The Improbability Pump
One type lies within the bodies of living organisms. In a wonderful chapter called "History Written All Over Us," Dawkins shows that animal anatomy is like a medieval palimpsest, carrying traces of our evolutionary ancestry. Human goose bumps, for instance, serve no function: they're remnants of the muscles used by our mammalian ancestors--and our living relatives like cats--to erect their fur, making them warmer and giving enemies the illusion of greater size. Modern genome sequencing has also uncovered vestigial DNA: useless, broken genes that are functional in our relatives and presumably were too in our ancestors. Our own genome, for instance, harbors nonfunctional genes that, in our bird and reptile relatives, produce egg yolk. Embryology--the study of development--brings more proof to the table. The pharyngeal arches of the early, fishlike human embryo are derived directly from the gill arches of fish, though they go on to become, among other things, our larynx and eustachian tube.
Even more evidence for evolution comes from the "bad designs" of animals and plants, which, Dawkins observes, look nothing like de novo creations of an efficient celestial engineer. His favorite example--and mine--is the recurrent laryngeal nerve, which runs from the brain to the larynx. In mammals it doesn't take the direct route (a matter of a few inches) but makes a curiously long detour, running from the head to the heart, looping around the aorta and then doubling back up to the neck. In the giraffe, this detour involves traversing that enormous neck twice--adding about fifteen feet of superfluous nerve. Anyone who's dissected an animal in biology class will surely agree with Dawkins's conclusion: "the overwhelming impression you get from surveying any part of the innards of a large animal is that it is a mess! Not only would a designer never have made a mistake like that nervous detour; a decent designer would never have perpetuated anything of the shambles that is the criss-crossing maze of arteries, veins, nerves, intestines, wads of fat and muscle, mesenteries and more."
Creationists often object to this sort of argument, saying that it's not scientific but theological. God is inscrutable, they claim, so how could we possibly know how he would or would not design creatures? But this misses the point, for the "bad design" we see is precisely what we'd expect if evolution were true. The laryngeal nerve takes that long detour because, in our fishy ancestors, it was lined up behind a blood vessel, with both nerve and vessel servicing the gills. As the artery moved backward during its evolution to the mammalian aorta, the nerve was constrained to move behind it, although its target (the larynx, an evolutionary descendant of the gill arch) remained up in the neck. If you insist that such designs reflect God's plan, then you must admit that his plan was to make things look as if they had evolved.
Finally, Dawkins provides evidence from a completely different realm: that of biogeography, the study of how plants and animals are distributed over the earth. Why do volcanic islands like Hawaii have plenty of unique plants, birds and insects (most resembling species from the nearest mainland) but no native amphibians, freshwater fish or land mammals? Such patterns defy explanation by any form of creationism. Instead, they bespeak long-distance migration of ancestors to newly formed islands, followed by the evolution of new species.
Fossils, embryology, bad design, molecular biology, vestigial traits, biogeography--all conspire to demonstrate the truth of evolution. All are described in Dawkins's famously lyrical prose and lavishly illustrated with color photos. Two chapters stand out. One, "You Did It Yourself in Nine Months" (the title is a reprise of evolutionist J.B.S. Haldane's reply to a woman who insisted that it was impossible for evolution to change a single cell into a complex human body), is simply the best existing description of how a linear DNA sequence codes for a three-dimensional body. The other deals with "Evolutionary Theodicy," Dawkins's idea that ecosystems reflect not harmonious central planning but inefficient natural selection. In an efficiently designed world, for instance, trees would be only a few feet tall; in an evolved one, natural selection among individuals competing for sunlight produces a lot of extra wood.
Dawkins's specialty has always been exploring and extolling natural selection, and this is where The Greatest Show on Earth really shines. But first, since selection is so uncontroversial to Dawkins yet so maligned by F&P, it behooves us to understand what it is. In principle, natural selection is simple. It is neither a "law" nor a "mechanism." It is, instead, a "process"--a process that is inevitable if two common conditions are met. First, some genes must harbor variation because of mutation; and second, some of those mutant genes must be better at replicating than others--usually because they improve the survival and reproduction of their carriers. Suppose, for instance, that the brown-colored ancestors of the polar bear included some carrying mutations in "pigment genes" that gave them lighter coats. These mutant bears would have an advantage: being more camouflaged in the snow than their darker confreres, they'd be able to sneak up on seals more easily and so get more to eat. Because well-fed individuals leave more offspring, over time the bear gene pool would become increasingly enriched in light-color genes. Eventually the species would evolve the familiar white polar bear coat. And this is the way, we think, that all organisms acquire that appearance of "design" that, before Darwin, was attributed to God.
Although we evolutionary biologists might describe the polar bear scenario as "natural selection acting on coat color," that's only our shorthand for the longer description given above. There is no agency, no external force of nature that "acts" on individuals. There is only differential replication of genes, with the winners behaving as if they were selfish (that's shorthand, too).
Dawkins describes selection as an "improbability pump," for over time the competition among genes can yield amazingly complex and extraordinary species. Here's how he describes the evolution of tigers:
A tiger's DNA is also a "duplicate me" program, but it contains an almost fantastically large digression as an essential part of the efficient execution of its fundamental message. That digression is a tiger, complete with fangs, claws, running muscles, stalking and pouncing instincts. The tiger's DNA says, "Duplicate me by the round-about route of building a tiger first."
Only Dawkins could describe a tiger as just one way DNA has devised to make more of itself. And that is why he is famous: absolute scientific accuracy expressed with the wonder of a child--a very smart child.