tions split from a common ancestor and diverge from each other under the influence of different selection pressures, they begin as 2 populations from the same variety, then they become 2 varieties of the same species, and finally they reach the point where they count as different species. It is convenience, not fact, that leads us to classify different degrees of divergence in different ways (1859, pp. 48–52). This vague boundary between variety and species is no reason to deny the existence of individual species, nor did Darwin do so (Ereshefsky, 2009; Sloan, 2009). This is the lesson we learn from other vague concepts—from rich and poor, hairy and bald, tall and short; a vague boundary does not entail that no one is rich, or hairy, or tall. Even so, “species” is not the central concept in Darwin’s theory. True, the process he describes produces species, but it produces traits and taxa at all levels of organization. For these reasons, Darwin’s theory is better described as “the origin of diversity by means of natural selection.”
Darwin’s concept of natural selection has several noteworthy features. Although the Origin introduced the idea of natural selection by first describing artificial selection, Darwin hastened to emphasize that natural selection is not an agent who intentionally chooses. When cold climate causes polar bears to evolve longer fur, the weather is not an intelligent designer who wants polar bears to change. The weather kills some bears while allowing others to survive, but the weather does not need to have a mind to do this. It is in this sense that natural selection is a mindless process (for a different assessment, see Richards, Chapter 16, this volume). So concerned was Darwin to emphasize this point that, in the 5th edition of the Origin, he followed Alfred Russel Wallace’s advice and used Herbert Spencer’s phrase “the survival of the fittest” to characterize his theory (Darwin, 1959, p. 164). Darwin hoped this new label would make it harder for readers to misunderstand his theory.
Another important feature of Darwin’s concept is that the direction in which selection causes populations to evolve depends on accidents of the environment. There is no inherent tendency for life to grow bigger or faster or harder or slimier or smarter. Everything depends on which traits do a better job of allowing organisms to survive and reproduce in their environments. This is the vital contrast that separates Darwin from Lamarck, who saw evolution as leading lineages to move through a pre-programmed sequence of steps, from simple to complex. Of course, if life starts simple, evolution by natural selection will lead the average complexity of the biota to increase. However, that is not because the “laws of motion” of natural selection inherently favor complexity. Parasites evolve from free-living ancestors, and the effect is often a move toward greater simplicity, with parasites losing organs and abilities possessed by their ancestors (Darwin, 1859, p. 148). Complexity increases from life’s beginning because of the initial conditions, not the laws. This is analogous to the