Michael J. Behe - The Edge of Evolution: The Search for the Limits of Darwinism

Life on earth developed over billions of years by utter chance, filtered through natural selection. So says Darwinism, the most influential idea of our time. If a rare random mutation in a creatures DNA in the distant past helped the lucky mutant to leave more offspring than others of its species, then as generations passed the species as a whole would have changed. Incessant repetition of this simple process over eons built the wonders of biology from the ground up, from the intricate molecular machinery of cells up to and including the human mind.

Thats the claim, at least. But is it true? To answer that question, Darwins theory has to be sifted carefully, because it isnt just a single conceptit actually is a mixture of several unrelated, entirely separate ideas. The three most important ideas to keep straight from the start are random mutation, natural selection, and common descent.

Common descent is what most people think of when they hear the word evolution. It is the contention that different kinds of modern creatures can trace their lineage back to a common ancestor. For example, gerbils and giraffestwo mammalsare both thought to be the descendants of a single type of creature from the far past. And so are organisms from much more widely separated categoriesbuffalo and buzzards, pigs and petunias, yaks and yeast.

Thats certainly startling, so its understandable that some people find the idea of common descent so astonishing that they look no further. Yet in a very strong sense the explanation of common descent is also trivial. Common descent tries to account only for the similarities between creatures. It says merely that certain shared features were there from the beginningthe ancestor had them. But all by itself, it doesnt try to explain how either the features or the ancestor got there in the first place, or why descendants differ. For example, rabbits and bears both have hair, so the idea of common descent says only that their ancestor had hair, too. Plants and animals both have complex cells with nuclei, so they must have inherited that feature from a common ancestor. But the questions of how or why are left hanging.

In contrast, Darwins hypothesized mechanism of evolutionthe compound concept of random mutation paired with natural selectionis decidedly more ambitious. The pairing of random mutation and natural selection tries to account for the differences between creatures. It tries to answer the pivotal question, What could cause such staggering transformations? How could one kind of ancestral animal develop over time into creatures as different as, say, bats and whales?

Lets tease apart that compound concept. First, consider natural selection. Like common descent, natural selection is an interesting but actually quite modest notion. By itself, the idea of natural selection says just that the more fit organisms of a species will produce more surviving offspring than the less fit. So, if the total numbers of a species stayed the same, over time the progeny of the more fit would replace the progeny of the less fit. Its hardly surprising that creatures that are somehow more fit (stronger, faster, hardier) would on average do better in nature than ones that were less fit (weaker, slower, more fragile).

By far the most critical aspect of Darwins multifaceted theory is the role of random mutation. Almost all of what is novel and important in Darwinian thought is concentrated in this third concept. In Darwinian thinking, the only way a plant or animal becomes fitter than its relatives is by sustaining a serendipitous mutation. If the mutation makes the organism stronger, faster, or in some way hardier, then natural selection can take over from there and help make sure its offspring grow numerous. Yet until the random mutation appears, natural selection can only twiddle its thumbs.

Random mutation, natural selection, common descentthree separate ideas welded into one theory. Because of the welding of concepts, the question, Is Darwinism true? has several possible answers. One possibility, of course, is that those separate ideascommon descent, natural selection, and random mutationcould all be completely correct, and sufficient to explain evolution. Or, they could all be correct in the sense that random mutation and natural selection happen, but they might be inconsequential, unable to account for most of evolution. Its also possible that one could be wholly right while the others were totally wrong. Or one idea could be right to a greater degree while another is correct to a much lesser degree. Because they are separate ideas, evidence for each facet of Darwins theory has to be evaluated independently. Previous generations of scientists readily discriminated among them. Many leading biologists of the late nineteenth and early twentieth centuries thought common descent was right, but that random mutation/natural selection was wrong.

In the past hundred years science has advanced enormously; what do the results of modern science show? In brief, the evidence for common descent seems compelling. The results of modern DNA sequencing experiments, undreamed of by nineteenth-century scientists like Charles Darwin, show that some distantly related organisms share apparently arbitrary features of their genes that seem to have no explanation other than that they were inherited from a distant common ancestor. Second, theres also great evidence that random mutation paired with natural selection can modify life in important ways. Third, however, there is strong evidence that random mutation is extremely limited. Now that we know the sequences of many genomes, now that we know how mutations occur, and how often, we can explore the possibilities and limits of random mutation with some degree of precisionfor the first time since Darwin proposed his theory.

As well see throughout this book, genetic accidents can cause a degree of evolutionary change, but only a degree. As earlier generations of scientists agreed, except at lifes periphery, the evidence for a pivotal role for random mutations is terrible. For a bevy of reasons having little to do with science, this crucial aspect of Darwins theorythe power of natural selection coupled to random mutationhas been grossly oversold to the modern public.

In recent years Darwins intellectual descendants have been aggressively pushing their idea on the public as a sort of biological theory-of-everything. Applying Darwinian principles to medicine, they claim, tells us why we get sick. Darwinian psychology explains why some men rape and some women kill their newborns. The penchant for viewing the world through Darwinian glasses has spilled over into the humanities, law, and politics. Because of the rhetorical fog that surrounds discussions of evolution, its hard for the public to decide what is solid and what is illusory. Yet if Darwinisms grand claims are just bluster, then society is being badly misled about subjectsranging from the cause of illnesses to the culpability of criminalsthat can have serious real-world consequences.