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Truth and Consequences

WHY EVOLUTION IS TRUE. Jerry A. Coyne. xxii + 282 pp. Viking, 2009. $27.95.

As a professional evolutionary biologist, I am sometimes asked to debate creationists—or as they are known in their newest incarnation, advocates of “intelligent design.” These invitations create a dilemma for me. The opportunity to communicate the excitement of modern evolutionary biology and to expose the vapid and depressingly anti-intellectual character of creationism is almost irresistible. But I am wary of lending credence to the idea that theological explanations for the nature of the living world are a worthy alternative to scientific ones.

I therefore tend to decline these debate invitations, but I am otherwise eager to speak about evolution to any audience that will have me, as are most of my colleagues. We’re excited about contemporary evolutionary biology. And we’re reluctant to give up on the idea that we can still make the undecided see the elegance and profundity of evolutionary thinking—even though we have learned that individuals committed to literal interpretations of religious texts will not be swayed by evidence, no matter how impressive it is or how eloquently presented. We know that the intelligent-design agenda is one manifestation of a much larger political movement that is based in part on the yearning for a simpler world of absolute certainties. And still we want to convey the exquisite nature of the workings of evolution, for we recognize that this debate is not simply a conflict between competing narratives about the origin and history of life, but a struggle about the role of reason in our understanding of ourselves and of the material world.

It is with this hopeful mind-set that Jerry Coyne appears to have written Why Evolution Is True, an engaging and methodical account of the evidence that has accumulated over the past 150 years in support of Darwin’s “Big Idea.” The book’s premise is that evolution is not only the simplest explanation of the known facts but also the best-supported and most illuminating scientific notion around today.

From the outset, Coyne separates the claim that evolution occurs—that species are not immutable and are all part of an unbroken genealogical web—from the question of what the mechanisms are that drive it. (The most notable mechanism, of course, is the process of natural selection.) Darwin underscored the same distinction in On the Origin of Species, knowing full well that the Victorian establishment needed first to be convinced of the reality of constant organic change, then to be reminded of the power of selective breeding to effect change, and finally to be introduced to the concept of natural selection.

Coyne spends the first four chapters of the book reinforcing the claim that evolution occurs. He reminds us that the evidence is everywhere. A stunning archive of fossil forms has been unearthed over the past several decades. Scientists have been astonished at the diversity and variety of fossils that date to the boundary between the Precambrian and Cambrian eras (some 570 million years ago)—the point at which complex multicellular life really began its romp. In much younger deposits, the hominin fossils from the past 5 million years of the record have led to a far more complex picture of our own evolution. And throughout the record we have uncovered many of the transitional forms that evolutionary logic predicted. What is surprising, of course, is not that the predicted transitional forms exist, but that through a combination of skill, effort and luck we have recovered them—from Tiktaalik roseae (which embodies the transition from lobe-finned fish to land-dwelling tetrapods 375 million years ago) to Rodhocetus balochistanensis (one of the transitional forms that, 330 million years later, mark the return trip to the ocean of a branch of land mammals—the branch that gave rise to whales).

The evidence for evolution from other facets of the modern life sciences is no less compelling. Biologists sequencing their way through the genomes of creatures large and small have found that organisms separated by vast evolutionary gulfs have many of the same genes, which were apparently bequeathed to their divergent lineages by a shared common ancestor. In some cases, the genes perform similar tasks in every lineage in which they are found. In others, the same genes are put to different uses. And sometimes, genes are mothballed by evolution; freed from the constant scrutiny of selection, these pseudogenes decay into frayed versions of their former selves.

Coyne also describes features whose presence is difficult to account for: an atavistic tail on a human infant, vestigial hindquarters on snakes and whales, human nerves that travel from the brain to the larynx via the aorta. Creationists dismiss these anomalies as the work of an inscrutable designer. But, more rationally, we can subsume all such features within a single, testable and coherent framework: descent with modification (Darwin’s preferred term for evolution).

