Evolution, 4th Edition

198 CHAPTER 8

populations is strong evidence for adaptation, and opens the possibility of finding

the genes that are involved [13].

Differences between populations can be caused by phenotypic plasticity as well

as by genetic divergence (see Chapter 6). A famous study by Clausen, Keck, and

Hiesey concerns yarrow (Achillea millefolium), a plant that grows from sea level up

to tree line in the mountains of California [7]. In their natural habitat, plants at high

elevations are much shorter than those at low elevations. This is likely adaptive

because tall plants attract more pollinators and have greater fecundity at low eleva-

tions, while short plants are able to flower before winter arrives at high elevations.

To determine if the differences in height are genetic or result from plasticity, the

investigators grew the plants under uniform conditions in a common garden experi-

ment (FIGURE 8.9). Plants grown from seeds sampled from high elevations grew to

shorter heights than those from lower elevations, but the differences were not as

great as what is seen in nature. These results show that both genetic variation and

plasticity contribute to the differences seen among populations growing at different

elevations. In the yarrow, both genes and plasticity contribute to local adaptation.

Gene flow and Selection

When selection favors different alleles or phenotypes in different places, a tension

develops between local selection, which enhances the genetic differences between

populations, and gene flow, which erodes them. Without gene flow, selection

would cause whatever alleles have highest fitness at any place to become fixed

there. Without selection (or genetic drift), gene flow would make allele frequencies

equal everywhere. The clines and other spatial patterns seen in nature are com-

promises between those extremes.

The tug-of-war between gene flow and selection plays out in the grass grow-

Futuyma Kirkpatrick Evolution, 4e ing on the mine in Wales that was discussed earlier. A striking observation comes

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Sierra Nevada range

Mather Aspen

Valley

Yosemite

Creek

Tenaya

Lake

Tuolumne

Meadows

Big Horn Lake Timberline Conway

Summit

Lee Vining

3600

2400

Elevation (m)

Plant height (cm)

1200

50

25

75

0 20 40

Distance (km)

60

FIGURE 8.9 arrow (Y Achillea millefolium)

is tall in populations near sea level and

short in populations at high elevation.

Seeds were sampled along a transect from

coastal California into the high mountains

inland (bottom), then grown in a com-

mon garden near sea level. The drawings

(top) show their heights. Plants grown

from high-elevation seeds were shorter

than those from low-elevation seeds.

This shows that some of the difference in

height seen in populations growing at dif-

ferent elevations is genetic. (After [7].)

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