Evolution, 4th Edition

122 CHAPTER 5

We can use that formula and the data on survivorship to predict how the allele

frequencies will evolve. The data show that wAA = 0.88 and wSS = 0.14. Plugging

those values into Equation 5.7 predicts that the equilibrium frequency of the S allele

will be pˆ = 0.12. That agrees well with the allele frequency along the coast of West

Africa near the equator (see Figure 5.18). The evolutionary genetics of sickle-cell

anemia have three general messages about evolution: overdominance maintains

genetic variation, population genetic theory makes testable predictions about evolu-

tion, and humans (like all other species) are still evolving by natural selection.

The three-spined stickleback (Gasterosteus aculeatus) is a small fish that lives in

oceans along the coasts of North America, Europe, and Asia. Remarkably, it has

independently colonized thousands of freshwater streams and lakes around the

world. Michael Bell and colleagues followed one of these invasive freshwater popu-

lations for 12 years in a lake in Alaska [5]. The bony plates that are typical of marine

sticklebacks became much less frequent (FIGURE 5.20A). Later research by Arnaud

Le Rouzic and colleagues suggested that most of this change resulted from evolu-

tion at a single locus called Eda [26]. They estimated the strength of selection on Eda

by finding the relative fitnesses that produce the observed changes in the morph

frequencies. The results suggested very strong positive selection that favored the

homozygote for the low-plated allele (adapted to fresh water): its fitness was esti-

mated to be more than eight times that of homozygotes for the high-plated allele

(adapted to marine environments). They then introduced plated sticklebacks into a

freshwater pond, which they monitored over the next 21 years. Here the fish evolved

rapidly to a polymorphic equilibrium indicative of strong overdominance (FIGURE

5.20B). This example shows again how overdominance maintains genetic varia-

tion. Furthermore, the difference in the results from the natural lake population

and the experimental pond underlines the key point that fitnesses depend on the

environment.

Other forms of balancing selection

Overdominance is one form of balancing selection. A second type can occur

with frequency-dependent selection, which occurs when the fitnesses of alleles

Futuyma Kirkpatrick Evolution, 4e

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1990 1992 1994 1996 1998 2000

0.25

0.50

0.75

1.00

(A) Natural population

0

Year

Low-plated

Low-plated

Completely plated

Morph frequency

1990 1995 2000 2005

(B) Experimental pond

Year

Completely plated

Partially

plated

Partially

plated

FIGURE 5.20 Strong selection on the bony plates that run along

the sides of the three-spined stickleback (Gasterosteus aculea-

tus) causes rapid evolution at the Eda locus, which controls the

plating. (A) Evolution of the frequencies of the three morphs for

bony plates following colonization of a lake in Alaska by a marine

population. The curves show the morph frequencies predicted

when the relative fitnesses are w 11 = 1, w 12 = 0.98, and w 22 = 0.12,

where allele A 1 at the Eda locus results in low plates and allele

A 2 results in high plates. Allele A 1 has the highest fitness and is

predicted to spread to fixation, eliminating the completely and

partially plated morphs. (B) The evolution of morph frequencies

in an experimental freshwater pond following the introduction

of plated sticklebacks. The curves are the predicted trajectories

when the fitnesses are w 11 = 0.83, w 12 = 1, and w 22 = 0.43. This

situation corresponds to overdominance, which results in a stable

polymorphism in which all three morphs are present. (After [26];

photos from [3b].)

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