Evolution, 4th Edition

THE GENETiCAl THEoRy of NATuRAl SElECTioN 129

into acorns.) As the mutation spreads, the mean fitness of individuals in the popu-

lation, shown in the cartoon by the number of fruits they produce, declines.

In this example, the mutation spreads because individuals with it have higher

fitness than their neighbors. That advantage, however, depends on the frequency

of the mutation. When the mutation first appears, it has high fitness because all the

neighboring individuals are small and easily shaded. As the mutation becomes more

common, tall plants are often shaded by other tall plants, and so the population’s

mean fitness declines.

Many animals and plants have traits that enhance their abilities to compete

with others. Even microbes have these kinds of nasty traits. Some bacteria secrete

compounds called bacteriocins that kill neighboring cells, giving them more nutri-

ents [44]. The bacteria that make bacteriocins also have genes that protect them

from the toxin, but both the poison and the antidote are expensive to make. A

benign population of microbes that does not make bacteriocins does not waste that

energy. Nevertheless, individuals that make bacteriocins become more frequent

because their individual fitness is higher. As they spread, however, the mean fit-

ness of the population declines.

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_05.26.ai Date 12-13-2016 01-06-17

(A)

(B)

(C)

Mutation introduced

that causes plants

to grow trunks,

shading out original

shrub population

Ancestral

population

Population has fewer

and smaller fruit per

individual when

compared to

ancestral population

FIGURE 5.26 Frequency-dependent selec-

tion resulting from competition can cause the

mean fitness of a population to decline. A mu-

tation in a population of bushes produces a

trunk. The mutation gains a fitness advantage

by shading neighbors, but it reduces fecun-

dity. As the mutation spreads, the mean fitness

(represented by the number of fruit) declines.

See the text for further details.

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