But isn’t there selection pressure for a two-sex species to evolve into a three-sex-species and so forth? why is the equilibrium 2 instead of 3 or 5 or different for different species? I guess you are saying the force of genetic drift is so strong that it overcomes the force pushing towards more sexes in pretty much every species ever… but since genetic drift is by definition a pretty weak force, I think that means you are saying that the pressure towards more sexes is extremely weak. Why is that? Is it not that beneficial to be able to mate with 100% of the population instead of merely with 50%?
The “random sampling” that causes genetic drift is applied once every generation, asexual or not, so the optimal number of types depends on the ratio of generations that are asexual vs sexual. The Constable & Kokko paper has a mathematical model to quantify how many asexual generations you need for 2 being the optimum, and it turns out that most isogamous species are well into that regime.
That being said, you’re entirely right when you ask “why is the equilibrium 2 instead of 3 or 5 or different for different species?” – Constable’s model and empirical data is only for isogamous species like baker’s yeast. It seems plausible that our isogamous ancestors were in the same regime, and then anisogamy evolved and kind of locked us into a 2-types configuration. But that’s mostly speculation, I don’t think we have any clear empirical data that confirms this hypothesis. That’s still open to investigation.
Another thing I didn’t mention is that the organelle-competition hypothesis naturally leads to 2 types, so it could simply be that.
But isn’t there selection pressure for a two-sex species to evolve into a three-sex-species and so forth? why is the equilibrium 2 instead of 3 or 5 or different for different species? I guess you are saying the force of genetic drift is so strong that it overcomes the force pushing towards more sexes in pretty much every species ever… but since genetic drift is by definition a pretty weak force, I think that means you are saying that the pressure towards more sexes is extremely weak. Why is that? Is it not that beneficial to be able to mate with 100% of the population instead of merely with 50%?
The “random sampling” that causes genetic drift is applied once every generation, asexual or not, so the optimal number of types depends on the ratio of generations that are asexual vs sexual. The Constable & Kokko paper has a mathematical model to quantify how many asexual generations you need for 2 being the optimum, and it turns out that most isogamous species are well into that regime.
That being said, you’re entirely right when you ask “why is the equilibrium 2 instead of 3 or 5 or different for different species?” – Constable’s model and empirical data is only for isogamous species like baker’s yeast. It seems plausible that our isogamous ancestors were in the same regime, and then anisogamy evolved and kind of locked us into a 2-types configuration. But that’s mostly speculation, I don’t think we have any clear empirical data that confirms this hypothesis. That’s still open to investigation.
Another thing I didn’t mention is that the organelle-competition hypothesis naturally leads to 2 types, so it could simply be that.