Some thoughts I had today on how sex determination can be a bottleneck for a species to evolve into a new niche rather than go extinct:
Temperature dependent sex determination in turtles (cold: more males, warm: more females) and crocodiles (more females if either too hot or too cold) seemed strictly worse than using a sex chromosome or anything else to determine sex. I thought the reason they do it might simply be that it was easy to develop millions of years ago, and most families who developed this way got extinct. Turtles and crocodiles happen to be the two that have made it this far. Then I remembered that both are cold-blooded, and their lifestyle is very dependent on temperature in any case. So if the climate is moving out of the optimal temperature for the species, it might make sense for you to produce more females, because they are a lower risk strategy? Not sure if that argument goes through. (Edit: No, it doesn't make sense for the individual and is selected against, but it does lead to a higher likelihood the species doesn't become extinct. Thanks to Dacyn for pointing out the above was confused!). In Crocodiles, that would make sense, but turtles produce few females when it is cold? That sounds suboptimal for the individual and worse than using sex chromosomes.
Regarding eusocial animals: Are ants more widespread and have more diverse strategies than termites, because ants are haploid-diploid and termites are diploid-diploid or is that a coincidence? In ants, male and female mate and then the female queen goes off to found a colony. In termites, both go off together, and they also keep mating together. This difference might be explained by sex chromosomes.
The idea: Haploid animals are sensitive to issues from deleterious mutations, so having the queen cooperate with the male is not worth the cost, because males are likely to be incompetent even if their genes are mostly fine. Instead, the queen keeps storing the sperm, even though that's worse sperm quality than if you went the termite way. Since ant queens found new colonies alone, it's easier for ants to adapt to new environments, which explains why we see ants in all kinds of niches, while termites mostly digest cellulose. In general, I'd expect to see haploid-diploid sex selection mostly in r-selected species and not in K-selected species (googling: seems to be true). For r-selected species like insects, I am unsure if being haploid-diploid is better than having a sex chromosome, unless you stumble into eusociality. Seems worse?
Haplodiploidy in hymenopterans (ants and wasps) has a huge impact on eusociality: All daughters of the queen get half their genes from their father's entire genome (the father's sperm are all identical), which means they share 75% of their genes with their sisters rather than the 50% in others like termites. For this reason, Eusociality has evolved independently multiple times in hymenopterans.
I would speculate that the colony is more cohesive for this reason, and this may have an impact on why ants are so successful in so many niches.
Termites, on the other hand, have one key thing going for them: the ability to digest wood. This allows them to fill their specific niche well.
I thought about this a bunch of years ago, and I am not quite sure how much this matters. Often ants get sperm from multiple fathers, and then they'd be less related than siblings (although maybe they only start doing that once they are out of the unstable intermediate period between being not eusocial and being eusocial). Also, this leads to colonies producing fewer males than would be game theoretically optimal for the queen and to conflict between queen and workers (50/50 would be optimal), although that might be helpful on the species level? So I think... (read more)
Some thoughts I had today on how sex determination can be a bottleneck for a species to evolve into a new niche rather than go extinct:
Temperature dependent sex determination in turtles (cold: more males, warm: more females) and crocodiles (more females if either too hot or too cold) seemed strictly worse than using a sex chromosome or anything else to determine sex. I thought the reason they do it might simply be that it was easy to develop millions of years ago, and most families who developed this way got extinct. Turtles and crocodiles happen to be the two that have made it this far. Then I remembered that both are cold-blooded, and their lifestyle is very dependent on temperature in any case. So if the climate is moving out of the optimal temperature for the species,
it might make sense for you to produce more females, because they are a lower risk strategy? Not sure if that argument goes through.(Edit: No, it doesn't make sense for the individual and is selected against, but it does lead to a higher likelihood the species doesn't become extinct. Thanks to Dacyn for pointing out the above was confused!). In Crocodiles, that would make sense, but turtles produce few females when it is cold? That sounds suboptimal for the individual and worse than using sex chromosomes.Regarding eusocial animals: Are ants more widespread and have more diverse strategies than termites, because ants are haploid-diploid and termites are diploid-diploid or is that a coincidence? In ants, male and female mate and then the female queen goes off to found a colony. In termites, both go off together, and they also keep mating together. This difference might be explained by sex chromosomes. The idea: Haploid animals are sensitive to issues from deleterious mutations, so having the queen cooperate with the male is not worth the cost, because males are likely to be incompetent even if their genes are mostly fine. Instead, the queen keeps storing the sperm, even though that's worse sperm quality than if you went the termite way. Since ant queens found new colonies alone, it's easier for ants to adapt to new environments, which explains why we see ants in all kinds of niches, while termites mostly digest cellulose. In general, I'd expect to see haploid-diploid sex selection mostly in r-selected species and not in K-selected species (googling: seems to be true). For r-selected species like insects, I am unsure if being haploid-diploid is better than having a sex chromosome, unless you stumble into eusociality. Seems worse?