The Edge of Evolution Read online

  ALSO BY MICHAEL J. BEHE

  Darwin’s Black Box:

  The Biochemical Challenge to Evolution

  FREE PRESS

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  New York, NY 10020

  Copyright © 2007 by Michael J. Behe

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  Library of Congress Cataloging-in-Publication Data

  ISBN-13: 978-1-4165-5904-7

  ISBN-10: 1-4165-5904-3

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  To my parents, Joseph and Helen Behe

  Contents

  1. The Elements of Darwinism

  2. Arms Race or Trench Warfare?

  3. The Mathematical Limits of Darwinism

  4. What Darwinism Can Do

  5. What Darwinism Can’t Do

  6. Benchmarks

  7. The Two-Binding-Sites Rule

  8. Objections to the Edge

  9. The Cathedral and the Spandrels

  10. All the World’s a Stage

  Appendix A—I, Nanobot

  Appendix B—Malaria Drug Resistance

  Appendix C—Assembling the Bacterial Flagellum

  Appendix D—The Cardsharp

  Notes

  Acknowledgments

  Index

  1

  THE ELEMENTS 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 creature’s 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.

  That’s the claim, at least. But is it true? To answer that question, Darwin’s theory has to be sifted carefully, because it isn’t just a single concept—it 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 giraffes—two mammals—are 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 categories—buffalo and buzzards, pigs and petunias, yaks and yeast.

  That’s certainly startling, so it’s 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 beginning—the ancestor had them. But all by itself, it doesn’t 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, Darwin’s hypothesized mechanism of evolution—the compound concept of random mutation paired with natural selection—is 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?

  Let’s 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. It’s 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 Darwin’s 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 descent—three 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 ideas—common descent, natural selection, and random mutation—could 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. It’s 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 Darwin’s 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, there’s 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 precision—for the first time since Darwin proposed his theory.

  As we’ll 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 life’s 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 Darwin’s theory—the power of natural selection coupled to random mutation—has been grossly oversold to the modern public.

  In recent years Darwin’s 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 k
ill 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, it’s hard for the public to decide what is solid and what is illusory. Yet if Darwinism’s grand claims are just bluster, then society is being badly misled about subjects—ranging from the cause of illnesses to the culpability of criminals—that can have serious real-world consequences.

  As a theory-of-everything, Darwinism is usually presented as a take-it-or-leave-it proposition. Either accept the whole theory or decide that evolution is all hype and throw out the baby with the bath water. Both are mistakes. In dealing with an often-menacing nature, we can’t afford the luxury of elevating anybody’s dogmas over data. The purpose of this book is to cut through the fog, to offer a sober appraisal of what Darwinian processes can and cannot do, to find what I call the edge of evolution.

  THE IMPORTANCE OF THE PATHWAY

  On the surface, Darwin’s theory of evolution is seductively simple and, unlike many theories in physics or chemistry, can be summarized succinctly with no math: In every species, there are variations. For example, one animal might be bigger than its brothers and sisters, another might be faster, another might be brighter in color. Unfortunately, not all animals that are born will survive to reproduce, because there’s not enough food to go around, and there are also predators of many species. So an organism whose chance variation gives it an advantage in the struggle to survive will tend to live, prosper, and leave offspring. If Mom or Dad’s useful variation is inherited by the kids, then they, too, will have a better chance of leaving more offspring. Over time, the descendants of the creature with that original, lucky mutation will dominate the population, so the species as a whole will have changed from what it was. If the scenario is repeated over and over again, then the species might eventually change into something altogether different.

  At first blush, that seems pretty straightforward. Variation, selection, inheritance (in other words, random mutation, natural selection, and common descent) seem to be all it takes. In fact, when an evolutionary story is couched as abstractly as in the previous paragraph, Darwinian evolution appears almost logically necessary. As Darwinian commentators have often claimed, it just has to be true. If there is variation in a group of organisms, and if the variation favorably affects the odds of survival, and if the trait is inherited, then the next generation is almost certain to have more members with the favorable trait. And the next generation after that will have even more, and the next more, until all members of the species have it. Wherever those conditions are fulfilled, wherever there is variation, selection, and inheritance, then there absolutely must be evolution.

  So far, so good. But the abstract, naive logic ignores a huge piece of the puzzle. In the real world, random mutation, natural selection, and common descent might all be completely true, and yet Darwinian processes still may not be an adequate explanation of life. In order to forge the many complex structures of life, a Darwinian process would have to take numerous coherent steps, a series of beneficial mutations that successively build on each other, leading to a complex outcome. In order to do so in the real world, rather than just in our imaginations, there must be a biological route to the structure that stands a reasonable chance of success in nature. In other words, variation, selection, and inheritance will only work if there is also a smooth evolutionary pathway leading from biological point A to biological point B.

