An excerpt from the preface of The Making of the Fittest

Over a century ago, William Bateson began one of the first important books on evolution to appear after Darwin’s time with the exhortation:

If facts of the old kind will not help, let us seek facts of a new kind. That the time has come for some new departure most naturalists are now I believe beginning to recognize.

With DNA science penetrating so many facets of everyday life, it is again time for a new departure and to seek facts of a new kind. My goal in this book is to present a body of new facts about evolution gathered from DNA evidence. Over the past few years, biology has gained unprecedented access to a vast amount of DNA evidence from all kinds of organisms, including humans and our closest relatives. In just twenty years, the amount of DNA sequences in our databases has grown 40,000-fold, with the vast majority of that coming in this new century. To put that number in perspective, in 1982 our total knowledge of DNA sequences from all living species amounted to less than one million characters. If printed onto pages like the ones you are reading, that amount of text would fit easily into one volume about the size of this book. If all of the DNA text that we now have was printed into volumes and stacked, they would reach more than double the height of the 110 story Sears Tower in Chicago. This library of life is growing by more than 30 stories per year.

Inside these books is the raw DNA code for the making of all sorts of bacteria, fungi, plants, and animals. The decoding of these texts, composed of almost infinite permutations of sequences of just four letters, A, C, G, and T, now presents one of the greatest opportunities in the history of evolutionary biology. Biologists are mining this rich new resource to investigate and solve some of the most fascinating mysteries in natural history and to reveal, in unprecedented detail, how all sorts of important traits have evolved in nature. In this book I will tell the story of how the new science of “genomics” — the comprehensive and, most especially, the comparative study of species DNA — is profoundly expanding our knowledge of the evolution of life.

Genomics allows us to peer deeply into the evolutionary process. For more than a century after Darwin, natural selection was observable only at the level of whole organisms such as finches or moths, as differences in their survival or reproduction. Now, we can see how the fittest are made. DNA contains an entirely new and different kind of information than what Darwin could have imagined or hoped for, but which decisively confirms his picture of evolution. We can now identify specific changes in DNA that have enabled species to adapt to changing environments and to evolve new lifestyles.

This new level of understanding provides more than just definitive forensic evidence, but some surprises that expand our picture of evolution. For example, in the DNA record of every species, we find fossil genes. These are bits of DNA text that were once intact and used by ancestors, but that have fallen into disuse and decay. These relics are an entirely new source of insights into traits and capabilities that have been abandoned as species evolved new lifestyles.

The DNA record also reveals that evolution can and does repeat itself. Similar or identical adaptations have occurred in the same way in species as different as butterflies and humans. This is powerful evidence that, confronted with the same challenges or opportunities, the same solution can arise at entirely different times and places in life’s history. This repetition overthrows the notion that if we rewound and replayed the history of life, all of the outcomes would be different.

DNA evidence is also revolutionizing the study and understanding of human origins and early civilization. While the sequencing of the human genome has grabbed most of the headlines, it is the decoding of the genes and genomes of other primates and mammals that enables us to interpret the meaning of the human text. Our genes contain tell-tale clues to both how we are different and how we evolved to be so. Many genes bear the scars of natural selection — of the battles our ancestors fought with the germs that have plagued human civilization for millennia.

I wrote the book with a variety of readers in mind. First, for those with a keen interest in natural history, I will roam the planet and explain how fascinating species have adapted to boiling hot springs, caves, jungles, lava formations, the deep ocean, and other remarkable places. There is grandeur in this new knowledge of how changing one or a few letters is in simple code can dramatically change the form or physiology of complex organisms. Second, for students and teachers, I have focused on what I believe are the best examples that illustrate the key elements of the evolutionary process, and which at the same time reinforce and expand our awe for the amazing diversity and adaptability of life. Most of the stories I tell are not yet in textbooks, but many will become key chapters in evolutionary science. And third, for those trying to sift through the rhetoric and pseudoscience of evolution’s opponents, I have provided some background to understand the tactics and arguments used to doubt and deny evolutionary science, and plenty of scientific evidence to vaporize those arguments.

The new DNA evidence has a very important role beyond illuminating the process of evolution. It could be decisive in the ongoing struggle over the teaching of evolution in schools and the acceptance of evolution in society at large. It is beyond ironic to ask juries to rely on human genetic variation and DNA evidence in determining the life and liberty of suspects, but to neglect or to undermine the teaching of the basic principles upon which such evidence, and all of biology, is founded. The anti-evolution movement has relied on entirely false ideas about genetics, as well as the evolutionary process. The body of new evidence I will describe in this book clinches the case for biological evolution as the basis for life’s diversity, beyond any reasonable doubt.