The theory of natural selection cites the fact that every organism produces more gametes and/or organisms than can possibly survive. If every gamete produced by a given species developed into offspring, the world would become so overcrowded in a short period of time that there would be no room for successive generations. This does not happen. There is a balance that is maintained in the reproduction of all species and therefore natural populations remain fairly stable, unless upset by a change in conditions. In the struggle for existence, some organisms die and the more hardy survive. The differences that exist between organisms of the same species, making one more fit to survive than another, can be explained in terms of variations. Variations exist in every species and in every trait in members of a species. Therefore some organisms can compete more successfully than others for the available food or space in which to grow, or they can elude their enemies better. These variations are said to add survival value to an organism. Survival value traits are passed on to the offspring by those individuals that live long enough to reproduce. As time goes on, these special adaptations for survival are perpetuated and new species evolve from a common ancestral species. The environment is the selecting agent in natural selection because it determines which variations are satisfactory for survival and which are not. The major weakness in Darwin's theory of natural selection is that he did not explain the source, or genetic basis, for variations. He did not distinguish between variations that are hereditary and those that are nonhereditary, making the assumption that all variations that have survival value are passed on to the progeny. Like Jean Baptiste Lamarck, Darwin believed in the inheritance of even acquired characteristics. Hugo De Vries (1845-1935), a Dutch botanist, explained variations in terms of mutations. His study of 50,000 plants belonging to the evening primrose species enabled him to identify changes in the plants that were passed on from parent to offspring. In 1901 De Vries offered his mutation theory to explain organic evolution. Today, we know that mutations are changes in genes that can come about spontaneously or can be induced by some mutagenic agent. Spontaneous mutation rates are very low, and mutations alone do not affect major changes in the frequencies of alleles. An important cause of variation within species is genetic recombination that results from sexual reproduction. The genes of two individuals are sorted out and recombined into a new combination, producing new traits―and thus variation. Gene flow is also responsible for the development of variations. It is the movement of new genes into a population. Genetic drift is a change in a gene pool that takes place in a population as a result of chance. An isolated smaller population has a different gene frequency than the larger population from which it came. Genetic drift and the random mutations are known as non-Darwinian evolution.

最後の行補足説明　 genetic driftとは、ある生物種のある集団において、偶然によって、ある種類の遺伝子（対立遺伝子―例えば色が黒い）の割合が高くなるということ。random mutation　は、やはり偶然による遺伝子の変異。ダーウィンの唱えたnatural selectionの正しい訳は自然選択であり、一般に言われている自然淘汰は誤訳。彼によれば、「自然環境」が、そこにいる生物種の進化を決定付けるという。genetic driftもrandom mutation　も、進化を引起すのは「自然環境」ではなく、「偶然」であるという立場なので、非ダーウィン的であるということ。