Preface

Like many children, I went through a dinosaur phase. I was legendary at nursery school for showing up every day with my basket full of plastic dinosaurs: Allosaurus , Stegosaurus , Ankylosaurus , Tyrannosaurus rex . I had ’em all, or at least all of the twenty or so species available back then (kids today have it so much better).

Unlike most kids, I never grew out of the phase. I still have my toy dinos, many more now; I still know their names, can even still pronounce Parasaurolophus (pair-uh-soar-ahl-oh-fuss). But my interests have shifted to living reptiles: snakes, turtles, lizards, and crocodilians.

To a large extent, this shift was prompted by a rerun of the old TV show Leave It to Beaver , specifically the episode in which Wally and the Beave buy a mail-order baby alligator and hide it in the bathroom. Needless to say, when Minerva the housekeeper finds it, hilarity ensues. Thinking that this was a great idea and knowing that pet stores in those days (the early 1970s) sold baby caimans, the Central and South American version of the alligator, I petitioned my mother. Not being the sort of woman who likes to say no, she suggested we contact a family friend, Charlie Hoessle, the deputy director of the Saint Louis Zoo, expecting him to put the kibosh on the idea. Every day when my father returned home from work, my first question was “Did you talk to Mr. Hoessle today?” By nature not especially patient (particularly not at age ten), I went from exasperation to annoyance to anger as the days passed. What was the problem? My father was waiting to see Hoessle at a meeting, rather than simply call him up. Would that meeting never happen? Just as I was giving up all hope of a crocodilian-in-residence, my father came home one evening and announced that he’d spoken with Mr. Hoessle. “And the answer?” I asked, as I fidgeted with hope and nervousness. Then elation: Hoessle said that it was a great idea, the same way he got his start in herpetology! My mother was stuck, and soon our basement was full of all manner of reptile. I was on my way to my own career in the field.

At the same time that I was tending my scaly charges, I was also an avid reader of the monthly magazine Natural History , put out by the American Museum of Natural History in New York. A highlight of every issue was the column This View of Life by the brilliant and erudite Harvard paleontologist Stephen Jay Gould. Its name lifted from the closing sentence of Darwin’s On the Origin of Species , the column regularly explored Gould’s heterodox ideas about the evolutionary process, often stressing the indeterminate and unpredictable nature of evolution. Elegantly written and mixing in vignettes from history, architecture, and baseball, Gould presented a compelling case for his worldview.

When I was accepted to Harvard in 1980, I looked forward to learning from the great man himself in his modestly entitled non-majors course, The History of Earth and of Life. And fascinating he was, as engaging in person as he was in print. But the faculty member who made the greatest impression on me was Ernest Williams, the Curator of Herpetology at Harvard’s Museum of Comparative Zoology (the position I now hold). An imperious elder scientist, he nonetheless was very welcoming to a young upstart with an interest in reptiles. Soon I found myself studying the particular type of lizards that had been the focus of his life’s work.

Small, usually green or brown, with sticky pads on their toes and an extensible flap of colorful skin under their throats, anole lizards are photogenic and captivating in their antics. But what has shot them to fame in scientific circles is their evolutionary exuberance. Four hundred species are known, and more are being discovered every year, making Anolis one of the largest genera of vertebrate animals. This immense diversity is accomplished by great local richness—as many as a dozen or more co-occurring species—combined with regional endemicity, most species being limited to a single island or small part of the tropical American mainland.

In the 1960s, Williams’ graduate student Stan Rand documented that anole species coexist by adapting to different parts of the habitat, some living high in the tree, others in the grass or on twigs. Williams’ great insight was to realize that the same set of habitat specialists had evolved on each island in the Greater Antilles (Cuba, Hispaniola, Jamaica, and Puerto Rico). That is, the lizards had diversified independently, yet had evolved to divvy up the available habitats in almost exactly the same way on all four islands.

As an undergraduate, I worked on a small part of this story, conducting an honor’s research project on the interactions between two species in the Dominican Republic. I graduated and headed to a Ph.D. program in California, vowing never to work on these lizards again because everything important had already been discovered by Williams and his lab.

Ah, the naïveté of youth. As anyone who’s done science knows, successful projects usually answer one question, but lead to three new ones. It took me two years of graduate school and a dozen failed projects, but I finally realized island anoles are a perfect group for studying how evolutionary diversification occurs.

