The facts of evolution are first, that it occurred and second, that it could occur because of the genotypic and phenotypic diversity both within species and between species. Charles Darwin's great contributions were to recognize the diversity and to explain it by the theory of natural selection. As currently understood, the theory is that given the variety of phenotypes in a species, some individuals will be more successful than others in surviving to produce offspring. Reproduction is the measure of success. The mean of the gene pool of the next generation shifts toward the mean of the successful phenotype. As the environment changes, the characteristics required to be successful change, and there is natural selection of individuals with those characteristics. This is a theory of the origin of species because there will eventually be enough change in the genotypic population to designate it as a new species.

There is so much support for the theory, in laboratory experiments and from field observations, that one might prefer a stronger word than "theory" to describe Darwin's integration. But there are disagreements among evolutionists, of course, which are sometimes taken incorrectly to be challenges to the credibility of the theory as a whole. The controversies are mainly about the relative importance of selection as opposed to random genetic drift, about the merits of various approaches to determining phylogenetic trees (cladistics), and about the rate of evolutionary change (gradualism versus punctuated equilibria). Despite their use and misuse in popular polemics, the controversies are on fairly technical questions and not on the fact of evolution.

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