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Speciation: Ernst Mayr (1 of 2)

Dobzhansky’s Genetics and the Origin of Species captivated biologists far beyond the confines of genetics. In the mountains of New Guinea, an ornithologist named Ernst Mayr (right) found the book to be an enormous inspiration. Mayr specialized in discovering new species of birds and mapping out their ranges. It is no easy matter determining exactly which group of birds deserves the title of species. A bird of paradise species might be recognizable by the color of its feathers, but from place to place, it might have a huge amount of variation in other traits—on one mountain it might have an extravagantly long tail while on another its tail would be cut square (below right).

Variation Between Populations
Biologists typically tried to bring order to this confusion by recognizing subspecies—local populations of a species that were distinct enough to warrant a special label of their own. But Mayr saw that the subspecies label was far from a perfect solution. In some cases, subspecies weren’t actually distinct from each other, but graded into each other like colors in a rainbow. In other cases, what looked like a subspecies might, on further inspection, turn out to be a separate species of its own.

The tails of birds of paradise living in the mountains of western New Guinea (A) are longer than those of birds living in the more central mountains (B).

Like many other naturalists of his day, Mayr suspected at first that some kind of Lamarckian heredity might be at work in evolution. But when he read Dobzhansky and other architects of the Modern Synthesis, he realized that it was possible to explain the origin of species with genetics. Mayr also realized that the puzzle of species and subspecies shouldn’t be considered a headache: they were actually a living testimony to the evolutionary process Dobzhansky wrote about. Variations emerge in different parts of a species’ range, creating differences between populations (see example at right). In one part of a range the birds may possess long tails, in others, square tails. But because the birds also mate with their neighbors, they do not become isolated into a species of their own.

The size and shape of Dicrurus paradiseus’ crest varies considerably across southeast Asia.

• Mayr image courtesy of the Ernst Mayr Library, Harvard University.
• Bird of paradise tails after an illustration in Mayr, E. 1942. Systematics and the Origin of Species. Columbia University Press, New York.
• Dicrurus graphic after an illustration in Futuyama, D.J. 1986. Evolutionary Biology. 2nd ed. Sinauer Associates, Inc.

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