The number of generations controls how long your experiment lasts. The longer (or more generations), the more drift you have, so the more likely for a given gene (or in this case—genders number) to take over. This effect will be weaker in larger populations, but unless you have an infinite population, given enough time (or generations), you’ll end up with the 2 sexes (except for fungi, of course, as always). Eukaryotes first appeared 2.2 billion years ago. For comparison, the Cambrian explosion, with the first complex life, was only ~500 million years ago. That’s a lot of time (or generations) for things to stabilize.
There are multiple mating types around. Mammals have the XY/XX chromosome thing going. Birds have a different chromosome set (denoted as ZW/ZZ). Some families use egg temperature to determine sex. Some fish have one male, and if it disappears, the next ranking individual becomes the male. Insects also have totally different mechanisms. But there are usually only the two sexes (apart from fungi), probably for the efficiency reasons outlined in the OP.
I believe there are also single-celled eukaryotes which have more than two mating types.
I think the key is that you have to have a system where a third mating type makes sense. Having fallen into the basin of attraction of anisogamy, and then later sexual differentiation of reproductive anatomy, it’s much harder to develop a new sex that could reproduce with existing males and females (but not itself).
The way the fungal system that leads to the claim of over 20,000 mating types for Schizophyllum commune is similar to how our pheremones (purportedly?) work; you just want to find someone who smells different (i.e., has a different set of MHC) from you, and there are many ways to have different MHC combinations. If someone develops a new one—good! They smell different than everyone (except their own children) and so they never end up stuck with a distantly related potential mate who nevertheless smells too much like them, and this improves their reproductive success until the new MHC is widespread in the genepool. Additionally (in the case of MHC but not mating types) there is purportedly actual immune benefits which drive this, in addition to the generally beneficial encouragement of “out-crossing”.
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.
The number of generations controls how long your experiment lasts. The longer (or more generations), the more drift you have, so the more likely for a given gene (or in this case—genders number) to take over. This effect will be weaker in larger populations, but unless you have an infinite population, given enough time (or generations), you’ll end up with the 2 sexes (except for fungi, of course, as always). Eukaryotes first appeared 2.2 billion years ago. For comparison, the Cambrian explosion, with the first complex life, was only ~500 million years ago. That’s a lot of time (or generations) for things to stabilize.
There are multiple mating types around. Mammals have the XY/XX chromosome thing going. Birds have a different chromosome set (denoted as ZW/ZZ). Some families use egg temperature to determine sex. Some fish have one male, and if it disappears, the next ranking individual becomes the male. Insects also have totally different mechanisms. But there are usually only the two sexes (apart from fungi), probably for the efficiency reasons outlined in the OP.
I believe there are also single-celled eukaryotes which have more than two mating types.
I think the key is that you have to have a system where a third mating type makes sense. Having fallen into the basin of attraction of anisogamy, and then later sexual differentiation of reproductive anatomy, it’s much harder to develop a new sex that could reproduce with existing males and females (but not itself).
The way the fungal system that leads to the claim of over 20,000 mating types for Schizophyllum commune is similar to how our pheremones (purportedly?) work; you just want to find someone who smells different (i.e., has a different set of MHC) from you, and there are many ways to have different MHC combinations. If someone develops a new one—good! They smell different than everyone (except their own children) and so they never end up stuck with a distantly related potential mate who nevertheless smells too much like them, and this improves their reproductive success until the new MHC is widespread in the genepool. Additionally (in the case of MHC but not mating types) there is purportedly actual immune benefits which drive this, in addition to the generally beneficial encouragement of “out-crossing”.
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.