Encyclopedia of Evolution.pdf - Online Reading Center

Beak depth (from the top of the beak to the bottom) is one of the
measurements of beak size in Darwin’s finches (Geospiza fortis) used by
Peter and Rosemary Grant to study natural selection. Across a wide range
of beak depths, from about 0. 0 to 0. inch ( to mm), the offspring
beak sizes were correlated with the parental beak sizes in . The
horizontal axis is the average of the parental beak sizes; the vertical
axis is the average of the offspring beak sizes. The slanted line is the
statistical regression line. Results for were similar.
the puncture vine. Then, during an El Niño Southern Oscillation
event, a tremendous amount of rain fell, and the island
was overgrown with plants that produced smaller seeds.
Almost immediately, the average sizes of the birds, and the
sizes of their beaks, began to decrease (see figure on page
115). When the drought returned, and the plants that produced
small seeds became progressively less abundant, the
average body and beak size began to increase in the G. fortis
population. This is an example of natural selection in action,
first one direction, then the other.
Character Displacement between Finch Species
Not only does natural selection occur in response to weather
patterns but also in response to contact between species of
finches. On the island of Genovesa, the large cactus finch
Geospiza conirostris and the large ground finch G. magnirostris
(which has a larger average beak size) live together. During
rainy seasons, when there are plenty of seeds, both species
exhibit a wide range of beak sizes, and there is overlap between
the beak sizes of the largest G. conirostris and the smallest G.
magnirostris. The finches in the zone of overlap would compete,
but they do not because there are plenty of seeds for both.
During droughts, when seeds become scarce, the two species
specialize: Natural selection favors the G. magnirostris with
the largest beaks but favors the G. conirostris with the smallest
Darwin’s finches
Beak sizes in Darwin’s finches (Geospiza fortis) on Isla Daphne Major
from to 00 . In response to the severe drought that began in ,
the relative beak size in the population increased dramatically. This
occurred because larger birds, with larger beaks, could eat the large,
hard seeds of plants that survived the drought. After the drought ended,
starting about , the average beak size decreased to its previous
levels. Natural selection caused beak sizes to fluctuate as climatic
conditions changed. The vertical axis is a mathematical combination of
various measurements of beak size. Small vertical lines indicate error
ranges of the data.
beaks. This is an example of character displacement between
two species that minimizes competition between them. Character
displacement is expected to occur only when two species are
in contact, and this is precisely what we see with these finches.
On the island of Española, the large ground finch G. magnirostris
is absent. On this island, without competition from the
large ground finch, the cactus finch G. conirostris has a much
greater range of beak sizes regardless of the weather conditions.
Competition between species has not only caused character displacement
but may have caused some local extinctions as well.
Generally speaking, finches that are very similar to one another
in size and the type of food they eat are not found on the same
island. An even more striking example of character displacement,
induced in G. fortis by G. magnirostris, was announced
in late 2006 (see character displacement).
Incipient Speciation
The Grants have also observed some possible examples of speciation
beginning to occur. During a dry year, they identified
two different groups of cactus finch Geospiza conirostris: One
group had shorter beaks than the other group. The differences
were only slight: The two groups differed by only 15 percent.
This was more than twice as much difference in beak size as
there was among individuals within each group. The birds
with the longer beaks extracted seeds from cactus fruits, while
those with shorter beaks frequently tore open cactus pads and