Evolution, 4th Edition

EVoluTioN iN SPACE 193

Other clines are very much shorter. A grass called the common bent (Agrostis

capillaris) grows on and around an abandoned surface mine in Wales (United King-

dom). An area of about 100 m × 300 m is heavily contaminated with copper, which

is lethal to most plants. Remarkably, plants growing on the mine have evolved much

higher tolerance to copper than plants growing nearby. Plants and seeds were sam-

pled from two transects, and their copper tolerance was measured by the rates at

which their roots grew in solutions with controlled concentrations of copper. The

results revealed clines in tolerance that are only dozens of meters long (FIGURE 8.3).

This is an example of local adaptation, in this case to copper in the soil, work-

ing on small spatial scales. Local adaptation of body size in moose and of copper

tolerance in the grass produced smooth clines. In other cases, the environment

varies in a more patchwork way, and local adaptation results in patterns more

like a mosaic than a smooth cline.

Gene flow

Clines, mosaics, and other patterns can evolve when selection pressures change in

space. A second evolutionary force that shapes these patterns is gene flow, which

is the mixing of alleles from different populations. Gene flow plays two impor-

tant roles in evolution. First, it equalizes allele frequencies, and so works to erode

genetic differences between populations. Natural selection can cause two popula-

tions to become either more similar or less similar, but gene flow can only make

them more similar. The second effect of gene flow is to introduce new alleles into a

population from other populations where they already exist. Here gene flow plays

a role similar to that of mutation.

Gene flow results from dispersal, which is the movement of individuals and

gametes. Some species disperse their genes passively. The pollen of plants such as

pines is blown many kilometers by the wind. Many plants have adaptations for dis-

persing their seeds. Some have fluffy plumes on the seeds that enable dispersal by

wind, while others have small hooks that attach the seeds to the fur of mammals or

the feathers of birds. Many plant lineages have independently evolved fleshy fruits

that are swallowed by animals and later defecated elsewhere. Some animals also

disperse passively. Spiders can “balloon.” The spider climbs to a high point, such

as the top of a plant, and spins out a thread of silk. When it feels the tug of a breeze

on the thread, the spider lets go and is carried away. Spiders can reach altitudes of

thousands of meters and can be blown downwind hundreds of kilometers. Corals

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_08.03.ai Date 11-17-2016

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40

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0

60 Adults

Grown from seed

0 50

m

Copper mine

100

Copper tolerance

Wind

FIGURE 8.3 A grass called common bent (Agrostis capillaris)

grows on and around an abandoned surface mine in Wales. Soil

on the mine has high concentrations of copper, and plants grow-

ing there have evolved tolerance of this toxic element. Tolerance

declines in clines over very short distances along two transects that

run from the mine into the surrounding meadow. On the mine, tol-

erance is greater among adult plants than plants grown from seed,

while off the mine the pattern is reversed. (After [21].)

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