system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Rather than have two separate sexes, why are not all species hermaphroditic?
gpt
Organisms that exchange genes change more per generation than organisms that don't. 

As such, sexual organisms tend to evolve a lot faster relative to generation length than asexual ones.

Not only does this allow them to more rapidly exploit ecological niches and adapt to change (thus standing a better chance of avoiding extinction), but it allows them to be *more different* than their peers - which means that a lot of the biological diversity out there *is the result of* sex-driven evolution. 

As with most evolutionary concepts, you kind of need to look at these things in hindsight. Species that haven't changed much over the eons... are the weird primitive things you find under rocks in ponds, because that's where they started, and they just haven't moved on. 

Any species that turns hermaphroditic (there are even some lizards that have) is retiring from the rat-race, because it lives in an environment so stable that mutations allowing self-fertilization provide less disadvantage than the cost of maintaining males. This doesn't tend to happen very often - and even when it does, it doesn't tend to spread, because a stable species doesn't split into new species.