Speciation: Ernst Mayr
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
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 below). 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.
Other modes of speciation
Others argue that organisms can diverge into genetically distinct populations even if they are living side by side. For example, females may be born with different preferences for mates, and those preferences may get strengthened over time into reproductive isolation. Even as biology's understanding of species formation evolves, Mayr's work remains hugely important to the understanding of how the millions of species on Earth came to be.
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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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