Nick Lane’s book Power, Sex, Suicide has an interesting model for this that seems pretty different from any of the ideas here. It has to do with maintaining alignment between the eukaryote genome and the mitochondrial genome, and having just one mitochondrial genome makes that problem tractable. So sexual reproduction needs a way to only inherit mitochondria from one gamete, to which sex is the solution.
At the organism level, I think sexual differentiation evolves to specialize in the interests of the different gametes. For plants, it’s straightforward to implement the different specialized strategies in parallel, but for animals its much more costly (they would have to carry around parallel adapted organs, and split time executing different behaviors).
I don’t think the parallel adapted organs would be that expensive. Perhaps if you needed the secondary sexual characteristics of both sexes—but as I argued, in the long term we should expect selection for secondary sexual characteristics (and mating strategies) that were more compatible with hermaphrodism.
Evolution sucks at long term though—that’s essentially your 4: they would be outselected before they would be frequent enough.
However, the “hermaphroditism is only found widely where, for one reason or another, finding a partner and finding them to be wrong sex can be really fatal” (sessile plants, slow snails) suggests it must be very costly indeed, and I’d bet on “simple” metabolic explanation: these adapted organs directly compete in terms of their influence on the body.
Nick Lane’s book Power, Sex, Suicide has an interesting model for this that seems pretty different from any of the ideas here. It has to do with maintaining alignment between the eukaryote genome and the mitochondrial genome, and having just one mitochondrial genome makes that problem tractable. So sexual reproduction needs a way to only inherit mitochondria from one gamete, to which sex is the solution.
At the organism level, I think sexual differentiation evolves to specialize in the interests of the different gametes. For plants, it’s straightforward to implement the different specialized strategies in parallel, but for animals its much more costly (they would have to carry around parallel adapted organs, and split time executing different behaviors).
I don’t think the parallel adapted organs would be that expensive. Perhaps if you needed the secondary sexual characteristics of both sexes—but as I argued, in the long term we should expect selection for secondary sexual characteristics (and mating strategies) that were more compatible with hermaphrodism.
Evolution sucks at long term though—that’s essentially your 4: they would be outselected before they would be frequent enough.
However, the “hermaphroditism is only found widely where, for one reason or another, finding a partner and finding them to be wrong sex can be really fatal” (sessile plants, slow snails) suggests it must be very costly indeed, and I’d bet on “simple” metabolic explanation: these adapted organs directly compete in terms of their influence on the body.