Evolution, 4th Edition

128 CHAPTER 5

an adaptive valley. If the allele frequency starts to the left of the low point, selection

drives the allele frequency toward 0. However, if the allele frequency starts to the

right of the valley, it evolves uphill toward 1. This visualization of how selection

causes gene frequencies to evolve is one of the most famous images in evolutionary

biology.

The fundamental theorem and the adaptive landscape make assumptions

that do not apply exactly to any natural populations. In many cases, though, they

give very good approximations that are useful to guide our thinking about evo-

lution. In other cases, the assumptions are violated in ways that make evolution

behave very differently. A particularly important situation where the fundamen-

tal theorem and adaptive landscape do not apply is when selection is frequency

dependent. In some cases, this can cause the mean fitness of a population to

decline [45].

FIGURE 5.26 shows a cartoon of an example. In a population of a bush, each

individual makes many fruits. One day, a mutation appears that makes individu-

als grow a trunk. Their neighbors are shaded out and die, which gives individuals

with the mutation more water and nutrients from the soil. The mutation therefore

spreads. But this fitness advantage comes at a cost: energy diverted into growing a

trunk causes individuals with the mutation to produce fewer fruits than did their

ancestors without the mutation. (Just imagine how much more fecund an oak tree

could be if all the energy devoted to growing its enormous truck were converted

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_05.25.ai Date 12-28-2016

0.2

A 1 A 1 A 1 A 2 A 2 A 2

0.4 0.6 0.8 1

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0

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Frequency of A 2

0.2 0.4 0.6 0.8 1

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Mean tness,

w

Relative tness,

w

A 1 A 1 A 1 A 2 A 2 A 2 A 1 A 1 A 1 A 2 A 2 A 2

(A) Positive (B) Overdominance (C) Underdominance

FIGURE 5.25 Wright’s adaptive landscape under positive,

overdominant, and underdominant selection. For each case, the

fitnesses of the three genotypes are shown at the top, and the

adaptive landscape that results from them at the bottom. The

landscapes are the population’s mean fitness, w—, plotted against

the frequency of allele A 2. From a given starting point, the allele

frequency evolves in the direction that increases w—. The ar-

rows show the direction of allele frequency change. The vertical

dashed lines correspond to peaks and valleys on the landscapes.

(A) Positive selection in which allele A 2 is favored. The allele

spreads until it is fixed, which corresponds to the frequency at

which mean fitness is maximized (the solid circle). (B) With over-

dominance (heterozygote advantage), the population evolves to

a stable polymorphic equilibrium, in this case with A 2 at a fre-

quency of 0.66. The mean fitness, w—, is again maximized at this fre-

quency (the solid circle). (C) With underdominance (heterozygote

disadvantage), if the frequency of A 2 starts below a threshold (here

at 0.33), its frequency declines until it is lost. If it starts above the

threshold, the frequency of A 2 increases until it becomes fixed. The

threshold represents an unstable equilibrium that minimizes mean

fitness, w— (the open circle). The loss of A 2 and fixation of A 2 corre-

spond to two local peaks on the adaptive landscape (solid circles).

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