In general, Darwin insisted on writing of evolution as very slow and gradual. But his own colleague T. H. Huxley pointed out that he was hobbling the process unnecessarily by referring to it as always gradual. It is slow, if we’re talking of the formation of most species under most natural conditions. I think that Darwin’s own cautious nature and hatred of conflict led him to characterize evolution as slow, on the grounds that it would be less upsetting for everyone if it were.
But when conditions change rapidly, especially if generations are short, species sometimes change rapidly as well. The British people inadvertently supplied a new environment for mosquitoes when they dug subway tunnels under the streets of London. And, obligingly, mosquitoes that found their way into the system and stayed there developed into a new species (Culex molestus), unwilling to breed with its surface-dwelling, bird-biting neighbours (Culex pipiens). The underground mosquitoes learned to feed on rats, mice, and people. (They tormented the people of London during the Blitz, when the subways were used as air raid shelters.) The first section of the London Underground opened in 1863, and the new species was noticed in 1988, a span of 125 years. Of course, a generation of mosquitoes is only 3 weeks long, so 125 years is about 1700 generations if the underground mosquitoes were able to breed all year ’round. There are even genetic differences between the mosquitoes on different subway lines. So a small population can diverge rapidly from its main branch. (See “Rapid Speciation” for more examples.)
In a stable environment, the long-term stability of forms is only to be expected, since organisms are well adapted and any change tends to make them less so. But under selective pressure from changed conditions, a group can change rapidly. If a population on the fringes of a territory has adapted earlier to conditions that become more common, the “fringe” group can spread back into the main group’s territory. That can explain why, sometimes, a different form appears in the fossil record but the few, isolated transitional fossils of their changing are hard to find and perhaps never formed. This, more or less, is punctuated equilibrium: long periods of well-adapted fossils enjoying their optimum fitness, followed by changing conditions and their rapid replacement by a different form. It’s still descent with modification by natural selection upon variation. It’s still a natural process; and in our eyes it takes a long time.
In spite of Darwin’s usual public insistence on “gradualism,” which became really prominent after the attacks on the first edition of On the Origin of Species, he recognized the punctuated process as a likely scenario in evolution. Here it is, in his own words, from “Transmutation Notebook E:”
The more I think, the more convinced I am, that extinction plays greater part than transmutation. — Do species migrate & die out.?—
In the place where any species is most common, we need not look for change, because its numbers show it is perfectly adapted; it [is] where few stray ones are, that change may be anticipated, & this would look like fresh Creation. The gardener separates a plant he wishes to vary…
U.S. National Public Radio program “Speaking of Faith” has worked with the Cambridge University Library and Darwin scholar David Kohn to provide an online tour of Darwin’s Notebooks and sketches (whence I borrowed this image).