The University of Chicago Magazine

Naturalist Trevor Price explores how bird species emerge.

Bordering the northern perimeter of the Indian subcontinent, the Himalayan Mountains stretch more than 1,500 miles across six nations and are home to roughly eight percent of the world’s more than 10,000 bird species. In a region where tropical climes exist alongside snow-capped peaks, the number of species varies widely. Follow the range from its western edge to its eastern edge and the number of bird species doubles.

A greenish warbler in the Himalayan forest.

For the past 35 years, Chicago ornithologist Trevor Price has worked in the western Himalayas, tracing such geographic distribution to pinpoint how species originate. His study of birds began at age 18, when he hitchhiked from his hometown south of London to volunteer at the Bombay Natural History Society. Price spent two months working with forest-dwelling leaf warblers, which constitute the genus Phylloscopus. He then spent his career studying the birds. This past summer the ecology & evolution professor published Speciation in Birds (Roberts & Company), the culmination of eight years researching key questions of speciation, the evolutionary process by which new species form.

While biologists have long recognized that one species can split into two, Price tackles the intricacies involved, such as how geographic isolation, ecological diversity, and natural and sexual selection interact. His findings also serve as a jumping-off point for understanding evolutionary processes in other animals.

Drawing on fieldwork in India, laboratory analysis of bird DNA, and more than 1,000 sources, he posits a three-stage cycle of speciation: barrier, differentiation, and expansion. At the core of this process is reproductive isolation, animals’ tendency to mate only within their species. For birds, cues such as coloration or the length and frequency of a song help members of a species identify each other and restrict their mate selection to like individuals. Without reproductive isolation, says Price, the bird species in a given area would interbreed, eventually producing a single species in the region and limiting the world’s total number of species.

Barrier, the first step in the speciation cycle, occurs when a reproductively isolated group becomes divided, usually by a geographical separation such as rising waters that split a peninsula into an island. In theory, speciation can occur quickly in a single locale if food and habitat resources are diverse, explains Price, but most speciation seems to be spurred by the emergence of a barrier.

Once populations diverge geographically, differentiation—the speciation’s second stage—may occur. The classic example is Darwin’s finches in the Galápagos Islands, where Price did his doctoral fieldwork at the University of Michigan. There DNA analysis links 14 recognized finch species dispersed through the Pacific archipelago to a common ancestor. They have evolved into ecologically diverse, reproductively isolated groups. And each species developed a specific beak size and shape adapted to its particular food.

With few exceptions, geographic separation appears critical to the initial stages of differentiation. Contrast the situation of the Galápagos finches, says Price, with those of Cocos Island, located between Costa Rica and the Galápagos. Rich with tropical forests, Cocos offers diverse ecological niches, yet only one species of Darwin’s finches exists there. Without surrounding yet separate landmasses to encourage secondary populations, the Cocos finch has continued to interbreed rather than differentiate into new species.

The third step in the process—one that has received less attention in traditional speciation discussions, says Price—is expansion. After differentiation occurs, a new species may fly back into its ancestral species’territory, particularly if the original geographic barrier disappears. Having become differentiated in separate areas, the new and old species can now share locations yet remain reproductively isolated. In the Galápagos, for example, seed-eating ground birds coexist, but don’t commingle, with fruit-and-leaf-eating tree dwellers. Alternatively, if the new species does interbreed with the ancestral species, a hybrid species may result. Eventually, the two may collapse back into one. As species accumulate in an area, Price discovered, the overall rate of speciation may slow as available niches become occupied.

His latest research, based in the small northeast Indian state of Sikkim—one of the world’s most species-diverse areas—delves further into the question of niche organization. “We’re trying to figure out why there are so many species there,” says Price, who has spent most of his field time cataloguing the local bird population. The answer, in part, lies in northeast India’s near-tropical habitat. Because tropical areas tend to be more stable than temperate ones, which suffer annual slings of winter cold, tropical species can accumulate over time and partition resources. Temperate species often die off because one species, in an effort to survive during climate fluctuations, monopolizes several resources. Price’s central question is whether tropical areas become species rich because they have more niches or because their niches have simply become more finely divided during their long life spans.

As Price and other ornithologists census birds in new areas, the number of recognized species continues to rise. A 1990 list tallied the world’s bird species at 9,672. A decade later, after a rise in fieldwork studying birdsongs, and sequencing DNA, the list had grown to nearly 10,000. Yet even Price, who has spent thousands of hours watching birds in the wild, admits that parsing species can be challenging. “It’s really amazing how similar two birds can be.”