Evolution, 4th Edition

192 CHAPTER 8

Patterns in Space

The geographic variation of cyanide-producing clover is an example of a cline,

which is a smooth change in space of a trait mean or an allele frequency. Some

clines extend over large geographical scales. Moose (Alces alces) are much heavier

in northern Sweden than they are farther south (FIGURE 8.2). Similar clines are

seen in many groups of animals: populations in colder climates tend to have

greater body mass than those in warmer climates. This pattern is so common

that it has a name (Bergmann’s rule). In the moose and other homeothermic ani-

mals, larger individuals lose heat less quickly in cold climates (because they have

lower surface-to-volume ratios), while smaller individuals can dissipate heat more

quickly in warm climates. Clines in body size like that in the moose can be hun-

dreds or even thousands of kilometers long.

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–13.3°C

–8.9°C

2.0°C –4.4°C

8.0°C

4.4°C

0°C

Plants produce

cyanide

Plants do not

produce cyanide

Trifolium repens

FIGURE 8.1 White clover (Trifolium repens)

has a smooth gradient (cline) in the frequency

of a gene needed to produce cyanide, which

protects the plant from herbivores. The white

portion of each pie diagram shows the fraction

of the population in which the gene CYP79D15

has been deleted [25]. Frequencies of deletions

are much higher in populations growing in cold

climates. The solid curve shows the 0°C isotherm

for January. (After [8].)

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Latitude (°N)

180 190 200 210 220 230

Average body mass of moose (kg)

Sweden

60

63

66

57

60°

63°

66°

69°

57°

54°

Alces alces

200 km

FIGURE 8.2 Moose (Alces alces) are larger

in the north of Sweden than in the south.

This cline is an example of Bergmann’s rule,

which says that the body sizes of mammals

and birds tend to increase with distance

from the equator. (After [31].)

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