Encyclopedia of Evolution.pdf - Online Reading Center

Darwin’s finches
has its own unique species of finch. Some species live on or
near the ground and eat seeds; others live on or in cactuses
and eat seeds, pollen, and nectar; the others live in trees. The
tree finches eat fruits, leaves, seeds, and insects; one of them
strips the outer bark and eats the rich inner bark. One group
of finches, though primarily consuming insects, occasionally
drinks the blood or eats the eggs of seabirds such as bluefooted
boobies. Some finches occasionally perch on iguanas
and eat the parasites from them. The finches have undergone
adaptive radiation in two ways: first, by evolving
on different islands; second, by specializing on different food
resources.
One of the earliest extensive studies of these finches,
which brought them to the attention of the world as an
excellent example of evolution in action, was the 1947 study
by David Lack. He visited for only four months, not long
enough to get extensive data regarding the ongoing processes
of evolution in these finch populations. He built an aviary,
in which he was able to confirm that the different kinds of
finches did not readily interbreed, therefore they were different
species (see speciation). He showed that evolution had
occurred, but his evidence did not show that evolution was
still occurring.
In order to obtain convincing evidence about natural
selection, it is important to closely monitor entire isolated
populations of organisms. If this is not done, the researcher
cannot determine whether the disappearance of individuals
is due to natural selection (the individuals die or fail to
reproduce) or due to migration (the individuals leave). This
was one of the problems with what had once been considered
a classic study of natural selection, zoologist H. B. D. Kettlewell’s
studies on the peppered moths. Darwin’s finches
provided a perhaps unique opportunity to study natural selec-
Darwin’s Finches
Ground finches
Small ground finch Geospiza fuliginosa
Medium ground finch Geospiza fortis
Large ground finch Geospiza magnirostris
Cactus finch Geospiza scandens
Large cactus finch Geospiza conirostris
Sharp-beaked ground finch Geospiza difficilis
Tree finches
Small tree finch Camarhynchus parvulus
Medium tree finch Camarhynchus pauper
Large tree finch Camarhynchus psittacula
Woodpecker finch Camarhynchus pallidus
Vegetarian finch Platyspiza crassirostris
Gray warbler finch Certhidea fusca
Olive warbler finch Certhidea olivacea
Cocos finch Pinaroloxias inornata
tion and other evolutionary processes, since they are isolated
on different islands, to and from which they seldom migrate.
A prodigious amount of work is required to keep track of
each individual finch, how many offspring it produces, how
it lives, and its physical characteristics, even with relatively
small populations on small islands. But that is precisely what
Peter and Rosemary Grant, evolutionary biologists at Princeton
University, and their associates, have been doing on some
of the islands since the 1970s. Having monitored the lives of
more than 20,000 individual finches, they have been able to
present convincing evidence of natural selection and other
evolutionary processes at work. They can recognize the individual
finches, and know the genealogies of each, the kinds
and sizes of seeds that these individuals eat, and their individual
songs, as well as the kinds of seeds available on the
islands and in what amounts.
The Grants studied the beak sizes of Geospiza fortis
ground finches, and the relationship between beak size and
the kinds of seeds that these finches ate. The ground finches
eat seeds. Some plants (for example, grasses) produce small
seeds that are relatively easy to crack. These plants grow
abundantly during rainy seasons. During dry seasons, large
seeds (such as those of the puncture vine) are the most abundant.
The large seeds also have very hard coats. Finches with
larger beaks can crack larger seeds. The finches with smaller
beaks can more easily handle the smaller seeds. A difference
of less than a millimeter in beak size can make a tremendous
difference on the food resources available to a finch. A finch
with a 0.44 inch (11 mm) beak can crack open a puncture
vine seed, while an individual with a 0.42 inch (10.5 mm)
beak may not be able to do so. The individuals with smaller
beaks may starve. Or they may eat seeds from the Chamaesyce
bush, which produces a sticky, milky sap. The sap gets
on the heads of the birds. The birds use their heads to push
gravel around as they search for seeds. When they do this,
the birds with sticky heads rub the feathers off of the tops of
their heads, leaving the skin exposed, which can lead to overheating
of the brain, followed by death. As Darwin wrote,
“the smallest grain in the balance, in the long run, must tell
on which death shall fall, and which shall survive.”
The Grants have gathered data about natural selection,
character displacement, and incipient speciation among
Darwin’s finches. The finches also provide evidence about the
limits to adaptive radiation.
Natural Selection in Finch Populations
In order to show that natural selection is occurring, the
Grants had to establish that the beak size differences
among the individual Geospiza fortis birds are genetically
based. To do this, the Grants switched eggs among nests
and found that individual birds had beak sizes similar to
those of their biological, not their adoptive, parents (see
figure on page 115).
Next, the Grants had to measure changes that occurred
in genetically based characteristics such as beak size. They
measured beak sizes of Geospiza fortis during a long drought
when vegetation was sparse, and most of the seeds came from