The claim that organisms evolve did not originate with Charles Darwin. Rather, it ran in the family: His grandfather Erasmus Darwin was one of many who argued for change as a fact of life. The radical claim made in On the Origin of Species was that natural and sexual selection propel that change. Coyne provides a lucid, if conventional, account of the evidence in support of selection as the shaping force behind evolutionary change. This middle section of the book ably demonstrates that heritable variation in combination with the link between particular variants and reproductive success invariably leads to change in the genetic composition of populations. Evolution is not a hypothetical outcome, but the inexorable consequence of the differential reproduction of genetically variable individuals.

But is all evolutionary change really adaptive? The bar should be set high for any claim that a particular feature of organisms is an adaptation. I wish Coyne had discussed more thoroughly the factors that constrain adaptation. Many forces other than selection (chance, or the role of speciation and extinction, for example) can propel traits to dominance and can account for the patterns in the fossil record.

To my surprise, Coyne barely mentions the many insights flowing from the comparative study of development at the molecular level. I wish he had given more attention to active controversies in our field: Whether adaptation is ubiquitous, whether evolutionary change is necessarily gradual and imperceptible, how to evaluate the relative roles of chance and selection in molding the world as we see it. Advocates of intelligent design seek in vain to portray any disagreement among evolutionists as evidence of a “theory in crisis” or “the end of Darwinism.” They do not understand that ferment and debate are the very heartbeat of science. Scientists are not discussing the reality of evolution; they are discovering its underpinnings and implications.

Given the many contributions Coyne’s lab has made to our understanding of speciation, it is not surprising that this book is at its strongest when discussing the mechanisms that underlie the diversity of life. Darwin knew that accounting for the variety of life forms populating our planet was at least as important as accounting for the apparent fit between organisms and their environments. In the absence of a theory of inheritance, however, there was little hope in his day for a comprehensive theory of diversification. More than 90 years later, Darwin’s theory of organic change was merged with Mendel’s theory of genetic transmission in the aptly named Modern Synthesis. Since that time, as Coyne details, we have come to understand the conditions that initiate the process of speciation, the forces that confirm it, and the consequences that follow from the reproductive isolation of gene pools.

This understanding of speciation has come about through painstaking observation and experimentation. Coyne conveys the power of close observation to illuminate phenomena when he discusses with affection the two species of Drosophila that he has been studying for the past decade on São Tomé, a volcanic island off the west coast of Africa. And when he describes with glee the antiaphrodisiac that a male fruit fly injects into a female during insemination to deter other males from mating with her, or when he discusses the “backward-pointing spines” on the penises of certain damselflies (used to “scoop out” the sperm of previous suitors), Coyne manifests the capacity for wonder that drives every scientist I know.

Why Evolution Is True is much more than a series of stunning examples of evolution at work. Coyne reminds us that the science of evolution today is not simply a collection of observations about the past. Although not predictive in the usual sense, evolutionary theory constantly makes predictions about the distribution of organisms in space and time—about the composition of flora and fauna to be found on continental and volcanic islands, for example. Evolution speaks to us about the relationships among organisms, about the content of genomes, about the response of bacteria to antibiotic overuse. And time and again, predictions based on the theory of evolution are borne out, suggesting that we hold in our hands a tool of immense explanatory power.

Will this book, or any book, finally make creationists see the light? Coyne is an optimist, but he is not naive. In the last chapter, he writes eloquently of the immense philosophical implications of the evolutionary worldview. Accepting the reality of evolution does not lead to a life of animalistic abandon, as the religious right would have it. “Although evolution operates in a purposeless, materialistic way,” Coyne points out,

that doesn’t mean that our lives have no purpose. Whether through secular or religious thought, we make our own purposes, meaning and morality.

I remain convinced that a commitment to evolution as the explanation for life on Earth is not incompatible with an equally strong commitment to religious belief as an organizing principle for personal behavior. But the insights from evolution, cosmology, physics, statistics, geology and more do require us to swallow hard. For modern science brings us face to face with the fact that our presence on Earth may, after all, be no more than an immense accident. Nevertheless, we have been endowed, however accidentally, with self-awareness and the power to understand our own origins. As this book makes clear, there is grandeur in that power.

Robert L. Dorit is an associate professor in the Department of Biological Sciences at Smith College. His work focuses on experimental evolution of molecules and bacteria, and on the design of novel antibiotics.

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