Alice and the Red Queen by John Tenniel

In Lewis Carroll’s Through the looking-glass– a whacky book if I ever read one – the laws of physics don’t really apply. Hills can become valleys, straight can become curvy, and forward is really backward.

In one scene, Alice chases after the Red Queen, both running as fast as they can, but when they stop Alice realises they are still right where they started. “Now, here, you see, it takes all the running you can do to keep in the same place” says the Red Queen.

And it might be the same with the evolution of predator and prey, host and parasite. Running doesn’t get you anywhere. So says the Red Queen Hypothesis.

C elegans embryo. Image by Monica Gotta

As the host adapts to fight the parasite, the parasite evolves to infect the host. It’s an endless race, and extinction faces the first organism to stop running.

So what’s this got to do with sex? Sex is evolution on turbo. Mixing and matching genes increases genetic diversity, giving a species more opportunities to outlast in the ultimate game of survivor.

Field data supports the Red Queen Hypothesis as describing an adaptive advantage of sex. Models and maths support the idea that coevolving species could select for rare genes and unusual combination randomly created by sex. Direct experimentation of coevolution and nookie is tricky business.

New research, published in Science, grew several populations of nematode worms (C. elegans, roundworms) which are usually asexually, but reproduce sexually 20% of the time.

The populations were differently exposed to bacterial parasites (Serratia marcescens) as shown.

C Elegans image by Bob Goldstein, University of Carolina, Chapel Hill, remixed by Science Journal. Creative Commons License

One population was given the parasites and left to their own devices. They and their bacteria could evolve together. These nematode worms increased their rate of sexual reproduction to 80-90% over time, and maintained a high level of sexy-times.

The other nematodes were given frozen stocks of bacteria every generation, so the parasites weren’t evolving as the worms did. At first, sexual reproduction increased in the worms, but then it dropped back down to 20% – the same level as nematodes which hadn’t been exposed to the bacteria at all.

Parasites on their own don’t increase sex – coevolution does.

A second experiment supported their conclusion. Nematodes mutated to be unable to reproduce sexually (asexual obligates) became extinct after 20 generations when exposed to the parasites. But mutants that always required sex to reproduce (sexual obligates) never became extinct.

When it comes to coevolution, it’s fall behind and be left behind.

Never stop running.

Morran, L. et al (2011). Running with the Red Queen: Host-Parasite Coevolution Selects for Biparental Sex Science, 333 (6039), 216-218 DOI: 10.1126/science.1206360
Brockhurst, M. (2011). Sex, Death, and the Red Queen Science, 333 (6039), 166-167 DOI: 10.1126/science.1209420