Sixty years ago a Carnegie Museum entomologist studied natural hybrid moths, and today his work on the fundamental process in evolution-speciation-is considered a classic.
The soft fluttering noise of their wings against the screen sides of the trap awakened me. Peering out of my sleeping bag, I could see the ghostly image of several large moths, distinctively marked with white bands and large eyespots against a background of rich burgundy. New arrivals gently drifted in, and then in a frenzy began flying and crawling around the caged female. Incredibly, these western relatives of the familiar eastern Cecropia moth are able to fly in the early summer predawn cold of the Bitterroot Mountains of Idaho and Montana. Their furry bodies hold in heat generated by flight muscles as they pursue the pheromone trails released by unmated females. But, beyond this surreal spectacle of flying creatures, it was the shape of the eyespots that arrested my sleepy concentration. I could tell even in the dim light that there was something unique about these beautiful insects.
I was first drawn into pursuing this particular group of wild silk moths a quarter of a century earlier by a small book, a monograph published by the Carnegie Museum, stapled between drab cardboard covers but telling a story that I found exciting as a bookish high-school student fascinated by collecting and rearing moths and butterflies. During the Depression the author, Walter R. Sweadner, undertook what must have been a difficult and probably dangerous journey. Driving a Model T truck alone from Pittsburgh to Coeur d'Alene, Idaho, he traveled along crude roads to collect these wild silk moths in rugged mountain terrain. Today, along interstate highways one can retrace Sweadner's journey, casually stopping to read historical markers describing the hardships of the Lewis and Clark expedition which traversed part of the same route. At Lookout Pass, where Sweadner made his base camp, a ski shop now offers café latte to weekend visitors from Seattle and Spokane.
Sixty years ago the loggers and miners Sweadner met along his route must have found his protective gear, collecting net and traveling laboratory a humorous diversion from their daily hardships. But, with no effective treatment for Rocky Mountain spotted fever available at that time, Sweadner knew that a bite from an infected tick could cost him his life. Prior to the young scientist's departure, a newspaper article entitled "Young Sharpsburg Scientist to Risk Life in Studies" included a photograph of Sweadner encased in a cumbersome outfit, a heavy coat with special elastic neck and wrist closures, long pants with leggings, high-top leather boots, and a gauze veil draped over his hat. Such garb in the early summer sun must have been unbearably hot, and Sweadner probably dressed more casually during most of his field work, keeping a watchful eye out for ticks. Later in his career, his early precautions against ticks yielded an unexpected reward.
Sweadner believed the moths he studied in the Bitterroot Mountains were natural hybrids, formed as their parent species reinvaded the Northwest after the Ice Age from their southern refuges in the Sierra Nevada of California, and from the southern Rocky Mountains. The West Coast species, known as the Ceanothus Silk Moth, is a bright red-brown with long, comma-shaped eyespots on its hind wings. The Rocky Mountain species is similar but easily distinguished by its darker wine-red color and small, kidney- shaped eyespots. Sweadner carefully measured the shape of these eyespots and recorded the wing colors, and plotted out figures and charts showing that the Bitterroot moths were intermediate between these two species for these characters. He also matched his wild naturally hybrid Bitterroot specimens with laboratory hybrids produced by the parental species.
His classic paper is the first detailed study of what are now called "hybrid zones."1 Hybrid zones intrigue evolutionary biologists because they occur only when two populations just on the verge of evolving into discrete species meet by chance and interbreed in nature. Matings between more closely related forms would likely produce only a subtle blending or intergradation of those features taxonomists might otherwise use to classify them as a separate species. True hybrid zones have distinct borders where such characters change abruptly. If allowed to evolve separately for a greater length of time, a contact between the same two forms would allow them to coexist side by side without interbreeding.
Ironically, in 1937, the same year Sweadner's monograph appeared, the noted Russian-born geneticist Theodosius Dobzhansky published Genetics and the Origin of Species, a landmark book on evolution. At the time the insights of Dobzhansky's book were unavailable to Sweadner, but subsequent editions cited Sweadner's work as a study of speciation in progress. As Darwin stressed in his Origin of Species, the formation of species is the fundamental branching point in evolution. Yet, without an understanding of the nature of inheritance, Darwin believed that the concept of the species was essentially a naturalist's abstraction, due to the gradual nature of evolution and the continuous variation seen among geographic races of many organisms. Dobzhansky defined species in terms of natural populations, united by the ability to interbreed and produce offspring, but reproductively isolated from closely related groups.
