The basic idea underlying natural selection is quite simple (once it is pointed out!). As it sometimes happens with discoveries whose time has come, the naturalist Alfred Russel Wallace independently formulated very similar ideas at about the same time. Wallace was nevertheless very clear on who he thought deserved most of the credit. In a letter to Darwin on May 29, 1864, he wrote:

As to the theory of Natural Selection itself, I shall always maintain it to be actually yours and yours only. You had worked it out in details I had never thought of, years before I had a ray of light on the subject, and my paper would never have convinced anybody or been noticed as more than an ingenious speculation, whereas your book has revolutionized the study of Natural History.

Let us attempt to follow Darwin’s train of thought: First, he noted, species tend to produce more offspring than can possibly survive. Second, the individuals within a given species are never all precisely identical. If some of them possess any kind of advantage in terms of their ability to cope with the adversity of the environment—and assuming that this advantage is heritable, and passed on to their descendants—then over time, the population will gradually shift toward organisms that are better adapted. Here is how Darwin himself put it, in chapter 3 of The Origin:

Owing to this struggle for life, any variation, however slight and from whatever cause proceeding, if it be in any degree profitable to an individual of any species, in its infinitely complex relations to other organic beings and to external nature, will tend to the preservation of that individual, and will generally be inherited by its offspring. The offspring, also, will thus have a better chance of surviving, for, of the many individuals of any species which are periodically born, but a small number can survive. I have called this principle, by which each slight variation, if useful, is preserved, by the term of Natural Selection.

Using the modern gene terminology (of which Darwin knew absolutely nothing), we would say that natural selection is simply the statement that those individuals whose genes are “better” (in terms of survival and reproduction) would be able to produce more offspring, and that those offspring will also have better genes (relatively speaking). In other words, over the course of many generations, beneficial mutations will prevail, with harmful ones eliminated, resulting in evolution toward better adaptation. For instance, it is easy to see how being faster could benefit both predator and prey. So in East Africa’s open plains of the Serengeti, natural selection has produced some of the fastest animals on Earth.