To explain sexual selection, Darwin used the familiar metaphor of artificial selection. Victorian England was still mostly agricultural and pastoral. People knew about artificial selection, in which farmers domesticate plants and animals by allowing some individuals to breed and others not. Darwin had already used this barnyard type of artificial selection as a metaphor to explain how natural selection worked. Sexual selection he compared to a rather different sort of artificial selection more familiar to the leisured classes, and more relevant to gorgeous ornamentation: breeding pet birds to make them look unusual and attractive. In The Origin he argued that "if man can in a short time give beauty and an elegant carriage to his bantams, according to his standard of beauty, I can see no good reason to doubt that female birds, by selecting, during thousands of generations, the most melodious or beautiful males, according to their standard of beauty, might produce a marked effect."
The analogy between artificial selection by human breeders and sexual selection by female animals may seem strained. But for Darwin there was no essential difference between human minds and animal minds: both could work as selective forces in evolution. As a dog-lover and an experienced horseman, Darwin felt comfortable attributing intelligence to animals. He reasoned that if humans can breed dogs, cats, and birds according to our aesthetic tastes, why shouldn't these animals be able to breed themselves according to their own sexual tastes?
Biology students now are usually taught that sexual selection is a subset of natural selection, and that natural selection is only loosely analogous to artificial selection by human breeders. This was not Darwin's view: he saw sexual selection as an autonomous process that was midway between natural and artificial selection. Darwin was fairly careful about his terms. For him, artificial selection meant the selective breeding of domesticated species by humans for their economic, aesthetic, or alimentary value. Natural selection referred to competition within or between species that affects relative survival ability Sexual selection referred to sexual competition within a species that affects relative rates of reproduction. Darwin knew that Herbert Spencer's term "survival of the fittest" could be misleading Heritable differences in reproduction ability were as important in evolution as heritable differences in survival ability.
However, whereas natural and artificial selection can apply equally well to mushrooms, lemon trees, and oysters, Darwin believed that sexual selection acts most strongly in the higher animals. This is because courtship behavior and selective mate choice behavior are best carried out by mobile animals with eyes, ears, and nervous systems. The mate choice mechanisms that drive sexual selection are much more similar to artificial selection by humans than to blind forms of natural selection by physical or ecological environments. Darwin understood that sexual selection's dependence on active choice might create distinct evolutionary patterns such as fashion cycles and rapid divergence between closely related species.
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