Evolution, 4th Edition

56 CHAPTER 3

surface area (over which water is lost by evaporation) to volume (FIGURE 3.2). In

some species, a coat of hair that reflects sunlight reduces body temperature.

An adaptation is a characteristic that enhances the survival or reproduction of

organisms that bear it, relative to alternative character states. Adaptations have

evolved by natural selection, which is the centerpiece of On the Origin of Species and

of evolutionary theory, and is perhaps the most important idea in biology. It is also

one of the most important ideas in the history of human thought—“Darwin’s dan-

gerous idea,” as the philosopher Daniel Dennett [10] has called it—for it explains

the apparent design of the living world without recourse to a supernatural, omnip-

otent designer.

For hundreds of years, it seemed that adaptive design could be explained only

by an intelligent designer. In fact, this “argument from design” was considered one

of the strongest proofs of the existence of God. The Reverend William Paley wrote

FIGURE 3.1 (A) Most snakes, such as this

egg-eating snake (Dasypeltis), can eat prey

much larger than their heads. (B) This ability is

enabled by loose connections among many

skull bones. The movable bones of the upper

jaw are shown in blue. (C) The head of a red

diamondback rattlesnake (Crotalus ruber) in

strike mode. (D) In vipers such as rattlesnakes,

rotation of the shortened maxilla erects its

single tooth, a hollow fang. (B and D after [41].)

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_03.01.ai Date 11-28-2016

Note: PU source line art has been drawn as vector art now so that we can colorize

what ever bones need to be called out. I made all bones a default neutral color with the

exception of the “Maxilla” since that was the only one labeled. OK?

(A) (C)

(B) (D)

Maxilla

Maxilla

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_03.02.ai Date 11-28-2016

(A) (B)

FIGURE 3.2 Many plants that live in arid environments have adaptations to

reduce water loss, as in these cacti, which lack leaves. The stems, where photo-

synthesis takes place, are thick, with a low ratio of surface area to volume. This

adaptation is taken to an extreme in almost globular barrel cacti (A; Echinocac-

tus grusonii). Some cacti, such as this cholla (B; Opuntia bigelovii), have hairs

that reflect light and so reduce the temperature of the plant body.

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