So he did an experiment with worms, which can reproduce sexually or asexually. The research team exposed the worms to a bacterial parasite that can do bad things to them.

“As gross as it sounds, it actually digests the host from the inside out,” Morran says. “So it’s a pretty nasty pathogen.”

This parasite is constantly evolving in ways that help it infect its worm host, Moran says. “And then you have the host evolving on the other side of the equation trying to evade the parasite,” he says.

As part of the experiment, the team used genetic engineering to create a group of worms that could only reproduce through self-fertilization.

When we allowed a parasite to co-evolve with them, they rapidly went extinct,” Morran says.

Then the team tried the same thing with another group of worms that could only reproduce by having sex.

“Even though their bacterial parasites became more infective,” Morran says, “those sexual populations were able to adapt and become better at evading their parasites.”