Yeah, this is pretty much the explanation I’m familiar with.
Gametes evolve, and two members of a species have to combine their gametes to reproduce
But having the same gamete that is both capable of moving out of the body and has the machinery for developing the embryo is inefficient.
So a division of labor evolves, where one gamete evolves to be easy to transfer (which causes it to be very small, which allows the production of much more of it), and one gamete evolves to receive the other gamete and have all the machinery required to create an embryo.
All other sex differences are then ultimately driven by this split—one sex has many small and cheap gametes they need to pass around to the other sex, and one sex has few big gametes that need to receive the other sex’s gamete.
As for why not hermaphrodites that have both type of gametes, I think it’s just inefficient, especially when you need many complex systems to support each one of the gametes. My understanding with hermaphrodite species like snails is that what drives it is their difficulty in finding a mate (because they’re so slow), so when they finally meet a mate, it’s best if they can just reproduce, and not have to go look for another.
Yeah, this is pretty much the explanation I’m familiar with.
Gametes evolve, and two members of a species have to combine their gametes to reproduce
But having the same gamete that is both capable of moving out of the body and has the machinery for developing the embryo is inefficient.
So a division of labor evolves, where one gamete evolves to be easy to transfer (which causes it to be very small, which allows the production of much more of it), and one gamete evolves to receive the other gamete and have all the machinery required to create an embryo.
All other sex differences are then ultimately driven by this split—one sex has many small and cheap gametes they need to pass around to the other sex, and one sex has few big gametes that need to receive the other sex’s gamete.
As for why not hermaphrodites that have both type of gametes, I think it’s just inefficient, especially when you need many complex systems to support each one of the gametes. My understanding with hermaphrodite species like snails is that what drives it is their difficulty in finding a mate (because they’re so slow), so when they finally meet a mate, it’s best if they can just reproduce, and not have to go look for another.