Evolution, 4th Edition

66 CHAPTER 3

The Nature of Adaptations

Adaptation is a central concept in biology. The word has two related meanings.

“Adaptation” means the evolutionary process by which, over the course of generations,

organisms are altered to become improved with respect to features that affect survival

or reproduction. “An adaptation” is a characteristic of an organism that evolved by

natural selection. Both meanings are difficult to define precisely [29, 43]. Most evo-

lutionary biologists think that for a character to be regarded as an adaptation, it must

be a derived character that conferred higher fitness than the ancestral character state

from which it evolved [25].

A preadaptation is a feature that fortuitously serves a new function. For instance,

parrots have strong, sharp beaks, used for feeding on fruits and seeds. When

domesticated sheep were introduced into New Zealand, some were attacked by an

indigenous parrot, the kea (Nestor notabilis), which pierced the sheeps’ skin and fed

on their fat (FIGURE 3.12). The kea’s beak happened to be useful for this new activ-

ity. Such a feature, if co-opted for a new function during evolution, is sometimes

called an exaptation [20]. For example, the wings of auks are exaptations for swim-

ming: these birds “fly” under water as well as in air (FIGURE 3.13A). An exaptation

may be further modified by selection so that the modifications are adaptations for

the feature’s new function: the wings of penguins have been modified into flippers

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_03.12.ai Date 11-28-2016

FIGURE 3.12 The long sharp bill of

the kea (Nestor notabilis) evolved for

functions such as cracking seeds, but

it can be used for many other things,

such as slicing into sheep skin and

ripping windshield wipers, rubber

gaskets, and other removable pieces

from parked automobiles. At sev-

eral sites in New Zealand, tourists are

warned to protect their cars against

keas. Why keas do this is not clear.

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_0313.ai Date 11-02-2016

(A) (B)

FIGURE 3.13 Exaptation and adaptation. (A) The wing might be called an exaptation for un-

derwater “flight” in members of the auk family, such as this Atlantic puffin (Fratercula arctica).

(B) The modifications of the wing for efficient underwater locomotion in penguins (these are

Humboldt penguins, Spheniscus humboldti) may be considered adaptations.

03_EVOL4E_CH03.indd 66 3/22/17 1:19 PM