Artificial selection

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Theory of Evolution > Selection > Artificial selection

Livestock, such as this Friesian dairy cow, have been subjected to artificial selection by farmers for millennia to increase yields.

Artificial selection, in contrast to natural selection, refers to intentional selection of characteristics. Humans have been exploiting genetics and heredity for centuries to increase production and profitability from agriculture. Crops and livestock are selected for their yield, still by differential reproduction, but this time because farmers choose which individual animals are allowed to reproduce, and which seeds are sown for the next crop. By choosing carefully the phenotypes we desire we have made tall cereal crops which produce lots of grain, fat cows which produce lots of milk and meat, sheep which produce lots of wool, strong horses and chickens which produce lots of eggs. Artificial selection has been taking place for 10,000 years, since the first farmers of the fertile crescent. Wheat and cows were first domesticated over 8,000 years ago. Nowadays farmers keep records of milk production and only the cows with the highest milk yields are allowed to mate. Genes for high milk yield spread through the gene pool not because they increase that individual's chances of survival, but because they increase that individual's chances of being chosen for reproduction.

In many cases, artificial selection by humans reverses past natural selection. The wild ancestors of cows produced only enough milk for their own young. Shorter varieties of wheat and rice have higher grain yield because they don't waste resources building a taller stem, but taller plants won under natural selection because they shaded out shorter competitors. It has been claimed PDF that most yield increases from genetic improvement of crops involved such trade-offs between the competitiveness of individual plants versus the productivity of a whole field of similar plants. There may be less room for improvement in processes like photosynthesis that are equally important to competitiveness and overall yield. If so, then yield increases from biotechnology may be disappointing.