NATuRAl SElECTioN ANd AdAPTATioN 73
if different closely related species coexist, those individuals that use
different resources from the other species would suffer less competi-
tion and have higher fitness. Consequently, the species will diverge
from the others. It took nearly a century for biologists to show that
Darwin was right. Today we know many cases of what is now called
character displacement: divergence of species as a consequence of their interaction
(see Chapter 13). For example, Peter and Rosemary Grant and their collaborators
have studied certain of the ground finches in the Galápagos Islands for more than
35 years (see Figure 2.2) [21, 23]. Among the seed-eating ground finches, those with
larger, deeper bills feed more efficiently on larger, harder seeds. Species with differ-
ent bill depth differ accordingly in diet, and the species that coexist on any island
differ, matching the availability of different seeds (FIGURE 3.22A). In a population
of one species, Geospiza fortis, there was high mortality of individuals with smaller
bills during a drought, in 1977, that caused a dearth of plants with small seeds. The
result was an increase in average bill size (FIGURE 3.22B). A few years later, Geospiza
magnirostris, with the large bill denoted by its name, invaded the island and slowly
grew in numbers until, in 2004, it depleted the supply of large seeds. The G. fortis
population then evolved smaller average bill size, as Darwin would have predicted.
The finch example shows the first stages of the evolution of diversity that is
seen in adaptive radiations, such as those described in Chapter 2 (see Figures 2.2,
2.24, 2.25). In each of those cases, the morphological differences are associated
with using different resources. The huge diversity seen among higher taxa, such as
the immense variety of flowers among plant families and of bills, legs, and wings
among the families and orders of birds, may be ascribed partly to the same principle.Futuyma Kirkpatrick Evolution, 4e
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Evolution4e_03.22.ai Date 11-28-2016(B)(A)Relative in bill size1975 1980 1985 1990 1995 2000 2005
Year–1.0–0.500.51.0–1.5Expected population density
1.7 2.2 2.7 3.2 1.7 2.2 2.7 3.2Log bill depth (mm)Daphne Wolf Marchena1.7 2.2 2.7 3.2DroughtArrival of G. magnirostrisFIGURE 3.22 Bill size in Galápagos ground finches
(Geospiza) is adapted to feeding on seeds, but com-
petition among species affects what kinds of seeds a
species eats. (A) Because of differences in abundance of
plant species with different seed sizes, different islands
would be expected to differ in the density that various
populations of finches would be expected to sustain,
as a function of their bill size. For example, Wolf Island
has only two abundant kinds of seeds, one small and
the other large; the jagged curve shows the theoretical
population density of a finch population, depending on
its log bill depth (a measure of size). This island has two
species of finches, with the predicted small and large
bill depths. (B) On the island of Daphne Major, the aver-
age bill size of G. fortis increased after a 1977 drought
that made smaller seeds less abundant than large seeds.
Bill size then evolved back to its original level until the
population of the large ground finch (G. magnirostris)
became large enough to deplete the supply of large
seeds. (A after [46]; B after [22].)03_EVOL4E_CH03.indd 73 3/22/17 1:19 PM