Evolution, 4th Edition

50 CHAPTER 2

Futuyma Kirkpatrick Evolution, 4e

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Evolution4e_02.23.ai Date 12-16-2016

Q

Q

D

D

G

N

Membrane

Extracellular

Cytoplasmic

NH 2 COOH

H

R

R

E D D

R

R

R R

L

L

Glutamine (Q) changed

independently to arginine (R)

in these four lineages.

2.23). In all of these cases, only four of the amino acids that compose the pro-

tein have been replaced, with exactly the same amino acid substitutions in some

cases. The Australasian lineage of Varanus lizards, including the giant Komodo

dragon, has lost resistance, and the two amino acid positions have reverted pre-

cisely to their ancestral states (glutamine and glycine) [32].

PHYLoGENiES DESCRiBE PATTERNS of DiVERSifiCATioN If the time of each

branching point in a phylogeny has been estimated by a calibrated molecular

clock, the phylogeny may suggest whether new lineages arose steadily over a

long period, or episodically, in one or more bursts of diversification. Divergent

evolution of numerous related lineages within a relatively short time is called

evolutionary radiation. In most cases, the lineages become modified for different

ways of life, and the evolutionary radiation may be called an adaptive radiation

[31]. The characteristics of the members of an evolutionary radiation usually

do not show a trend in any one direction. Evolutionary radiation, rather than

sustained, directional evolutionary trends, is probably the most common pattern

of long-term evolution. The most famous example is the adaptive radiation of

finches in the Galápagos Islands. These finches, descendants of a single ances-

tor that colonized the archipelago from South America, differ in the morphol-

ogy of the bill, which provides adaptation to different diets (see Figure 2.2).

The vangas, a family of birds restricted to Madagascar, provide an even more

dramatic example, in which species differ greatly in bill morphology, foraging

behavior, diet, and habitat (FIGURE 2.24) [16, 30]. Another example of adaptive

radiation is the Hawaiian silverswords and their close relatives, members of the

sunflower family. They occupy habitats ranging from exposed lava rock to wet

forest, and their growth forms include shrubs, vines, trees, and creeping mats

(FIGURE 2.25).

FIGURE 2.23 Rampant convergent

evolution in a protein. A diagram of

part of the sodium-potassium pump

protein, showing the four positions

at which amino acids in the extracel-

lular domain (open circles) have been

independently substituted in diverse

animals. For example, a change

from glutamine to arginine occurred

independently in four lineages (red

arrows). Amino acids are designated

by their single-letter abbreviations: D,

aspartic acid; E, glutamate; G, glycine;

H, histidine; L, leucine; N, asparagine;

Q, glutamine; R, arginine. (After [32].)

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