Sweadner's work was significant because he revealed that during speciation two forms can evolve many obvious physical differences, yet retain the ability to interbreed and produce hybrids in the wild.
He recognized that in these cases the concept of the species is arbitrary: "Man has always tried to classify the many and diverse forms of life into discrete groups. When the species...tend to intergrade so that it becomes impossible to fix limits...a new basis for differentiation is proposed." In his concluding remarks he notes that, "The genus...is an excellent example to illustrate the futility of attempting to set up rigid universal criteria for defining the limits of species." Sixty years later controversy continues to stimulate intense research on the process of speciation, and hybrid zones are seen as important laboratories for this work.
Sweadner's path in Pittsburgh soon crossed that of another Russian émigré biologist, Andrey Avinoff, who was born into wealth, and once served a diplomatic role in the czar's court.2 Sweadner was born in 1903 in more humble circumstances in Beaver, Pennsylvania, where he graduated from high school. In 1927 he earned a bachelor's degree in electrical engineering from Carnegie Tech. During college he suffered from rheumatic fever and spent a year recovering in Long Beach, California, with his aunt and uncle. For relaxation he resumed an early interest in collecting butterflies, and upon his return to Pittsburgh changed his major to biology, earning a master's degree in biology in 1931, and a Ph.D. in 1934 based on his research with hybridization in wild silk moths. Sweadner met his future wife, Marie Whaling, at the Erie campus, where he taught biology (from 1935 until 1941) and where she was an instructor in English. They were married in June 1941 and in July he was offered $2500 per year to be a curator of Entomology at the Carnegie Museum. He continued as an assistant professor of Zoology at the University of Pittsburgh.
During the war years, in addition to his other responsibilities, Sweadner taught the mathematics of navigation to Army Air Corps trainees, putting in 70 to 80 hours a week on occasion. Eventually, this stressful schedule took its toll and Sweadner suffered a heart attack. Although he recovered completely, Marie told me during an interview that his sacrifices made him "as much a war casualty as anyone." After the war, the Sweadners bought a house in the Point Breeze section of Pittsburgh. Remodeling the house was an escape from the tedium of his profession, and Sweadner, an accomplished craftsman, built custom cabinetry, including unusual kitchen cabinets with roll-up doors, a central heating system built into the various fireplace chimneys, and replicas of early American furniture built without modern fasteners. Examples now in Marie Sweadner's home in southern California include a bedstead beautifully decorated with inlaid designs of butterflies and a table with an intricate spiral mosaic of wood pieces. Sweadner housed his personal Lepidoptera collection of 40,000 specimens in cabinets and drawers of his own making.
Sweadner gained a reputation as a scientist and educator, and worked hard to promote Carnegie Museum in exhibits he helped create. He wrote several articles for Carnegie Magazine and stories about him appeared in the local papers over the years. Through their mutual interest in Lepidoptera, Sweadner and Avinoff became close associates. Avinoff had earlier served on Sweadner's thesis committee and was director of the Natural History Museum from 1928 to 1945. On March 1, 1941, Avinoff appointed Sweadner assistant curator of Entomology and advanced him to curator the next year. The noted lepidopterist Charles Remington wrote of Sweadner concerning this period that "He was faced with the heavy task of organizing and arranging the lepidoptera collection-left in a chaotic state by its principal assembler, W. J. Holland."
Avinoff had earlier amassed a valuable and unique collection of Old World butterflies, including an extensive series from Central Asia. Forced to leave Russia following the revolution, Avinoff had to abandon most of his possessions, and his Lepidoptera collection was appropriated by the Soviets. Following World War II, negotiations led to a plan to retrieve the collection, the trip to be funded by the prominent Mellon family, but Cold War tensions caused the Soviets to renege. Through trading and purchase Avinoff managed to build up a near duplicate collection, most of which was donated to Carnegie Museum.
