Evolution, 4th Edition

Some people are thrilled by snakes, while others are repelled and fearful, but

no one can deny that they are fascinating animals. Depending on the spe-

cies, they can crawl, burrow, swim, climb trees, and even glide, all without

benefit of legs. Perhaps most amazing, they can swallow prey, whole, that are

much larger than their heads (FIGURE 3.1A). Snakes can do this because, unlike

humans, they have movable skull bones. Their lower jawbones (mandibles)

are joined to a long, movable bone so that they can drop away from the skull,

and their front ends are not fused, but are joined by a stretchable ligament.

The tooth-bearing maxilla bones of the upper jaw can be flexed outward, fur-

ther increasing the mouth opening. The upper and lower jaw bones on both

sides can be independently moved forward and backward to pull the prey

into the throat (FIGURE 3.1B). Rattlesnakes and other vipers take this appara-

tus a step further: their maxilla is short and bears only a long, hollow fang—a

natural hypodermic needle—to which a duct leads from the massive poison

gland (a modified salivary gland). The fang lies against the roof of the mouth

when the mouth is closed. When the snake opens its mouth, the short maxilla

is rotated 90 degrees, so that the fang is fully erected (FIGURE 3.1C,D).

Snakes’ skulls, like many anatomical features, are complex mechanisms that

look as if they had been designed by engineers to perform a specified function.

They are said to be adapted to the animal’s way of life: swallowing large prey

whole. Every species has features—adaptations—that are thought to enhance

survival in its environment. For example, cacti that grow in arid environments

lack leaves and have thick, sometimes globular stems that reduce the ratio of

Natural Selection and Adaptation

and Adaptation

3

Wallace’s flying frog (Rhacophorus nigropalmatus), which inhabits the rain forest canopy in

southeastern Asia, glides between trees with the aid of its extensive toe webbing. This adap-

tation resulted from natural selection among individuals in ancestral populations that varied

genetically in the extent of their webbing. Modification of ancestral features to serve new

functions—such as gliding—is a common theme in evolution.

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