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.

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.

Geographic isolation

A population of birds, or any organism, can speciate if isolated from its neighbors. In his 1942 book, Systematics and the Origin of Species, Mayr argued that the most significant way to cut off a population is by geographical isolation (see illustration at right). For example, a glacier may thrust down a valley, creating two separate populations, one on either side of the glacier. A rising ocean may turn a peninsula into a chain of islands, stranding the beetles on each of them. This sort of isolation doesn’t have to last forever; it needs only form a barrier long enough to let the isolated population become genetically incompatible with the rest of its species. Once the glacier melts, or the ocean drops and turns the islands back into a peninsula, the animals will be unable to interbreed. They will live side by side, but follow separate evolutionary fates.

Other modes of speciation

Today, scientists studying the origin of species can compare not just the bodies of species, but their genes as well. Geographic isolation remains a crucial element in forming new species, but a number of biologists now argue that the formation of species can take several different paths. It may be possible, for example, for a population to continue breeding with other members of its species — and trading genes — while still diverging into a distinct group. All that may be required is that a few of its genes diverge, thanks to strong natural selection. If the conditions are right, this genetically distinct population may then become a new species.

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.