Sex against parasites

Image acquired from Flickr under a Creative Commons 2.0 license.

Image acquired from Flickr under a Creative Commons 2.0 license.

Sex may seem like all fun and games, but evolutionarily speaking, sexual reproduction has perplexed biologists for decades. It’s a question of math—why have a population in which only 50% of people can reproduce? In other words, why do men exist? Other than killing bugs and lifting heavy things that you could probably lift yourself, men, and sexual reproduction, confers an important evolutionary advantage: protection from pathogens.

The generation time of a human, other animal, or even a plant, is far greater than that of a bacterium. Think years, versus hours (or even minutes). Bacteria, and other pathogens, also acquire mutations at a much higher rate than humans per generation. Although mutating doesn’t sound like a benefit, it actually allows the bacteria to evolve as it is able to find mutations that better suit the particular environment in which it finds itself.

With bacteria acquiring new mutations so often, and evolving so rapidly, how are we humans supposed to keep up? This is where sex comes in. While we aren’t able to reproduce every hour, sexual reproduction allows us, as a species, to be constantly mixing our genetic material. Asexual reproduction, as occurs in bacteria, involves a single organism making an almost exact copy of itself. Any mutations that arise are random, and useful ones are just lucky. Sexual reproduction, on the other hand, always involves mixing information of two parents, so each generation is an opportunity for the acquisition of lots of new traits.

The idea that sexual reproduction might provide protection from pathogens is not a new one. This theory has its roots in a set of ideas known as the Red Queen Hypothesis. In Lewis Carroll’s Through the Looking Glass, the Red Queen says: “Now, here, you see, it takes all the running you can do, to keep in the same place. If you want to get somewhere else, you must run at least twice as fast as that!” In evolutionary biology, this translates to the idea that pathogens (e.g. viruses, bacteria, fungi, and parasites) and their hosts are engaged in a constant race against one another where the pathogens want to remain in their hosts and hosts want to eliminate them. Fortunately for the pathogens, they’re able to reproduce and evolve more rapidly than complex multicellular organisms like us. Each new generation, which occurs on the scale of hours, is an opportunity for a species of bacteria to acquire new mutations that could fortuitously render it less susceptible to attack from an animal’s immune system.

Multicellular organisms cannot mutate themselves on a per infection basis, so we depend on other mechanisms of battling quickly mutating bugs. The genetic variation that we, as a species, get from sexual reproduction is particularly important for the ability of our immune system to fight pathogens. In fact, the most variable set of genes in the human genome encode proteins that determine what kinds of pathogens an individual is best at fighting. This variation affords our species widespread protection from pathogens in general—even if one person is particularly susceptible to a certain viral infection, for example, the likelihood of everyone being susceptible to this virus is made extremely low by our extraordinary genetic diversity. This diversity is afforded by sexual selection that allows humans to acquire new traits with every generation.

There’s no guarantee that newly acquired traits will be useful, and many of them can be neutral, like eye color, or detrimental, like genetic diseases. Over evolutionary time, however, sexual reproduction is hypothesized to give organisms a leg up in the arms race with pathogens. So in addition to allowing you to make babies and enjoy yourself at the same time, sex may also play an important role in protecting species from extinction.