Origin of species
How we decide which birds are, and which birds aren't, new species
Published: July 1, 2006
|Species are the currency of birdwatching and the basic unit of avian taxonomy. When you identify a bird, you specify its species. Chances are, you use a common name, such as American Robin, rather than its scientific name, Turdus migratorius (meaning migrating thrush). But what is a species? |
Think of species as different kinds of birds. Beginning in the mid-1700s, kinds of birds were described and given names based on size, bill shape, color, markings, and other physical characteristics. A bird was determined to belong to a new species if it looked different from birds already described.
The system worked fairly well for species with well-defined characteristics, such as kingfishers, but not so well for similar-looking species, such as some of the flycatchers and many gulls. Moreover, in species in which males and females don't look alike (that is, in dimorphic species), females were sometimes described as new species. A better system was needed.
In the early 1940s, Harvard biologist Ernst Mayr proposed a radical concept that considered species not as individuals, but as populations, and focused on reproduction. The concept held that individuals should interbreed with birds from populations of the same species, and that they should not be able to interbreed with (that is, they should be reproductively isolated from) birds from populations of other species.
When individuals of distant populations of a species do not regularly exchange genetic material by interbreeding, each population can take on characteristics of its own because of differences in genetic changes (mutations) and/or selection pressures that favor divergent traits. Populations that are different are often identified as subspecies or races.
Numbers of subspecies vary. Mallards and the Canada/Cackling Goose complex are extreme examples. Mallards are distributed widely, and all are genetically similar. No subspecies are officially recognized on the North American mainland. That is because females on the wintering ground pair with males from all over the continent and usually return to their nesting area. The result is a thorough mixing of genes throughout the Mallard's range.
Canada and Cackling Geese, on the other hand, migrate in family units and return to their nesting area the following spring. Pairing is done within the population. The result is a kind of inbreeding. Any genetic change in the population tends to be intensified and is restricted to that population. Consequently, differences between populations are common, and about 11 subspecies of Canada and Cackling Geese are currently recognized.
Products of isolation
So how do new species arise? If individuals of populations change so much that they can no longer interbreed with their sibling populations, the changed populations are recognized as different species. Genetic changes that prevent interbreeding - whether structural, behavioral (including displays and song), or temporal (timing of reproduction) - serve as reproductive isolating mechanisms. Almost always, such isolating mechanisms develop as a result of geographic isolation. The keys are isolation, which prevents gene exchange, and sufficient time to allow changes to take place.
A classic case of geographical isolation occurred in the middle of North America about 10,000 years ago. The Wisconsin lobe of Pleistocene glaciation slowly snaked south like a giant wedge, destroying the landscape and dividing a number of widely distributed bird species into eastern and western forms. After the glacier receded, the birds moved back to the center, and their ranges overlapped with those of their former siblings.
At least 12 species were so divided. We now recognize three of the species pairs as Eastern and Western Meadowlarks, Indigo and Lazuli Buntings, and Rose-breasted and Black-headed Grosbeaks.
Members of each pair will occasionally interbreed where their ranges overlap, because the species' isolating mechanisms are not fully developed. Each pair can produce viable offspring, but the meadowlark offspring appear to be sterile. The frequency of these events is low, however, and all are considered to be good species even though they hybridize rarely.
Baltimore or Northern?
Two oriole species, Baltimore and Bullock's Orioles, have a more complicated history. They hybridize more often than the meadowlark, bunting, and grosbeak pairs and also produce viable offspring. Because of the amount of their interbreeding, they were combined into a single species and named Northern Oriole. The terms Baltimore and Bullock's were assigned to the eastern and western subspecies.
Analysis later revealed that the frequency of these pairings was not as great as would be expected if they were a good single species, and that Bullock's appears to be more closely related to Streak-backed than to Baltimore Oriole. Consequently, their status has reverted back to two species, and their original names have been restored.
The Marsh Wren is currently considered to be one good species that extends across North America. Eastern and western forms differ slightly in physical appearance but have very different songs.
Studies in their range of overlap show that eastern birds breed with eastern and western birds with western, indicating that the forms are reproductively isolated and should be considered as two different species. Look for a change in the future.
The species concept is our way of trying to understand and explain what birds are doing reproductively. Birds know nothing about the species concept, but they know a lot about the birds they select for breeding. We continue to study and observe them as we try to unravel their amazing secrets.