Species and Speciation
photographs of Townsend's Warbler (top) and Yellow-rumped Warbler (bottom) by Rohan Kamath	Species are distinctly different kinds of organisms. Birds of one species are, under most circumstances, incapable of interbreeding with individuals of other species. Indeed, the "biological species concept" centers on this inability to successfully hybridize, and is what most biologists mean by "distinctly different." That concept works very well when two different kinds of birds live in the same area. For example, Townsend's and Yellow-rumped Warblers are clearly distinct kinds because their breeding ranges overlap, but they do not mate with one another. If they did, they might produce hybrid young, which in turn could "backcross" to the parental types, and (eventually) this process could cause the two kinds of warblers to lose their distinctness. On the other hand, when relatively similar populations occur in different areas, it is much more difficult to decide whether to classify them as different species. For example, the western populations of the Yellow-rumped Warbler (which have yellow throats) were previously considered a species, Audubon's Warbler, distinct from the eastern Myrtle Warblers (which have white throats), largely because of differences in appearance. Then it was discovered that the breeding ranges of Audubon's and Myrtle Warblers overlap broadly in a band from southeastern Alaska through central British Columbia to southern Alberta, and that the two "species" hybridize freely within this area. The forms intergrade, and taxonomists now consider them to be subspecies of a single species, the Yellow-rumped Warbler. Subspecies are simply populations or sets
of populations within a species that are sufficiently distinct that taxonomists have found it convenient to formally name them, but not distinct enough to prevent hybridization where two populations come into contact. Judgments about whether two populations should be considered different species or just different subspecies may be very difficult to make. For instance, in some areas where populations of Red-breasted and Yellow-bellied Sapsuckers meet, they hybridize, whereas in other areas of overlap they do not. As a result, ornithologists do not agree upon whether to consider the two forms as separate species or as subspecies of the same species. In situations where differentiated, but clearly closely related forms replace one another geographically, taxonomists often consider them to be separate species within a superspecies. These complications are a natural result of applying a hierarchical taxonomic system, developed a century before Darwin, to the results of a continuous evolutionary process. Geographic variation -- birds showing different characteristics in different areas -- is inevitable among the populations of all species with extensive breeding distributions. It is largely the result of populations responding to different pressures of natural selection in different habitats. If populations of a single bird species become geographically isolated, those different selection pressures may, given enough time, cause the populations to differentiate sufficiently to prevent interbreeding if contact is reestablished. In nature, degrees of differentiation and of abilities to hybridize fall along a continuum, so one finds what is expected in an evolving avifauna -- some populations intermediate between subspecies and species, populations (members of superspecies) that have differentiated to the point where they will not hybridize but have not yet regained full contact, and populations so distinct that they can be recognized as full species whether or not they occur together. As an example of the latter, the Three-toed Woodpecker and the Red-cockaded Woodpecker have separate distributions, but clearly are separate species. Biologists differ on the details of both the definition of species and the mechanisms of speciation, and our treatment is necessarily simplified. Changing views of the specific status of North American birds can be an annoyance, since they may result in new names for familiar birds. But changes reflect increasing knowledge of the biology of the birds involved, and by careful observations of hybrids, mating pairs, and species distributions, you should be able to add to that knowledge.
SEE: Taxonomy and Nomenclature; Natural Selection; Superspecies; Sibling Species; Great Plains Hybrids; Hybridization. Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.