So I spent four years traipsing through the Caribbean, climbing trees, catching lizards, and sipping the occasional piña colada. And by the end, I had shown, using the latest analytical techniques, that Williams was exactly right. Anatomically and ecologically very similar species had evolved independently on the different islands. Moreover, my studies of biomechanics—how the lizards run, jump, and cling—revealed the adaptive basis for anatomical variation, explaining why features such as long legs or big toepads evolved for species using particular parts of the habitat.

The ink was barely dry on my dissertation when Wonderful Life: The Burgess Shale and the Nature of History , arguably Stephen Jay Gould’s greatest work, appeared in bookstores. I read it voraciously and found the argument persuasive. Evolution’s path is quirky and unpredictable, he argued; replay life’s tape again and you’d get a very different outcome.

But hold on. Gould’s conceit of turning back the clock and replaying the evolutionary tape of life is impossible (at least in nature), but another way to test the repeatability of evolution would be to play the same tape in multiple locations. Aren’t Caribbean islands, each seeded with an ancestral anole lizard, essentially the same as replaying the tape of life? Assuming that the islands have more or less the same environments, doesn’t this constitute a test of evolutionary repeatability?

Indeed it does, and I found myself in an intellectual conundrum. Gould convincingly argued that evolution shouldn’t repeat itself, yet my own research showed that it did. Was Gould wrong, or was my work the exception that somehow proved the rule? I opted for the latter explanation, embracing the Gouldian worldview even as my own work provided a counterexample.

The last quarter century has been challenging for this perspective. An intellectual counterpoint to Gould’s emphasis on unpredictability and non-repeatability has emerged. This alternative view emphasizes the ubiquity of adaptive convergent evolution: species living in similar environments will evolve similar features as adaptations to the shared natural selection pressures they experience. My Anolis lizards are an example of such convergence. Proponents of this view argue that convergence demonstrates that evolution, far from being quirky and indeterminate, is actually quite predictable: there are limited ways to make a living in the natural world, so natural selection drives the evolution of the same features time and time again.

Evolutionary biology has advanced considerably since Wonderful Life was published and I received my doctorate. New ideas, new approaches, and new methods of data collection have emerged. The number of scientists studying evolution has increased enormously. We’ve cracked the genome, mapped the tree of life, learned about the evolving microbiome. Spectacular fossil discoveries have clarified much of evolution’s history.

These data have much to say about the predictability of evolution. The more we learn about life’s history on this planet, the more we see that convergence has occurred, that very similar outcomes have evolved repeatedly. My anoles seem less exceptional, Gould’s rule more in doubt.

But we now know that there’s another way to study evolution besides documenting what has happened through the ages. We’ve discovered that we can study evolution as it occurs, right before our eyes. And that means that we actually can replay the tape by harnessing the power of the experimental method—the hallmark of laboratory science—to address the question of evolutionary predictability.

Experiments are a powerful way to study evolution. And they’re also a lot of fun. You may remember experiments from high school chemistry class. Mixing chemical reagents in beakers and pouring them into test tubes was not particularly enjoyable—at least it wasn’t for me. But when your test tubes are Bahamian islands and your reagents are lizards, it’s a completely different story. Sure, sometimes the Sun is a bit strong, and there’s nothing more frustrating than failing to catch an important lizard because you were distracted by a dolphin swimming by. But experimental evolution is at the cutting edge of evolutionary biology, allowing us to actually test our ideas about evolution, out in nature, in real time. What could be more exciting? Evolution experiments are now going on all over the world—from the montane rainforests of Trinidad to the Sandhills of Nebraska to British Columbian ponds—and they’re letting us directly investigate whether evolution can be predicted.

Oh to be a graduate student again. It’s a glorious time to be an evolutionary biologist, a golden era. With the tools available, from genome sequencing to field experiments, we can finally answer the questions that have bedeviled our field for the last century.

I set out to write a book about the ongoing work to answer one such question: how predictable is evolution? But while writing, I discovered that this book had to be about much more than just what the science is telling us. Scientific knowledge doesn’t just appear out of nowhere; it’s a result of scientists toiling away, using their creativity and insight to learn about the natural world. And the people studying evolutionary predictability are a particularly fascinating lot.

In this light, Improbable Destinies will be not just what we know about evolution, but how we know what we know. Not just the technology and theories of science, but where the ideas come from—how researchers think them up, how they are honed by experiences in the field, how much of science is the serendipitous juxtaposition of disparate ideas brought together by unexpected observations. Moreover, the seemingly esoteric academic questions they study turn out to actually matter, both to our understanding of our own place in the universe and how life around us is coping with a changing world. As a result, Improbable Destinies is a story of people and places, plants and animals, big questions and pressing problems. And it begins, like my love of the natural world, with dinosaurs.