The Evolution of Darwinism

W hen biologists refer to the theory of evolution nowa­ days, they are usually think­ ing not of Charles Darwin'S original statement of the theory but of a modi­ fied and expanded version of Darwin­ ism that took shape in the 1930's and 1940's. First known as the Neo-Dar­ winian theory and later as the synthet­ ic theory, it affirmed the fundamental tenets of Darwinism. It held that evo­ lution proceeds through the natural se­ lection of heritable differences arising at random in each generation. Differ­ ences that render their carriers better adapted to the environment are multi­ plied and those that are harmful are eliminated. Like Darwinism, the syn­ thetic theory stressed that evolution through natural selection is opportun­ istic, in that variations arise by chance and are selected in accordance with the demands of the environment, and that it takes place steadily. To this Darwinian foundation the architects of the synthetic theory, who included the geneticist Theodosius Dobzhansky, the biogeographer and systematist Ernst Mayr, the paleontol­ ogist George Gaylord Simpson, the biologist Julian Huxley and one of us (Stebbins), added new elements. The science of genetics made it possible to identify the determinants of the traits on which natural selection acts as genes: heritable units of information governing structure, development and function. Variant traits were held to result from mutations, or lasting alter­ ations that arise at random in individ­ ual genes. From population biology came a second new feature of the syn­ thetic theory: an emphasis on the im­ portance of popUlation structure and distribution in the development of new species. The synthetic theory also incorporated the biological concept of species, which leans more heavily on reproductive isolation (an inability to interbreed) than on visible differences in distinguishing species. Although