Encyclopedia of Evolution.pdf - Online Reading Center

a book stall on a street Edinburgh in 1818. In little over a
decade, W. & R. Chambers had become one of the most successful
publishing houses in Britain. They published science
books for the general public and a weekly magazine.
Little of Chambers’s book was of lasting scientific value.
In particular, it provided no explanation for what caused evolution
to occur. Without a mechanism, no evolutionary theory
could be credible to scientists. It was a mechanism, natural
selection, that Darwin provided and Mr. Vestiges did not.
The one lasting effect of Vestiges is that it vividly demonstrated
to Darwin, who was secretly working on his theory,
that publishing an ill-formed and unsupported evolutionary
theory would be a disaster. It was perhaps because of Chambers
that Darwin waited until 1858 before presenting even a
summary of his ideas, and Darwin would not have done so
even then had it not been for a letter from a young British naturalist
who had come up with the same idea as Darwin (see
Wallace, Alfred Russel). Chambers died March 17, 1871.
Further Reading
Eiseley, Loren. Darwin’s Century: Evolution and the Men Who Discovered
It. New York: Anchor, 1961.
Secord, James A. “Behind the veil: Robert Chambers and Vestiges.”
In J. R. Moore, ed., History, Humanity and Evolution. Cambridge,
U.K.: Cambridge University Press, 1989.
character displacement Character displacement is an evolutionary
change that occurs within a trait when species compete
for the same resource. It is called character displacement
because, with respect to that character or trait, the species
evolve to become less similar to one another, thus displacing
the character from its average state. Charles Darwin, in Origin
of Species (see Darwin, Charles; origin of species
[book]) stated that natural selection should favor divergence
between closely related species that had similar diet and
habitat. Competition is strongest when the species are most
similar in that trait. Each species benefits from evolving versions
of the trait that minimize competition.
Extensive observational evidence to support this concept
was not available until the 20th century. Ornithologist David
Lack studied Darwin’s finches on the Galápagos Islands.
The beaks of the small ground finch Geospiza fuliginosa were
smaller, on the average, than those of the medium ground
finch G. fortis. There was considerable overlap between the
two species. The amount of overlap in beak sizes between the
species was much greater when populations were compared
from islands on which only one of the species occurred. There
was much less overlap in beak sizes on islands on which the
two species competed for seeds. Evolutionary biologists William
Brown and E. O. Wilson (see Wilson, Edward O.)
presented other examples in the 1956 paper in which they
presented the term character displacement. The problem with
observational evidence is that evolutionary processes other
than character displacement may be able to account for them.
In some cases, what appears to be character displacement
results from plasticity rather than from true evolutionary
adaptation. Many proposed examples of character displace-
character displacement
ment, including that of Darwin’s finches, have withstood the
test of alternative explanations.
Experiments have begun to confirm observations of character
displacement. Evolutionary biologist Dolph Schluter
studied three-spined sticklebacks, which are freshwater fish in
coastal lakes of British Columbia. When two species of sticklebacks
are present in the same lake, one of them (the limnetic
form) is small, slender, and feeds mainly on small organisms
throughout the depths of the lake, while the other (the benthic
form) is larger and feeds mainly on animals that live at the
bottom of the lake. The limnetic and benthic forms have recognizable
differences in the structures of their mouths. When
a stickleback species occurs alone in a lake, its characteristics
are more intermediate between the limnetic and benthic
extremes. In one experiment, Schluter placed individuals of
an intermediate species (the target species) on both sides of a
divided experimental pond, then added individuals of a limnetic
species to one side. Because of competition, the individuals
of the target species that fed in the limnetic zone had lower
fitness than the individuals that fed on the bottom, where
there were no competitors. In another experiment, Schluter
placed individuals of the target species on both sides of the
pond, then added a limnetic species to one side and a benthic
species to the other. The individuals of the target species that
had highest fitness were those that were different from the
species that were added to their half of the pond. Schluter had
demonstrated that natural selection favored displacement.
For over 30 years, Peter and Rosemary Grant have
studied evolutionary changes in Darwin’s finches on the
Galápagos Islands. During a prolonged drought beginning
in 1977, average beak size in the medium ground finch
(Geospiza fortis) on the island of Daphne Major increased,
an example of directional selection (see natural selection).
This shift allowed the birds to consume larger seeds.
When the rains returned, natural selection favored a return
to smaller beak sizes in this species. At the time, there was
no other species of bird that ate seeds on the ground on this
island. Then, in 1982, G. magnirostris, a species of finch
with larger beaks, invaded Daphne Major. When another
drought struck in 2003, the beaks of G. fortis evolved to be
smaller, not larger. This occurred because in 2003, unlike
1977, G. fortis had to compete with G. magnirostris, which
could more effectively consume the large seeds. The evolutionary
shift in 1977 was due directly to drought, while
the 2003 evolutionary shift was due to competition during
drought—making it perhaps the best example of yet documented
of character displacement in the wild.
Further Reading
Brown, William W., Jr., and E. O. Wilson. “Character displacement.”
Systematic Zoology 5 (1956): 49–64.
Grant, Peter R. “The classic case of character displacement.” Evolutionary
Biology 8 (1975): 237.
Grant, Peter R., and B. Rosemary Grant. “Evolution of character
displacement in Darwin’s finches.” Science 313 (2006): 224–226.
Summary by Pennisi, Elizabeth. “Competition drives big beaks
out of business.” Science 313 (2006): 156.