Figure 2

Most of the intellectual heavy lifting on evolution, however, had already been achieved in The Origin. In one blow, Darwin disposed of the notion of design, dispelled the idea that species are eternal and immutable, and proposed a mechanism by which adaptation and diversity could be accomplished.

In simple terms, Darwin’s theory consists of four main pillars that are supported by one remarkable mechanism. The pillars are: evolution, gradualism, common descent, and speciation. The crucial mechanism that drives it all and glues the different elements into cooperation is natural selection, which, we know today, is supplemented to some degree by a few other vehicles of evolutionary change, some of which could not have been known to Darwin.

Here is a very succinct account of these distinct components of Darwin’s theory. The description will mostly trace Darwin’s own ideas rather than updated, modernized versions of these concepts. Still, in a few places, it will be essentially impossible to avoid the delineation of evidence that has accumulated since Darwin’s time. As we shall discover in the next chapter, however, Darwin did make one serious error that could have negated entirely his most important insight: that of natural selection. The root of the error was not Darwin’s fault—nobody in the nineteenth century understood genetics—but Darwin did not realize that the theory of genetics with which he was operating was lethal for the concept of natural selection.