Evolution is the unifying theory of the biological sciences. Evolutionary biology aims to discover the history of life, the causes of the diversity and characteristics of organisms, and the mechanisms that underlie evolutionary change.
Charles Darwin’s major work, On the Origin of Species, published in 1859, contains two major hypotheses: first, that all organisms have descended, with modification, from common ancestral forms of life, and second, that the chief agent of modification is natural selection.
Darwin’s hypothesis that all species have descended with modification from common ancestors is supported by so much evidence that it has become as well established a fact as any in biology. His theory of natural selection as the chief cause of evolution was not broadly supported until the evolutionary synthesis that occurred in the 1930s and 1940s.
Modern evolutionary biology is based on the evolutionary synthesis, which united Darwin’s ideas with Mendelian genetics. The major causes of evolution within species are those that change the frequencies of alleles, and hence of the phenotypes they may affect. Different populations of a species may experience different genetic changes, and ultimately become different species. Over long time periods, many slight changes accumulate to yield large genetic and phenotypic differences among species and their ancient an-cestors.
Evolutionary biology makes important contributions to other biological disciplines and to social concerns in areas such as medicine, agriculture, computer science, and our understanding of ourselves.
The implications of Darwin’s theory, which revolutionized Western thought, include the ideas that change, rather than stasis, is the natural order; that biological phenomena, including those seemingly designed, can be explained by purely material causes rather than by divine creation; and that no evidence for purpose or goals can be found in the living world, other than in human actions.
Like other sciences, evolutionary biology cannot be used to justify beliefs about ethics or morality. Nor can it prove or disprove theo-logical hypotheses such as the existence of a deity. Many people hold that evolution is compatible with religious belief. However, evolution is incompatible with a literal interpretation of some passages in the Bible. Evolutionary biology and other sciences can test and reject claims for supernatural causes of observed phenomena.