There are many things I consider beautiful. I like a colourful dance, a haunting song or an intriguing face. But I couldn't always consciously tell you why.
Peacock spiders are much the same. The males have beautiful rainbow-coloured abdomens, which they show off by performing some serious dance moves. They go to all this trouble to win the approval of a female.
The peacock spider and I are far from alone. When it comes to choosing a mate, many animals seem to have a clear sense of what is beautiful.
These preferences may seem arbitrary. It's hard to see how it would benefit a female peacock spider to choose a colourful male who can dance. But in fact these preferences may have had a profound effect on the course of evolution.
The idea that animals have "beautiful" traits to attract mates was first put forward by Charles Darwin. He proposed that one sex, often males, competes for the attention of the other.
Darwin called this "sexual selection". He wrote that it "depends on the advantage which certain individuals have over other individuals of the same sex and species, in exclusive relation to reproduction."
This competition is not to the death. Instead the unsuccessful suitor ends up with fewer offspring. What's more, mate choice is crucial to the theory. The sex that's being courted, often the female, will prefer a mate with the most desirable traits.
This is quite different to natural selection, otherwise known as survival of the fittest. Animals with poor-quality genes, for instance that make them more prone to disease, tend to die young, so only the best genes are passed on to future generations. Sexual selection and natural selection push animals to evolve in different ways, in a sort of tug-of-war.
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