As Darwin was formulating his theory of evolution by natural selection, he was influenced by the vast body of knowledge on the domestication of plants and animals. Artificial selection refers to the selective process when humans are acting as a selective agent. Darwin was aware of the power of artificial selection to affect genetic changes in domestic animals over a relatively short period of time. Imagine, he perhaps exclaimed, what sort of changes might occur over the history of life on Earth!
Charles Darwin wrote On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life. In this book, one of his key insights was to recognize that the struggle for life had winners and losers, and that it would result in changes in populations through time as the winners contributed their genes to the next generation but the losers didn't.
Darwin saw that the process of natural selection, where the environment in which a population lived could impact what genes made it into subsequent generations, was very similar the process used in animal breeding. The key difference is that in husbandry, humans — and not nature — decide who the winners and losers are.
When nature is the selective agent, the process is called natural selection. When humans are the selective agent, the process is called artificial selection. We can use artificial selection to examine the process of evolution in the laboratory, and we can observe natural selection occurring in the wild.
The process of artificial selection isn't exactly identical to what happens in the natural environment because humans can get pretty creative in their animal and plant breeding. A particular breeding endeavor, for example, could require a cocktail of approaches: perhaps a little directional selection, just a touch of genetic drift, and a dash of in-breeding followed by some more selection. The result is that allele frequencies of the domestic population change, but it's not strictly identical to the natural process.
Evolution by natural selection (or any other mechanism) is a property of groups, not individuals. Have you ever seen that cartoon of the fish that grew legs and crawled up onto the land? If that seemed confusing, it's because it is. No one fish grows legs. It lives and dies with the same fins it always had. But if, in a population of fish, there is heritable variation in fin structure (and remember, there is heritable variation for pretty much everything) and fish with stiffer, stubblier fins leave more offspring, then the population at some future time will have, on average, stiffer fins. If mutations that results in even stiffer fins are selectively favored, the process keeps going until you have fish that waddle around in the mud (such fish exist), and up up up we go from there.
The following sections tease apart in more detail some of the different ways that selection can act.
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