Published on July 30, 2025 8:27 AM GMT
Epistemic status: Some armchair reasoning about evolution. I might be overlooking basic facts about reptiles and termites below. I know more about ants.
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. 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?
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