Why are there so many kinds of organisms (but especially salamanders)?

The vitality and dynamism of contemporary evolutionary biology owe much to the efforts and creativity of David Wake, who died of renocardiac syndrome on April 29, 2021. He was 84 years old. As one of the foremost biologists of his generation, Dave pursued a highly synthetic, integrative research program that sought to explain the evolution of diversity. But he maintained a particular focus on processes, such as organismal development, that had been underemphasized, if not entirely neglected, by the modern synthesis achieved in the middle of the 20th century. Over the last half of his career, Dave drew the world’s attention to the precipitous decline of amphibian populations, including his beloved (and it was love) salamanders of the family Plethodontidae, and promoted and organized a global scientific response to diagnose and ultimately address the problem.

David Wake. Reprinted with the permission of The Museum of Vertebrate Zoology, University of California, Berkeley.

Dave was born on June 8, 1936, in Webster, South Dakota. He was raised in nearby Pierpont, a small farming community—326 people according to the 1940 census—filled mostly with Norwegian immigrants (1, 2). Education was highly prized; of the 11 students in his high school class, 3 went on to earn doctorates and become college professors. Before Dave’s final year, the family relocated to Tacoma, Washington, where he and two siblings could attend Pacific Lutheran College (now Pacific Lutheran University) while living at home, the only financial arrangement his parents could afford. It was here, while collecting insects for an entomology course, that Dave discovered plethodontid salamanders, but especially Ensatina, a remarkable species endemic to the Pacific Coast ranges and Sierra Nevada that is best known to evolutionary biologists as a classic example of Rassenkreis, or ring species (3). Any prior fascination with insects, or even plants, was quickly cast aside; salamanders would preoccupy Dave for the next 60+ years, including more than a dozen papers that analyzed geographic variation in Ensatina and its implications for our understanding of speciation.

After graduating in 1958, Dave moved on to the University of Southern California, where he worked with Jay Savage for both master’s (1960) and doctoral (1964) degrees. Savage, a tropical ecologist and biogeographer, was at that time launching a broad research effort on amphibians and reptiles of Central and South America, and he promoted Dave’s interests in plethodontids, which have their principal species diversity in the Neotropics. Savage’s close association with nearby Los Angeles County Museum of Natural History also introduced Dave to research collections in herpetology and facilitated his early studies in taxonomy and systematics. It was at the University of Southern California, too, that Dave would meet Marvalee Hendricks, then a premed undergraduate, who chose instead to pursue a doctorate with Savage studying caecilians, the least well known of living amphibians. Dave and Marvalee celebrated their 58th wedding anniversary in 2020.

Dave led a charmed existence as an academic. Nearing the end of graduate school, but at a time when most faculty searches weren’t advertised and on-site job interviews were rare, he mailed letters to a host of colleges and universities simply asking if they had a job for which he would qualify. He quickly received seven offers, including one from the University of Chicago, which he accepted. His move to Berkeley 5 years later began with a phone call inviting him to fly out and give a seminar. The morning he arrived on campus, his host casually mentioned that the Museum of Vertebrate Zoology (MVZ) had a vacancy, and before his visit ended the Zoology Department had voted to offer him a job as assistant professor of zoology and assistant curator at MVZ. A few days later, now back in Chicago, Dave reluctantly declined the offer, citing the 4 years he’d accumulated toward tenure at Chicago and Marvalee’s academic appointment at the University of Illinois, Chicago. Two weeks later, his seminar host called back to say that Berkeley would offer Dave a tenured position. Within a year, Dave and Marvalee would swap the South Side for the Berkeley Hills, and soon Dave was appointed director of MVZ, a position he would hold for the next 27 years. Later, Marvalee would receive her own faculty appointment in zoology and ultimately chair the department.

