As you add mating types, the number of individuals of each type becomes increasingly small, and it’s increasingly likely that all individuals of a type will be lost to random sampling.
In practice, the number of mating types depends on how many generations of asexual reproduction happen between two sex events, as this governs the relative importance of genetic drift compared to the benefits of an extra mating type. As it turns out, our unicellular ancestors probably did a lot of asexual generations between two mating events, so the number of mating types was pushed to the minimum of two. (
I am confused. If I think “Genetic drift means that many-typed populations tend to collapse towards fewer types as some types randomly die off,” which is putting your hypothesis in my words, I feel like this predicts that we should see some sort of distribution over N-typed species—with a bunch having 2 genders, a smaller bunch having 3, a smaller bunch having 4, etc. But instead it seems like they almost all have 2. Which I suppose is what you’d expect if the genetic drift effect was VERY strong. But it doesn’t seem plausible to me that it would be that strong. Moreover, wouldn’t this effect be weaker in larger populations? (idk just attempting to reason about something I have no expertise in here!) Also what does # of generations of asexual reproduction have to do with it? And why does it matter that our unicellular ancestors had only two mating types; surely that was long enough ago that we’d have time to evolve three or four types by now?
I have the same concern as Daniel Kokotajlo, but for a different reason.
Mr Malmesbury considers the gradual extinction of genders beyond 2, but he never mentions the injection of fertile new genders into the population through mutations.
In order to make the case convincing case for unique suitability of exactly two genders, we should look for reasons why systems with three or more genders would be unstable. Here is a hint: consider a third gender entering into an established species with two genders: one with a huge gametes, the other with tiny gametes. Where does the gamete size of the new entry fit in?
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.
Arguably you do in fact see this, at least as to 3-type species. Many bees, ants and other eusocial insects have queens, drones, and workers. Workers are often described as sterile females, but it seems like that’s us 2-typers imposing a 2-type frame on what are clearly 3 types.
Workers are only sterile in the most eusocial of species. In others, being a worker vs. queen is something of a choice, and if circumstances change a worker may start reproducing. There isn’t a sharp transition between cooperative breeding and eusociality.
Even in very eusocial haplodiploid species (so ants and bees, but not termites), unmated workers may reproduce after the death of the queen. They can only produce sons, but it’s still reproduction. .
In others, being a worker vs. queen is something of a choice, and if circumstances change a worker may start reproducing. There isn’t a sharp transition between cooperative breeding and eusociality.
Yep. Once the old naked mole-rat queen dies, the remaining female naked mole-rats have a dominance contest until one girl emerges victorious and becomes the new queen.
Drones are haploid and develop from unfertilized eggs. Queens and workers are diploid.
Stingers are ovipositors, and obviously workers have them. (It makes sense if bees and ants evolved from parasitoid wasps.)
There are degrees of eusociality, and workers are only mostly sterile. When they do reproduce, they lay eggs. Queens may suppress this tendency with dominance behaviors, pheromones, and may eat eggs laid by the workers.
For these reasons it makes sense to call the workers female like the queens.
I would instead characterise the workers as asexual—not a third gender, but a “defective” female gender - and eusocial insects as an excellent demonstration why asexual/agender/queer folks with these defects are in fact a benefit and hence kept in the gene pool, despite the fact that you’d intuitively think they would instantly die out as their core difference means they tend not to reproduce; namely, that they can play excellent support roles. The only way for the workers to spread their genes is through supporting the queen, who they are very closely related to; hence, they show extreme loyalty. A queen by herself would be unable to survive. If she only bore queens, those queens would not support her, but compete with her, taking resources for their kids. Having a bunch of asexual kids and only rarely raising a new queen when a whole new hive can be supported is ideal for the queen.
I’ve wondered whether this, in a more minor form, still holds true in mammals. It stands out that gay/ace animals do turn up in quite regular intervals, when it seems such an obvious bug. And then I think of humans, where they gay uncle gives you the best presents, because he doesn’t have kids of his own to raise, and where your lesbian aunt chips in with childcare, because she has no kids of her own. Mammal offspring often need a hell of a lot of care to be successful—you don’t win by having as many as possible that are fertile, they just fight each other. You want a few great fertile ones, and then arrange things so they make it—you want more labour to support, but not more competition. That is also likely why women go through menopause—if they kept reproducing, their children would be in competition with their children’s children, and as a result, their recent offspring would be neglected, and their earlier offspring would be pushed out of reproduction. Instead, they stop reproducing about the age their own kids start cranking out kids, and instead go for quality over quantity, support their kids and grandchildren. Basically, having genes that make it likely that your sibling is gay might be neat in some situations, especially environments with limited resources and demanding young. You can basically raise a free worker to hunt for food.
