The Evolutionary
A year after winning a MacArthur “genius” grant, UT biologist David Hillis is still tracing branching patterns on the tree of life—and chasing frogs.
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The entire Hillis family seems to have been standing near the front of the line when scientific talents were handed out: David’s mother, Argye, holds a Ph.D. in biostatistics; his younger sister, Beth, is a neurologist at Johns Hopkins, while his older brother, Danny, is a prominent computer scientist—known, among other things, for having built a computer out of Tinkertoys when he was an undergraduate at MIT and for starting the company Thinking Machines. (After the MacArthur grants were announced, the Dallas Morning News managed to contact William Hillis before he’d heard the news; when the reporter said, “I’d like to congratulate you on your son’s having won the MacArthur,” he replied, “Which one?”)
While Danny was the more serious one, David was “Huck Finn,” recalls Argye Hillis. “He was always out in the woods, turning over rocks, going fishing, having fun.” After Africa, the Hillises went on to live in Louisiana, Maryland, India, and then Maryland again, where David went to junior high and high school. William joined the faculty at Johns Hopkins, and David joined the Maryland Herpetological Society.
He would eventually publish eight scientific papers based on research he did while in high school. In one of his projects, designed to determine how environmental conditions affected the breeding patterns of salamanders, he enlisted the help of friends to construct a fence, several hundred feet long and ten inches high, by the edge of a pond where he had seen salamanders breeding the year before. The fence served to block the salamanders from entering the pond, guiding them instead into pitfall traps—cans or buckets that Hillis and his friends had buried at ten-foot intervals along the fence. From February until April of 1976, the seventeen-year-old Hillis checked the traps before school and again at dusk, counting and measuring the salamanders, noting the weather conditions, and recording these and other observations in one of the field journals he’d been keeping since junior high. (The fact that he liked to spend rainy nights watching salamanders breed, says Hillis, “probably explains why I didn’t have much of a social life in high school.”)
He chose to go to Baylor because it had a good natural history museum, and met his future wife, Ann Mackie, his first semester (“I spent the first six months trying to set him up with my roommate, since she was into frogs and so was he,” says Ann Hillis, now the director of U.T.’s Speech and Hearing Center). He also met Henry Fitch, the father-in-law of one of his professors and a University of Kansas herpetologist. In 1979, when he was twenty, Hillis took a semester off to accompany the 69-year-old Fitch on a four-month expedition to Mexico and Central America, during which they examined hundreds of species of plants and animals, including frogs of the genus Rana, which would later be the subject of Hillis’ Ph.D. dissertation.
In his study of Rana he combined more-traditional research with molecular phylogenetic methods, examining enzymes and DNA in the various frogs. It was an uncommon approach at the time, but the use of phylogenetics has since become widespread. Phylogenetic analysis is based on comparisons: Often, the reason related species share similar “characters,” such as wings or lungs, is that the present-day species are descended from a common ancestor who possessed some primitive version of that character; by looking at the species’ similarities and differences in the present, inferences about their evolutionary history can be drawn. What has changed recently is that scientists are now able to use genetic information in trying to puzzle out evolutionary relationships, comparing gene sequences and inferring how those sequences mutated over time. “So for instance, if you find out that all mammals have a particular nucleotide at a particular position [in a gene], then you can infer that the most recent common ancestor of mammals also probably had that same nucleotide at that position,” says Hillis. “If you see variation in that information, then you have to construct where the changes occurred in time.”
In other words, little differences in genes can indicate branching patterns on the tree of life. To trace those patterns requires not only recently developed methods for extracting, cloning, and sequencing genes but also considerable computing power. “We’re dealing with, oftentimes, tens of thousands of nucleotides in a given gene, and with changes across many hundreds or thousands of individuals or species, so it can be computationally really complex,” says Hillis. Much of his work is figuring out how to approach the computational problems: what the best methods are for tracing evolution by computer, and how to apply those methods to large sets of data.
The range of applications of these techniques gives the lie to the notion of evolutionary biology as a musty, museum-bound enterprise. “An unusual example of an application that people hadn’t really thought about until recently is viruses,” says Hillis. “Viruses change very quickly, and so one of the big applications of phylogenetic analysis is tracing the origin of new human diseases—looking at how they’re spreading through populations. HIV is a good example of that; it changes extremely quickly. So you can trace back in time and figure out when it came into human populations, how many times it has come into human populations, and what’s happening now. That information is really important for developing new AIDS vaccines and also can be used for actually tracing individual infections.” Hillis testified in the 1998 Louisiana criminal case against Dr. Richard Schmidt, who was accused of injecting Janice Trahan, his former lover, with HIV from one of his patients, after she ended their affair. Scientists at the University of Michigan and Houston’s Baylor College of Medicine determined the genetic sequences of virus samples from Trahan, Schmidt’s patient, and other HIV-infected individuals in the local population. Hillis then analyzed those sequences to infer the evolutionary history of Trahan’s virus strains, finding that they were indeed closely related to the patient’s strains.
On a day in February when I visited Hillis’ office, three jars were arrayed on his desk, each containing a pale, spindly-legged, flat-snouted salamander preserved in ethanol. Hillis was one of the scientists who identified the famous Barton Springs salamander, an endangered species that has played a central role in Austin’s development wars; more recently he has co-authored papers, to be published later this year, identifying several new salamander species and tracing the phylogenetic relationships among the different species of Central Texas.
The same techniques that helped incriminate Schmidt can be used to infer the history of the Edwards Aquifer, which feeds the region’s springs. Present-day salamanders are descendants of the few ancient ones who survived, millions of years ago, when the Southwest was transformed from a cool, wet region to a hot and arid one. “Most places like this, the salamanders have gone extinct,” Hillis says. “In Texas, what happened is they managed to find these little isolated cool, wet environments, mainly these springs and cave systems.” Those springs and caves might as well be desert islands as far as the salamanders are concerned, since they can’t get from one to another. So, to take one example, populations north of the Colorado River have been separated from those to the south for some twenty million years, ever since the river cut down through the limestone strata of the surrounding plateau. Determining how the different salamanders are related and how they evolved yields insights into the shaping of the land itself. “The history and relationships of these salamander populations record the whole geological history of the Edwards Plateau over the past several tens of millions of years,” says Hillis.
Biologists are finding history revealed at the molecular level, a record of life on earth inscribed in genetic material, and Hillis is at the forefront of this endeavor. “He’s an absolutely brilliant person,” says his friend Robert Baker, a mammologist at Texas Tech. “People often have said that scientists are characteristically childlike in their behavior, but of all the people that I hang around with, he is the one most like a kid. He’s like a four-year-old. He goes to work and has a great time. It’s just a joy to see how his mind works.”
Karen Olsson has written for the Texas Observer and Salon.com.
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