That being said, climate tends to induce a “directional” evolution, and so do predators. Wouldn’t parasites just encourage change? So if the host changes physiologically to inhibit a parasite and the parasite changes to take advantage of the new host, the host changing back to the original configuration can be just as useful in inhibiting the parasite. So parasite would encourage the evolution of the ability to make small but significant changes rapidly. Which I guess is one of the main benefits of sexual selection…I think I painted myself into a corner.

Other species have them. The only immediate one I can think of is “SIV,” Simmian Immunodeficiency Virus. The thing is, it’s not sex that makes diseases transmittable, it’s the fact sex involved body fluids. If you’re taking about genital-related parasites, infections, etc, then that’s something different. I don’t really know.

Err is that really true? Many plants for example have both male and female parts.

[CZ: Other species can pose a more intense challenge than an environmental condition like climate. A parasite can evolve in response to a new host defense, while the climate does not. Parasite pressure is particularly intense for several reasons. For example, a single virus can produce billions of new viruses in a matter of days, speeding up their rate of evolution. ]