A theory in biolgy postulating that the various types of animals and plants have their origin in other preexisting types and that the distinguishable differences are due to modifications in successive generations. The theory of evolution is one of the fundamental keystones of modern biological theory.

The 19th-century English naturalist Charles Darwin argued that organisms come about by evolution, and he provided a scientific explanation, essentially correct but incomplete, of how evolution occurs and why it is that organism have such features--such as wings, eyes, and kidneys--clearly structured to serve specific functions. Natural selection was the fundamental concept in his explanation. Genetics, a science born in the 20th century, reveals in detail how natural selection works and led to the development of the modern theory of evolution. Since the 1960s a related scientific discipline, molecular biology, has advanced enormously knowledge of biological evolution and has made it possible to investigate detailed problems that seemed completely out of reach a few years earlier--for example, how similar the genes of humans and chimpanzees might be (they differ in about 1 or 2 percent of the units that make up the genes).

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