What Evolution Is

Here is a great summary of evolution theory from Ernst Mayr's book What Evolution Is, Basic Books, 2001, page 116.

Box 6.1 Darwin's Explanatory Model of Natural Selection

Fact 1: Every population has such high fertility that its size would increase exponentially if not constrained. (Source: Paley and Malthus)
Fact 2: The size of populations, except for temporary annual fluctuations, remains stable over time (observed steady-state stability). (Source: universal observation)
Fact 3: The resources available to every species are limited. (Source: observation, reinforced by Malthus)
- Inference 1: There is intense competition (struggle of existence) among members of a species. (Source: Malthus)
Fact 4: No two individuals of a population are exactly the same (population thinking). (Source: animal breeders and taxonomists)
- Inference 2: Individuals of a population differ from each other in the probability of survival (i.e., natural selection). (Source: Darwin)
Fact 5: Many of the differences among individuals of a population are, at least in part, heritable. (Source: animal breeders)
- Inference 3: Natural selection, continued over many generations, results in evolution. (Source: Darwin)