In the purest terms, Dave’s research addressed a fundamental question in evolutionary biology: Why are there so many different kinds of organisms? His work remains influential because it exemplifies a comprehensive, multidimensional approach to analysis of evolutionary diversification (4). It simultaneously considered a wide range of biological attributes, from molecules to morphology, including behavior, biogeography, and ecology, all underlain by careful attention to phylogenetic relationships and species-level taxonomy. For nearly half a century following the modern synthesis crafted by Mayr, Simpson, Dobzhansky, and other evolutionary all-stars in the 1930s and 1940s, extrinsic factors were regarded as the primary determinants of evolutionary change. This view emphasized, for example, the role of the external environment in mediating natural selection for specific adaptations. Dave’s work highlighted the need to also consider intrinsic factors, such as lineage-specific developmental traits or conserved anatomical features, which may both facilitate and constrain diversification in significant ways, often in opposition to natural selection (5, 6). Achieving a satisfactory understanding of the evolution of any taxon requires such a holistic approach, which incorporates all relevant information. Indeed, largely through the efforts of Dave and his numerous students and collaborators, plethodontid salamanders are one of the most comprehensively investigated and fully documented instances of adaptive radiation in the history of evolutionary biology, a model taxon (7, 8).

Why salamanders? To study diversification, evolutionists look for lineages whose species collectively exhibit a broad range of size, shape, physiology, ecology, and so forth, features that they can really sink their analytical teeth into (think Darwin’s finches or, better yet, Australian marsupials). However, you don’t want to choose a taxon that has so many species or different forms that it’s impossible to wrap your arms around it, one that is beyond the ability of a single researcher or even an organized research group. Plethodontids, while constituting only one of the world’s 10 salamander families, comprise nearly two-thirds of the world’s salamander species. Plus, they’re found on four continents and are tremendously diverse in form and function. Dave knew all of this when he chose them, but he had another, more pragmatic reason: “At that time there were about as many species as there were people in my home town and I knew I could easily remember that many names and a great deal about each one” (9).

Dave’s passion for biology, but especially salamanders, was infectious. If you sat next to him during dinner or a long car ride and asked what’s new, it would take only a few minutes before you’d be willing to drop everything and commit the rest of your life to studying plethodontids. He also had a knack for seeing things on the horizon before other people did, of sensing important trends that others had missed. Such was the case with global amphibian declines, which Dave brought to the world’s attention beginning in 1989. At the first World Congress of Herpetology, held that summer in Canterbury, England, Dave heard numerous anecdotal accounts of the palpable decline, or even disappearance, of natural populations of several amphibian species around the world, a phenomenon he’d seen afflicting plethodontids during fieldwork in Central America over the preceding 10 years. Yet, no one had suggested that this was a global phenomenon, let alone one that deserved a coordinated response. Fortuitously, Dave returned to Berkeley via Washington, DC, to attend a meeting of the National Academy of Sciences’ Board on Biology, where he shared what he’d learned in Canterbury as a topic deserving urgent attention. In response, only a few months later the National Research Council convened a meeting in Irvine, California, to highlight the problem and recommend next steps. The meeting attracted tremendous press coverage of global amphibian declines, which remained a focus of Dave’s activities for the rest of his life (10).

I joined Dave’s laboratory as a beginning doctoral student in 1974; not surprisingly, I worked on plethodontid salamanders. Only with his passing did I stop to calculate that we had maintained our collaboration for the next 47 years. Like so many of his more than 80 graduate students and postdocs, I regularly called on him for professional (and sometimes personal) advice. My favorite memory is from spring 1983, when I phoned Dave from Halifax, Nova Scotia, where I was a postdoc, to relay the news that I’d just received a call from the University of Colorado offering me a faculty position, but with all pertinent details (salary, start-up, space, and such) still to be determined. Dave’s response: “You said yes, didn’t you?” So much for his training me in hardball negotiating tactics.

Thanks to David Wake, we have a richer understanding of the mechanisms that mediate diversification of lineages and an appreciation of the benefits of embracing a model-taxon approach exemplified by his lifelong study of plethodontid salamanders. The onus now falls on the rest of us to help conserve Earth’s biological heritage, which he held so dear.