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Theories of sex

Why is sexual reproduction so important?

The key advantage is thought to be the way it leads to the shuffling of genes, increasing genetic variability. Oddly, though, it is not totally clear why this is so important.

Natural selection underlies the main theories of sexual reproduction. Although in the short term asexual reproduction might be favoured, over longer periods sexual reproduction has a distinct advantage.

Why might this be so? An early model known as ‘Muller’s Ratchet’ imagines a small population reproducing asexually. As individuals multiply, mutations and variations accumulate – or ratchet up – generation by generation, until eventually so many damaging mutations accumulate that the population becomes extinct.

But sexual reproduction could keep a gene free of harmful mutations, as its shuffling of genetic material creates offspring with variable levels of mutation in their genes, avoiding a ratchet effect.

There is also an alternative model known as ‘Kondrashov’s Hatchet’, which suggests that the effect of lots of accumulated mutations is greater than the sum of those same mutations individually – their effects are compounding. (At a certain point these become lethal, and down comes the hatchet.) If this theory is true, then sexual reproduction, in allowing harmful mutations to accumulate in some offspring but not others (rather than in all, as asexual reproduction), is even more beneficial.

Perhaps the strongest theory is that the variation provided by sex helps species to adapt in response to a constantly changing environment, or to pathogens. In nature there is a constant ‘arms race’ between predator and prey, and between host and intruder, as each tries to gain the upper hand. Sex gives species the flexibility to evolve defences against the predators and pathogens that attack them. This idea is often called the Red Queen hypothesis, referring to Lewis Carroll’s book ‘Through the Looking Glass’, where the Red Queen says: “It takes all the running you can do to keep in the same place.”

Lead image:

Jamie/Flickr CC BY


About this resource

This resource was first published in ‘Sex and Gender’ in January 2006 and reviewed and updated in November 2014.

Statistics and maths, Ecology and environment, Genetics and genomics, History
Sex and Gender
Education levels:
16–19, Continuing professional development