I am confused. If I think “Genetic drift means that many-typed populations tend to collapse towards fewer types as some types randomly die off,” which is putting your hypothesis in my words, I feel like this predicts that we should see some sort of distribution over N-typed species—with a bunch having 2 genders, a smaller bunch having 3, a smaller bunch having 4, etc. But instead it seems like they almost all have 2. Which I suppose is what you’d expect if the genetic drift effect was VERY strong. But it doesn’t seem plausible to me that it would be that strong. Moreover, wouldn’t this effect be weaker in larger populations? (idk just attempting to reason about something I have no expertise in here!) Also what does # of generations of asexual reproduction have to do with it? And why does it matter that our unicellular ancestors had only two mating types; surely that was long enough ago that we’d have time to evolve three or four types by now?
I have the same concern as Daniel Kokotajlo, but for a different reason.
Mr Malmesbury considers the gradual extinction of genders beyond 2, but he never mentions the injection of fertile new genders into the population through mutations.
In order to make the case convincing case for unique suitability of exactly two genders, we should look for reasons why systems with three or more genders would be unstable. Here is a hint: consider a third gender entering into an established species with two genders: one with a huge gametes, the other with tiny gametes. Where does the gamete size of the new entry fit in?
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.
Arguably you do in fact see this, at least as to 3-type species. Many bees, ants and other eusocial insects have queens, drones, and workers. Workers are often described as sterile females, but it seems like that’s us 2-typers imposing a 2-type frame on what are clearly 3 types.
Workers are only sterile in the most eusocial of species. In others, being a worker vs. queen is something of a choice, and if circumstances change a worker may start reproducing. There isn’t a sharp transition between cooperative breeding and eusociality.
Even in very eusocial haplodiploid species (so ants and bees, but not termites), unmated workers may reproduce after the death of the queen. They can only produce sons, but it’s still reproduction. .
Yep. Once the old naked mole-rat queen dies, the remaining female naked mole-rats have a dominance contest until one girl emerges victorious and becomes the new queen.
Drones are haploid and develop from unfertilized eggs. Queens and workers are diploid.
Stingers are ovipositors, and obviously workers have them. (It makes sense if bees and ants evolved from parasitoid wasps.)
There are degrees of eusociality, and workers are only mostly sterile. When they do reproduce, they lay eggs. Queens may suppress this tendency with dominance behaviors, pheromones, and may eat eggs laid by the workers.
For these reasons it makes sense to call the workers female like the queens.
Workers don’t mate. I don’t think they count for purposes of this discussion.
I would instead characterise the workers as asexual—not a third gender, but a “defective” female gender - and eusocial insects as an excellent demonstration why asexual/agender/queer folks with these defects are in fact a benefit and hence kept in the gene pool, despite the fact that you’d intuitively think they would instantly die out as their core difference means they tend not to reproduce; namely, that they can play excellent support roles. The only way for the workers to spread their genes is through supporting the queen, who they are very closely related to; hence, they show extreme loyalty. A queen by herself would be unable to survive. If she only bore queens, those queens would not support her, but compete with her, taking resources for their kids. Having a bunch of asexual kids and only rarely raising a new queen when a whole new hive can be supported is ideal for the queen.
I’ve wondered whether this, in a more minor form, still holds true in mammals. It stands out that gay/ace animals do turn up in quite regular intervals, when it seems such an obvious bug. And then I think of humans, where they gay uncle gives you the best presents, because he doesn’t have kids of his own to raise, and where your lesbian aunt chips in with childcare, because she has no kids of her own. Mammal offspring often need a hell of a lot of care to be successful—you don’t win by having as many as possible that are fertile, they just fight each other. You want a few great fertile ones, and then arrange things so they make it—you want more labour to support, but not more competition. That is also likely why women go through menopause—if they kept reproducing, their children would be in competition with their children’s children, and as a result, their recent offspring would be neglected, and their earlier offspring would be pushed out of reproduction. Instead, they stop reproducing about the age their own kids start cranking out kids, and instead go for quality over quantity, support their kids and grandchildren. Basically, having genes that make it likely that your sibling is gay might be neat in some situations, especially environments with limited resources and demanding young. You can basically raise a free worker to hunt for food.