Evolution under Domestication and Evolution by Polyploidy
When The Origin of Species was published in 1859, neither Darwin nor any one else claimed to have witnessed one species giving rise to another. Darwin's theory was a scientifically legitimate inference from indirect evidence. Numerous facts in biology made sense on the hypothesis that species evolve by gradual change and differentiation from races. Nevertheless, Darwin collected with great care the observed facts which showed that organisms undergo genetic changes and may become altered in the course of time. He found many such facts recorded by the breeders of domesticated animals and plants. The breeders produce new varieties of animals and plants to suit their needs or fancies. Moreover, they do so by means of artificial selection, often following hybridization of diverse varieties or races. Darwin's natural selection was a counterpart of artificial selection which had been known to be effective in domesticated forms. In 1868, Darwin summarized the evidence concerning evolution under domestication in a book entitled The Variation of Animals and Plants under Domestication.
Experimental evidence of the occurrence of evolution is at present much greater than it was in Darwin's time. Among microorganisms, evolutionary changes due to interaction of mutation and selection can be observed under controlled experimental conditions (see Chapter 5). To be sure, these changes involve alterations of single genes or, at most, of small numbers of genes. They are called microevolutionary changes, to distinguish them from macroevolution, which results in production of new genera, families, and classes. Macroevolution entails changes in many, perhaps in all, genes composing the genotype. Evolution under domestication continues to be interesting to modern