Dawkins begins his case for evolution in the same place as Darwin himself: by discussing the myriad successes of plant and animal breeders. Whereas Darwin was very taken with pigeons, however, Dawkins prefers dogs, cabbages and cattle.
The chapter opens with a brief discussion of essentialism in biology, and how evolution shows it to be false. The following paragraph provides a well-written summary of the main point:
If there’s a `standard rabbit’, the accolade denotes no more than the centre of a bell-shaped distribution of real, scurrying, leaping, variable bunnies. And the distribution shifts with time. As generations go by, there may gradually come a point, not clearly defined, when the norm of what we call rabbits will have departed so far as to deserve a different name. There is no permanent rabbitiness, no essence of rabbit hanging in the sky, just populations of furry, long-eared, coprophagus, whisker-twitching individuals, showing a statistical distribution of variation in size, shape, colour and proclivites. What used to be the longer-eared end of the old distribution may find itself the centre of a new distribution later in geological time. Given a sufficiently large number of generations, there may be no overlap between ancestral and descendant distributions: the longest ears among the ancestors may be shorter than the shortest ears among the descendants. All is fluid, as another Greek philosopher, Heraclitus, said; nothing fixed. After a hundred million years it may be hard to believe that the descendant animals ever had rabbits for ancestors. Yet in no generation during the evolutionary process was the predominant type in the population far from the modal type in the previous generation. This way of thinking is what Mayr called population thinking. Population thinking, for him, was the antithesis of essentialism. According to Mayr, the reason Darwin was such an unconscionable time arriving on the scene was that we all — whether because of Greek influence of for some other reason — have essentialism burned into our mental DNA. (pp. 22-23).
He moves on to a similarly lucid explanation of the idea of a gene pool. For example:
Now let’s return to the remark that opened my discussion of gene pools. I said that if human breeders are to be seen as sculptors, what they are carving with their chisels is not dog flesh but gene pools. It appears to be dog flesh because the breeder might announce an intention to, say, shorten the snouts of future generations of boxers. And the end product of such an intention would indeed be a shorter snout, as though a chisel had been taken to the ancestor’s face. But, as we have seen, a typical boxer in any one generation is a sampling of the contemporary gene pool. It is the gene pool that has been carved and whittled over the years. Genes for long snouts have been chiselled out of the gene pool and replaced by genes for short snouts. Every breed of dog, from dachsund to Dalmation, from boxer to borzoi, from Poodle to Pekinese, from Great Dane to chihuahua, has been carved, chiselled, kneaded, moulded, not literally as flesh and bone but in its gene pool. (p. 34)
If this sort of “artificial selection” can cause such massive changes in organisms in periods of time that are short relative to geological history, then the common descent of all organisms does not seem so farfetched. In terms of the broader case for evolution this should be viewed as a plausibility argument. By itself it tells us nothing about natural history, but it does earn evolution a hearing.
Creationists have two standard replies to this sort of thing. The first is to assert that animal breeding is an example of intelligent design, since it is human breeders doing the selecting. This completely misses the point, of course. The success of artificial selection in animal breeding shows that the non-random selection of randomly occurring genetic variations can cause impressive physical changes in organisms, even in relatively short periods of time. That it was intelligent human breeders, as opposed to mindless nature, that was doing the selecting in this case is neither here nor there.
It is important to keep in mind that artificial selection is not merely an analogy for natural selection. It is precisely the same process. Natural selection is what happens when heritable differences among organisms have consequences for their level of reproductive success. The only significance of the term “artificial” is to make clear that the basis for the differing reproductive success is the whims of human breeders as opposed to the demands of survival in nature.
The second reply is that, as impressive as the differences between, say, a Boston Terrier and a Great Dane might be, they are both still dogs. That hardly implies that human beings can be the evolutionary descendants of ancient single-celled organisms.
There is some truth to this. There could, in principle be natural barriers that put strict limits on the amount of change that can occur via natural evolutionary mechanisms. It would indeed be a great leap to say that the accomplishments of animal breeders prove that the evolution of humans from simpler ancestors is a live possibility.
It is fortunate, therefore, that no one is saying that. As I said, animal breeding provides a plausibility argument, nothing more. It shifts the burden of proof back to the creationists. No longer can you make a dogmatic assertion about the extent of change that is possible in principle. If you claim nature imposes some impassable barrier to the amount of change that can occur, you will have to present some serious evidence to show that is the case. Considering what human breeders have achieved, it is decidedly not obvious.
The grand claim of evolution is, of course, that the barriers separating different kinds of living organisms are actually not as fundamental as they at first appear. Whether there really could be a chain of ancestors leading back from humans all the way to the bizarre, worm-like critters of the Cambrian is something that can only be resolved by the meticulous collection of evidence. Does the fossil record suggest such a chain of ancestors? Do comparisons of organisms, both at the level of gross anatomy and at the level of biochemistry, reveal patterns that are well-explained by common descent?
When we study complex adaptations in organisms, do we see pristine creations from nothing, or do we find Rube Goldberg contraptions that seem to be cobbled together from simpler precursors. Neuroscientist David Linden writes, in his book The Accidental Brain, “The difference between the lizard brain and the mouse brain does not involve wholesale redesign. Rather, the mouse brain is basically the lizard brain with some extra stuff on top. Likewise, the human brian is basically the mouse brain with still more stuff piled on top.” Is that a good description of complex adaptations generally? Or do we sometimes find complex structures with no good candidates for precursors?
I suspect most of the people reading this already know the answers. I also suspect Dawkins will answer these questions himself in due course. Stay tuned!