Among these specimens were long series of Karanasa, a satyrid butterfly from the mountains of Tibet and nearby regions. The pattern of geographic variation in this group suggests an interesting evolutionary history. Repeated cycling of glaciation followed by periods of warmth during the Pleistocene epoch appears to have continually split these butterflies into small populations, isolated from each other by rugged terrain. During warmer periods these populations expanded and interbred with their neighbors.
The Karanasa were another example of hybridization among borderline species in mountainous terrain, and Sweadner drew upon his earlier work to begin an eight-year collaboration with Avinoff on an extensive monograph of the systematic relationships among these butterflies. Sweadner's analytic and quantitative approach, based on his engineering background, contrasted with and complemented Avinoff's worldly, more philosophical intellect. Both men were unfettered by the narrow viewpoint of many contemporary taxonomists, who regarded animal species as essentially uniform throughout their range, and unchanging over time until replaced by a new species. In April 1945 Avinoff suffered a heart attack, and had to retire from the museum. Although he continued work on the monograph as its principal author, it was still unfinished when Avinoff died in July 1949.
Using Avinoff's notes and manuscripts, Sweadner completed the Karanasa monograph in 1950. He was aided in his efforts to secure funding for this project by an unlikely agent, a lowly wood tick. A prominent trustee of the Carnegie Museum, James C. Rea, brought in a tick that had fastened itself to his skin. He was referred to Sweadner, who knew from his research in connection with preparations for the Bitterroot field trip that the tick was not of a type that transmitted serious disease. After reassuring the man, the two exchanged pleasantries and Sweadner discussed his work at the museum, including his project with the Karanasa. Later during a meeting with fellow prominent Pittsburghers, Rea was asked if he knew of any useful museum projects in need of funding. He related the story of Walter Sweadner and the tick, and subsequently a grant was given to Carnegie Museum by Mrs. Henry R. Rea to support the publication of the Karanasa monograph. Today the legacy of the "tick grant" continues in the form of the Rea Postdoctoral Fellowship.
Near the end of this project, Sweadner's health deteriorated and he was diagnosed with colon cancer. Although an operation to remove the cancer was considered successful, a postoperative infection led to complications and Sweadner was again hospitalized. Proofs of the color plates of the Karanasa paper were brought to his bedside for correction and comment. Despite a rushed publication schedule, the book was not printed and bound until just after Sweadner's death, at age 47, on January 13, 1951. Marie Sweadner eventually moved to California, where she resumed her teaching career and raised their two children. Retired now, she is an avid amateur naturalist and bird watcher.
Just four years after Sweadner's death the chemical structure of DNA was discovered, and the new discipline of molecular genetics began to develop. With the advent of computers, population geneticists were able to study the origin and maintenance of the unexpected genetic variation the molecular geneticists were finding in natural animal populations. By the late 1960s the concept of the population as the unit of evolution began to be accepted, just as Sweadner and Avinoff had proposed for the Karanasa. Hybrid zones were now seen as natural laboratories for the study of the process of speciation.
Inspired by Sweadner's earlier work, I discovered and analyzed a hybrid zone between the same two silk moths on the east slope of the California Sierra Nevada, near Lake Tahoe. Unlike laboratory hybrids between these species, in which the female is barren of eggs, I found that natural hybrid females here and in the Bitterroot Mountains are fertile. Within the bounds of the hybrid zone, evolution has partly reversed the trend toward the formation of distinct species.
Such a discovery would have fascinated Sweadner. One wonders what contributions he would have made during our current renaissance in evolutionary biology.
Michael M. Collins is a scientific research associate of the section of Invertebrate Zoology at Carnegie Museum of Natural History. He is coauthor, with Paul Tuskes and James Tuttle, of Wild Silk Moths of North America (Cornell University Press, 1996) and is publishing his recent research on natural hybrids in the Annals of Carnegie Museum.
The museum has in all of its scientific departments a total of 149 research associates around the world, who share their research and contribute scientific information with the museum.
1. Hybridization and the phylogeny of the genus Platysamia, Annals of the Carnegie Museum, 1937.
2. "Andrey Avinoff Remembered," by Nicholas Shoumatoff, Carnegie Magazine, March/April 1995.