Evolution, 4th Edition

NATuRAl SElECTioN ANd AdAPTATioN 67

that enhance swimming but cannot support flight in air (FIGURE 3.13B). Exaptation

is a very common early stage in the evolution of new adaptations.

Selection of and selection for

To say that a feature is adaptive is unsatisfying unless we have some idea of what it is

adaptive for: by what mechanism did it increase fitness? What is its function?

In the child’s “selection toy” pictured in FIGURE 3.14, the holes in each partition

are smaller than in the one above. Balls of several sizes, when placed in the top com-

partment, fall through the holes in the partitions. If the smallest balls in the toy are

all red, and the larger ones are all other colors, the toy will select the small, red balls.

Thus we must distinguish selection of objects from selection for properties [48]. Balls are

selected for the property of small size—that is, because of their small size. They are not

selected for their color, or because of their color; nonetheless, there is selection of red

balls. Natural selection may similarly be considered a sieve that selects for a certain

body size, mating behavior, or other feature. There may be incidental selection of

other features that are correlated with that feature. We will return several times in

the book to the theme that selection on one trait has side effects on others.

The importance of this semantic point is that when we speak of the function

of a feature, we imply that there has been natural selection for the feature itself:

that the feature caused its bearers to have higher fitness. The feature may have side

effects, other consequences that were not its function, and for which there was no

selection. For instance, a fish species may be selected for coloration that makes

it less conspicuous to predators. The function of the coloration, then, is predator

avoidance. An effect of this evolutionary change might well be a lower likelihood

that the population will become extinct, but avoidance of extinction is not a cause of

evolution of the coloration.

Recognizing adaptations

Not all traits are adaptations. There are at least four other possible explanations of

organisms’ characteristics. First, a trait may be a necessary consequence of physics or

chemistry. Hemoglobin gives blood a red color, but the redness is not an adaptation;

it is a by-product of the protein’s structure. (However, this feature has been co-opted

for various functional roles in the evolution of many species of vertebrates, such as

the white-winged chough [FIGURE 3.15].)

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_03.14.ai Date 11-28-2016

FIGURE 3.14 A child’s toy that selects

small balls, which drop through smaller and

smaller holes from top to bottom. In this

case there is selection of red balls, which

happen to be the smallest, but selection is

for small size. (After [48].)

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_0315.ai Date 11-02-2016

(A) (B)

FIGURE 3.15 (A) The Australian white-winged chough (Corcorax melanorhamphos) normally has

predominantly yellow eyes. (B) During aggressive displays, the bird shows brilliant red, bulging eyes,

using the red color that is a nonadaptive property of hemoglobin for an adaptive function.

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