Evolution, 4th Edition

NATuRAl SElECTioN ANd AdAPTATioN 69

locomotion or defense) in a way that is likely to increase fitness, relative to indi-

viduals with other features. For example, several floral characters have evolved

convergently in the many plant lineages that have shifted from insect pollination

to bird pollination (FIGURE 3.18A). Maria Castellanos and colleagues tested the

hypothesis that some of these features are advantageous because they facilitate

bird pollination, and others because they discourage bees, which are less effec-

tive pollinators because they comb much of the pollen into a mass that they feed

to their larvae [7]. The researchers surgically altered several features of flowers

on a bee-pollinated plant to resemble those of related hummingbird-pollinated

species (FIGURE 3.18B–E). They then measured pollen transfer from the altered

flowers by bumblebees and hummingbirds. The researchers concluded that the

lower “lip” typical of bee-pollinated flowers, which bees use as a landing plat-

form (see Figure 3.18B), has been reduced or lost in some bird-pollinated species

because its absence discourages bees (see Figure 3.18C). The projecting anthers of

bird-pollinated plants also seem to be an “anti-bee” adaptation (see Figure 3.18D),

and the narrowly constricted corolla tube (see Figure 3.18E) is both “pro-bird” and

“anti-bee”: it forces hummingbirds to remove more pollen, but prevents bees from

easily obtaining nectar.

THE CoMPARATiVE METHod A powerful means of inferring the adaptive sig-

nificance of a feature is the comparative method, which consists of comparing

sets of species to pose or test hypotheses on adaptation and other evolutionary phe-

nomena [13]. This method takes advantage of “natural evolutionary experiments”

provided by convergent evolution. If a feature evolves independently in many

lineages because of a similar selection pressure, we can often infer the function of

that feature by determining the ecological or other selective factor with which it is

correlated. For instance, a long, slender beak has evolved in at least six lineages of

birds that feed on nectar, and many plants that are pollinated by such birds have

independently evolved attractive red or orange coloration and a tubular form that

restricts access by bees (see Figure 2.22). Among fishes, open-water, fast-moving

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_03.17.ai Date 11-02-2016

(A) (B)

FIGURE 3.17 Functional morphological analyses have shown

that small surfaces shed the hot “boundary layer” of air that forms

around them more readily than do large surfaces. Many tropical

and desert-dwelling plants have large leaves that are broken up

into leaflets, as in Acacia karroo (A), or split into small sections, as

in the banana (B). The form of these leaves is therefore believed

to be an adaptation for reducing leaf temperature.

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