% males?
B: Maybe, but then they'll come to dominate the population with those better-male traits, and once those traits spread population-wide, they'll no longer confer advantage. Then the parents will no longer be able to have better-than-average males. Those who have more females will then have the adaptive advantage, and the population will return to the 50-50 equilibrium.
A: This suggests that rather than seeing a stable 50-50 equilibrium, we should sometimes see bursts when species have lots of males, followed by eventual return to having equally many females. Do we observe this in practice?
B: Dunno.
A: Hmm, I see. Ok, another possibility: What if different parents specialized in different genders? Say some parents were really good at having good males, so they had mostly males. Other parents were really good at having fertile females, so they had mostly females. Why couldn't this kind of specialization by absolute advantage happen? You could have specialization without it spreading population-wide. The population could split into two halves, with some parents being better at each gender.
B: Suppose it did happen. Eventually you'd have most of the females born to the parents who are good at having females and most of the males born to the parents that are good at having males. But then those two types of children would have to start mating with each other, and once they do, the good-at-having-female genes would get mushed together with the good-at-having-male genes. Maybe they'd cancel each other out.
A: Yeah, what happens when someone good at males mates with someone good at females? Which gender does the child turn out to be? If it's not 50-50, then that couple will be at a disadvantage in the long run.
B: Still, it seems like you could have this kind of specialization latent. That is, if someone good at males mates with someone good at females, the result is 50-50, but if two people both good at males mate, they have mostly males, and if two people mostly female mate, they have mostly females. If the ratios of those good at males vs. good at females remained 50-50, then specialization would never start hurting you. But if this were the case, we should expect to see 25% of all couples having all males and 25% having all females, but we don't.
A: So probably this kind of specialization never happened in the first place.
B: Yep, or else there's something else wrong with the logic.
Following is a hypothetical conversation, although it has roots in an actual conversation I had with a friend on 28 Sept 2012. Most of it is speculation that I have not confirmed with people who know what they're talking about, so don't take it too seriously.
A: The long pole in producing offspring is the 9 months of female pregnancy. During those 9 months, males sit around idle, except for providing resources. If a species as a whole could program its genome to maximize offspring, presumably it would have almost all females and just a few males.
B: Yes, but each organism is selfish. Females can only have a bounded number B of offspring -- say B = 20 or however many they can fit into their fertile lifetime. The best males can have almost unboundedly many offspring -- even hundreds or thousands in rare cases. So for any given animal, it might be better to have a male.
A: Okay, but then why aren't there mostly males? Even if there were only, say, f = 25% females in the population, and m = 75% males, parents that were really good at having males could still do better to have another male if it could have more than B kids.
B: Yes, but on average across the whole population, that won't be true. With only 25% females, on average, each male has only (25/75)*B kids. So there's pressure to have more females. In fact, (f/m)*B < B whenever f < m, so this pressure continues until f = m.
A: And then when f > m, each organism wants to cheat by having more males, because the males have an expected (f/m)*B kids, while the females have only B kids, so there's downward pressure until f = m once more.
B: Exactly.
A: Okay, but why are we only looking at averages? Maybe some parents are really good at having successful males -- e.g., because the parents are very masculine. Why won't they have more than 50% males?
B: Maybe, but then they'll come to dominate the population with those better-male traits, and once those traits spread population-wide, they'll no longer confer advantage. Then the parents will no longer be able to have better-than-average males. Those who have more females will then have the adaptive advantage, and the population will return to the 50-50 equilibrium.
A: This suggests that rather than seeing a stable 50-50 equilibrium, we should sometimes see bursts when species have lots of males, followed by eventual return to having equally many females. Do we observe this in practice?
B: Dunno.
A: Hmm, I see. Ok, another possibility: What if different parents specialized in different genders? Say some parents were really good at having good males, so they had mostly males. Other parents were really good at having fertile females, so they had mostly females. Why couldn't this kind of specialization by absolute advantage happen? You could have specialization without it spreading population-wide. The population could split into two halves, with some parents being better at each gender.
B: Suppose it did happen. Eventually you'd have most of the females born to the parents who are good at having females and most of the males born to the parents that are good at having males. But then those two types of children would have to start mating with each other, and once they do, the good-at-having-female genes would get mushed together with the good-at-having-male genes. Maybe they'd cancel each other out.
A: Yeah, what happens when someone good at males mates with someone good at females? Which gender does the child turn out to be? If it's not 50-50, then that couple will be at a disadvantage in the long run.
B: Still, it seems like you could have this kind of specialization latent. That is, if someone good at males mates with someone good at females, the result is 50-50, but if two people both good at males mate, they have mostly males, and if two people mostly female mate, they have mostly females. If the ratios of those good at males vs. good at females remained 50-50, then specialization would never start hurting you. But if this were the case, we should expect to see 25% of all couples having all males and 25% having all females, but we don't.
A: So probably this kind of specialization never happened in the first place.
B: Yep, or else there's something else wrong with the logic.