  The question of the pathway is as critical in evolution as it is in everyday life. In everyday life, if you had to walk blindfolded from point A to point B, it would matter very much where A and B were, and what lay between. Suppose you had to walk blindfolded (and, to make the example closer to the spirit of Darwinism, blind drunk) from A to B to get some reward—say, a pot of gold. What’s more, suppose in your sightless dizziness the only thought you could hold in your head was to climb higher whenever you got the chance (this mimics natural selection constantly driving a species to higher levels of fitness). On the one hand, if you just had to go from the bottom of a single enclosed stairwell to the top to reach the pot of gold, there might be little problem. On the other hand, if you had to walk blindfolded from one side of an unfamiliar city to the top of a skyscraper on the other side—across busy streets, bypassing hazards, through doorways—you would have enormous trouble. You’d likely stagger incoherently, climb to the top of porch steps, mount car roofs, and so on, getting stuck on any one of thousands of local high points, unable to step farther up, unwilling to back down. And if, just trying to climb higher whenever possible, you had to walk blindfolded and disoriented from the plains by Lubbock, Texas, to the top of the Sears Tower in Chicago—blundering randomly over flatlands, through woods, around canyons, across rivers—neither you nor any of billions of other blindfolded, disoriented people who might try such a thing could reasonably be expected to succeed.

  In everyday life, the greater the distance between points A and B, and the more rugged the intervening landscape, the bleaker are the odds for success of a blindfolded walk, even—or perhaps especially—when following a simple-minded rule like “always climb higher; never back down.” The same with evolution. In Darwin’s day scientists were ignorant of many of the details of life, so they could reasonably hope that evolutionary pathways would turn out to be short and smooth. But now we know better. The great progress of modern science has shown that life is enormously elegant and intricate, especially at its molecular foundation. That means that Darwinian pathways to many complex features of life are quite long and rugged. The problem for Darwin, then, as with a long, blindfolded stroll outdoors, is that in a rugged evolutionary landscape, random mutation and natural selection might just keep a species staggering down genetic dead-end alleys, getting stuck on the top of small anatomical hills, or wandering aimlessly over physiological plains, never even coming close to winning the biological pot of gold at a distant biological summit. If that is the case, then random mutation/natural selection would essentially be ineffective. In fact, the striving to climb any local evolutionary hill would actively prevent all drunkards from finding the peak of a distant biological mountain.

  This point is crucial: If there is not a smooth, gradually rising, easily found evolutionary pathway leading to a biological system within a reasonable time, Darwinian processes won’t work. In this book we’ll examine just how demanding a requirement that is.

  A BRIEF LOOK BACK

  As a practical matter, how far apart do biological points A and B have to be, and how rugged the pathway between them, before random mutation and natural selection start to become ineffective? How can we tell when that point is reached? Where in biology is a reasonable place to draw the line marking the edge of evolution?

  This book answers those questions. It builds on an inquiry I began more than a decade ago with Darwin’s Black Box. Then I argued that irreducibly complex structures—such as some stupendously intricate cellular machines—could not have evolved by random mutation and natural selection. To continue the above analogy, it was an argument that the blindfolded drunkard could not get from point A to point B, because he couldn’t take just one small step at a time—he’d have to leap over canyons and rivers. The book concluded that there were at least some structures at the foundation of life that were beyond random mutation.

  That conclusion stirred a lot of discussion. In particular, a lot of heat was generated in the scientific community by my inference that the structures are intelligently designed. Many people are viscerally opposed to that conclusion, for a variety of reasons. In this book, although my conclusions are ultimately the same, and will undoubtedly be opposed by some, I spend the bulk of the chapters drawing on molecular evidence, genomic research, and—above all—crucial long-term studies of evolutionary changes in single-celled organisms to test Darwinism without regard to conclusions of design. Readers who cannot accept my final conclusions should still be able to consider the evide
nce presented in the bulk of these chapters, before taking issue with my conclusions in the final three chapters of the book. As I will argue, mathematical probabilities and biochemical structures cannot support Darwinism’s randomness, except at the margins of evolution. Still, as we seek to find the line marking the edge of randomness, there is no need to infer design.

  BREAKING THE LOGJAM

  Darwin’s Black Box was concerned to show just that some elegant structures in life are beyond random mutation and natural selection. This book is much more ambitious. Here the focus is on drawing up reasonable, general guidelines to mark the edge of evolution—to decide with some precision beyond what point Darwinian explanations are unlikely to be adequate, not just for some particular structures but for general features of life. This can be compared to the job of an archeologist who discovers an ancient city buried under sand. The task of deciding whether random processes produced things like intricate paintings on walls of the city buildings (perhaps by blowing sand) is pretty easy. After all, elegant paintings aren’t very likely to be made by chance processes, especially if the paintings portray not just simple geometric patterns, but images of people or animals.

  But once the cherry-picking is over, the going gets tougher. Are the dark markings at the side actually a part of a painting, or just smudges? Is a pile of stones next to an exterior wall a table or an altar of some sort, or just a random collection of rocks? Is ground near the wall the remnant of a tilled field? Where lies the border of the city? Where does civilization stop and raw nature begin? Deciding on marginal cases like those is harder work, and the conclusions will necessarily be more tentative. But at the end of the study the archeologist will be left with a much clearer picture of where the city leaves off and random natural processes take over.

  In a way, archeologists have it easy. Although they have to worry about the effects of physical processes on artifacts they study, they don’t usually concern themselves much with biological ones. In puzzling out where might lie the far boundaries of Darwinism, uniquely biological processes of course come strongly into play. Random mutations of DNA might be likened to random accidents that befall inanimate objects. But plants and animals reproduce, stones don’t. Natural selection works on living objects, not on nonliving ones. Darwin’s theory claims that random genetic accidents and natural selection working over eons will yield results that don’t look at all like the effects of chance.