There are several elements that combine effectively to create the complete picture of natural selection. First, natural selection takes place in populations—communities of interbreeding individuals at given geographical locations—not in individuals. Second, populations typically have such high reproduction potential that if unchecked they would increase exponentially. For example, the female of the ocean sunfish, Mola mola, produces as many as three hundred million eggs at a time. If even just 1 percent of those eggs are fertilized and survive to adulthood, we soon would have oceans filled with Mola molas (and the average weight of an adult ocean sunfish exceeds two thousand pounds). Fortunately, due to competition for resources within the species, struggles with predators, and the environment’s other adversities, from a set of parents belonging to any species, an average of only two offspring survive and reproduce.

This description makes it clear that the word “selection” in Darwin’s formulation of natural selection really refers more to a process of elimination of the “weaker” (in terms of survival and reproduction) members of a population, rather than to a selection by an anthropomorphic nature. Metaphorically, you could think of the process of selection as one of sifting through a giant sieve. The larger particles (corresponding to those that survive) remain in the sieve, while the ones that pass through are eliminated. The environment is the agent that does the shaking of the sieve. Consequently, in a letter that Wallace wrote to Darwin on July 2, 1866, he actually suggested that Darwin should consider changing the name of the principle:

I wish, therefore, to suggest to you the possibility of entirely avoiding this source of misconception . . . and I think it may be done without difficulty and very effectually by adopting Spencer’s term (which he generally uses in preference to Natural Selection), viz. “Survival of the Fittest.” This term is the plain expression of the facts; “Natural Selection” is a metaphorical expression of it, and to a certain degree indirect and incorrect, since, even personifying Nature, she does not so much select special variations as exterminate the most unfavourable ones.

Darwin adopted this expression, coined in 1864 by the polymath Herbert Spencer, as a synonym for natural selection in his fifth edition of The Origin. However, present-day biologists rarely use this term, since it may give the wrong impression that it means that only the strong or healthy survive. In fact, “survival of the fittest” meant to Darwin precisely the same as “natural selection.” That is, those organisms with selectively favored and heritable characteristics are the ones who most successfully pass those to their offspring. In this sense, even though Darwin admitted to having been inspired by ideas of philosophical radicals such as the political economist Thomas Malthus—some sort of biological economics in a world of free competition—important differences exist.

A third and extremely important point to note about natural selection is that it really consists of two sequential steps, the first of which involves primarily randomness or chance, while the second one is definitely nonrandom. In the first step, a heritable variation is produced. In modern biological language, we understand this to be a genetic variation introduced by random mutations, gene reshuffling, and all the processes associated with sexual reproduction and the creation of a fertilized egg. In the second step, selection, those individuals in the population that are best suited to compete, be it with members within their own species, with members of other species, or in terms of their ability to cope with the environment, are more likely to survive and reproduce. Contrary to some misconceptions about natural selection, chance plays a much smaller role in the second step. Nevertheless, the process of selection is still not entirely deterministic—good genes are not going to help a species of dinosaurs wiped out by the impact of a giant meteorite, for instance. In a nutshell, therefore, evolution is really a change over time in the frequency of genes.