Encyclopedia of Evolution.pdf - Online Reading Center

Dawkins, Richard
ate the pulp. Moreover, the two groups tended to breed separately;
each group had its own type of mating song. This had
the elements—genetic variation and reproductive isolation—
needed to begin the formation of separate species. When the
rains returned, the two groups began to blend back together.
Computer simulation can retrace the pathway of evolution
for the seed-eating finches of the Galápagos. A program
designed by evolutionary biologist Dolph Schluter begins with
the ranges of beak sizes within each species, the range of seed
sizes available on each island, and the presence or absence
of competition, and calculates the expected ranges of beak
sizes on each of the islands. The result of Schluter’s analysis
matches the real pattern of beak sizes very closely.
Limits to Adaptive Radiation
When the finches first arrived on the Galápagos Islands, there
were no other similar kinds of birds with which to compete.
The islands represented a new world open to them for adaptive
radiation. They were able to exploit new ways of making
a living, without having to compete with other birds species
that already possessed superior adaptations. The woodpecker
finch (Camarhynchus pallidus), for example, would not have
been able to evolve on an island that already had woodpeckers.
Some of the cactus finches eat nectar from the flowers of Waltheria
ovata, and smaller finches are able to obtain and use nectar
more efficiently. (Most birds that consume nectar, such as
hummingbirds, are small.) On islands that do not have native
bees, some of the finches specialize upon nectar, although not
exclusively: Nectar comprises 20 percent of their diet, and they
are smaller than the finches that do not eat nectar. On islands
that already have populations of the native bee Xylocopa darwini,
nectar makes up only 5 percent of the finch diet, and the
finches do not differ in size from those that do not eat nectar.
hybridization is rare between species of Galápagos
finches, but when it does occur, the Grants often make note
of it. In more than one case, hybrids have proven fertile for
several more generations, during wet seasons; but under dry
seasons, when the successful individuals are the ones that
specialize, the hybrids (with intermediate beak sizes) have
inferior fitness. It appears that it is the dry seasons in which
natural selection enforces postzygotic reproductive isolation
(see isolating mechanisms) and keeps the species apart.
In the Pacific Ocean, the island of Cocos is the home of
one of the same species of finches that is found in the Galápagos:
the Cocos Island finch, Pinaroloxias inornata. Unlike
the Galápagos Islands, the island of Cocos is not large enough
to promote reproductive isolation among populations of the
finches. Speciation is not occurring in these finches. Rather
than specializing on food resources, the Cocos finches have
remained generalists, exploiting many different food resources,
and learning how to do so by watching other finches.
Further Reading
Grant, Peter R. Ecology and Evolution of Darwin’s Finches. Princeton,
N.J.: Princeton University Press, 1999.
Sato, A., et al. “On the origin of Darwin’s finches.” Molecular Biology
and Evolution 18 (2001): 299–311.
Weiner, Jonathan. The Beak of the Finch: A Story of Evolution in
Our Time. New York: Knopf, 1994.
Dawkins, Richard (1941– ) British Evolutionary scientist
Richard Dawkins is one of the leading spokespersons
in the world today for the understanding of evolution by the
general public. He has generated numerous original ideas that
have proven controversial and productive.
Dawkins was born in Kenya on March 21, 1941. He left
when he was too young to see many of the wild animals of
Africa but developed an interest in animal behavior. When
he became an undergraduate at Oxford University in 1959,
he studied with the eminent ethologist Niko Tinbergen, who
helped to establish the modern study of behavior (see behavior,
evolution of). Tinbergen said that the study of behavior
required an interdisciplinary approach, involving psychology,
physiology, ecology, sociology, taxonomy, and evolution. This
was the kind of approach Dawkins was later to take in all of
his work. At this time, the repercussions of the discovery of
DNA as the basis of the inheritance of all organisms were being
felt throughout all fields of biology (see DNA [raw material
of evolution]). As Dawkins studied bird behavior, learned
how to use computers, and read about DNA, he began to put
all three perspectives together and arrived at an understanding
that genes, like animals, have behavior. Dawkins remained
with Tinbergen for his doctoral research. Dawkins spent two
years at the University of California at Berkeley before returning
to Oxford University, where he has been on the faculty
ever since. Since 1995 he has held the Charles Simonyi Chair
of Public Understanding of Science at Oxford.
Two of Dawkins’s ideas have had a particular impact
on evolutionary science. One is that natural selection
operates primarily on the genes; genes use bodies as their
machines for recognition, survival, and propagation. Dawkins
presented this idea in his first book, The Selfish Gene, in
1976. Since that time, many researchers have found examples
of selfish genetic elements that pervade the natural
world at the molecular level. Molecular biologists consider
the phenotype of an organism to be the chemicals that make
up its cells, which are produced under the direction of DNA.
In his book The Extended Phenotype, Dawkins expanded the
phenotype concept. He argued that bodies are not the only
machines that genes use for their benefit, but the immediate
environment that the animal manipulates, such as the nests of
birds, the dams of beavers, and the effects of animal signals
on other animals, even of other species.
A second and related major idea is that genes are not the
only replicators. Ideas and cultural innovations can spread
through the minds of an animal population just as genes can
spread through any biological population (see evolution).
Dawkins called these replicating ideas memes and speculated
that they have been crucial in the astonishing evolutionary
development of humans. Others have gone further in suggesting
that memes are part of the extended phenotype of
humans. Humans create technology but must also adapt to
it; anyone who does not adapt to technological innovation is
at an evolutionary disadvantage. This idea is very similar to