Not a biologist, but there's nothing that novel about 3 sexes as such. Some fungi have thousands of sexes[1] for example. The way I heard it explained is from

Not a biologist, but there's nothing that novel about 3 sexes as such. Some fungi have thousands of sexes[1] for example. The way I heard it explained is from an evolutionary point of view there is selection pressure towards either 2 or very large numbers of sexes. Very large numbers of sexes means it's very likely that when 2 individuals meet they can sexually reproduce (whereas if you have only 2 sexes half the time they cannot). Say there are 100 sex types of a species it might well be that 80% of random couples could sexually reproduce together because there are far more compatible combinations. On the other hand, evolutionary pressure towards 2 sex types comes from specialisation. It's very helpful for the gamete to either be large and full of food for the developing embryo or to be small and fast to win the race to fertilize. Once the process of specialisation starts, the largest "foodie" gamete and smallest "fast" gamete tend to beat out all the others so you end up with 2 sexes one of which produces eggs and the other sperm. [1] https://earthsky.org/earth/fungus-sexes-chromosomes-thousands https://earthsky.org/earth/fungus-sexes-chromosomes-thousand...