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What is a Species?Carl Linnaeus, an 18th Century Swedish scientist interested in life forms, developed a system of classifying plants and animals. (He invented the practice of giving each species a two-word scientific name, such as Turdus migratorius for the American robin.) Linnaeus considered each species to be fixed and unchanged since the moment of creation. Although we still use the system that Linnaeus devised, science has refined the concept of species. We now know that species are not absolute. They change and evolve. Sometimes one species gives rise to two. Occasionally the lines between species blur. But we still understand the species as the fundamental unit of biology. Scientists are still working on the question of what a species is. They argue about it at length and with surprising heat. The prevailing point of view in ornithology at the moment is that a bird species is a natural population of birds who breed with each other, or could potentially breed with each other, but not with members of other species. This concept, first defined by Ernst Mayr in 1942, is called the Biological Species Concept (BSC). According to the BSC, if two species interbreed but their offspring are not fertile, then the two are separate species. However, if the offspring of the cross can reproduce, then the two parents belong to one species. As the BSC gained scientific acceptance, some bird groups that had been considered separate were lumped together. That is why northern flicker is now considered a single species. Formerly it was two species: the red-shafted flicker of western North America and the yellow-shafted flicker of eastern North America. However, the two forms interbreed where they come into contact in the Great Plains, and so the American Ornithologists' Union (A.O.U.) ruled that the flickers are one species. If you were keeping a life list when that ruling happened, you lost a bird. A thorn in the shoe of the BSC is how to say whether two groups of birds that are separated geographically could interbreed under natural conditions (not under forced association in captivity). Scrub-jays live in western North America and in Florida. There is no way to test whether they can interbreed naturally, because a vast continent separates them. Since the late 1980s, the Phylogenetic Species Concept (PSC) seems to be gaining support. According to Mary McKitrick, Robert Zink, and other exponents of the PSC, a bird species is a group of birds that share distinguishing characteristics and have a separate evolutionary history. Under the PSC, it's not an issue whether the two groups of birds could interbreed. If they haven't been doing so for a long time and there are ways to tell the two apart, then they have separate histories and are separate species. This definition avoids the problem of deciding whether or not separated groups of birds could breed if they hypothetically got in touch. The PSC has influenced our current take on a number of birds. For example, the second edition (1987) of the National Geographic Society Field Guide to the Birds of North America describes the scrub-jay as one species. It shows pictures of three races, or subspecies—"Florida," "west coast," and "interior." The idea was that if scrub-jays from the Florida race and scrub-jays from the west coast race were not separated by most of North America, they would breed, and therefore they were considered a single species. However, according to the PSC, since the Florida birds differ in appearance and behavior, and since they clearly have separate evolutionary histories, they should be considered a distinct species. The PSC wins this one. Starting with the third edition (1999) of the NGS field guide, the corresponding section shows three species, Western Scrub-Jay (which includes the west coast and the interior forms), Florida Scrub-Jay, and Island Scrub-Jay. If you saw scrub-jays in Florida, Los Angeles, and Santa Cruz Island (off the California coast near Santa Barbara) before the species was split, your life list gained two birds from the decision. The birds did not change perceptibly between 1987 and 1999. What changed was the way ornithologists think about species. If the drift toward the PSC definition of species continues, applying the separate-evolutionary-history principle may one day give us at least twice as many bird species in the world as we have now. Field guides could get so big they'll come equipped with wheels and handles. -- Diane Cooledge Porter Article first appeared in Bird Watcher's Digest, September, 2007.
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