Deciding whether a given population constitutes a species can be difficult in part because there is no single accepted definition of the term. Years ago, evolutionary biologist Ernst W Mayr propounding what is called the biological species concept, and proposed that the definition be based on reproductive compatibility. Specifically, he considered a species to be a group of animals that can mate with one another to produce fertile offspring but cannot mate successfully with members of a different group.
Yet this idea can be too restrictive. First, mating between species (hybridization), as often occurs in the canine family, is quite common in nature. Second, in some instances the differences between two populations might not prevent them from interbreeding, even though they are rather dissimilar in traits unrelated to reproduction; one might question whether such disparate groups should be considered a single species. A third problem with the biological species concept is that investigators cannot always determine whether two groups that live in different places are capable of interbreeding.
When the biological species concept is difficult to apply, some investigators use phenotype, an organisms observable characteristics, as a surrogate. Two groups that have evolved separately are likely to display measurable differences in many of their traits, such as the size of the skull or width of the teeth. If the distribution of measurements from one group does not overlap those of the other group, the two groups might be considered distinct species. Another widely discussed idea designates a species based on the presence of some unique characteristic not found in another closely related organism—for example the upright posture of humans—or a distinguishing sequence of nucleotides DNA (building blocks) in a gene.
4. Proving that the red wolf fits any of these descriptions has been extremely challenging. For instance, the red wolf is not a species by Mayrs definition, because it can breed extensively with the coyote and the gray wolf. And efforts to classify the red wolf based on its phenotypic traits have yielded ambiguous results. John James Audubon and John Bachman, who described the red wolf in their classic 1851 book, Viviparous Quadrupeds of North America, had difficulty distinguishing the red wolf from the physically similar coyote and gray wolf. Modern researchers looking at phenotypic traits have variously concluded that the red wolf, a hybrid of the coyote and the gray wolf, is a full-fledged species.
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