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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
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| 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). |
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.
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| The size and shape of Dicrurus paradiseus' crest varies considerably across southeast Asia. |
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.
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