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.