126 CHAPTER 5is shown in Figure 5.19C. The key feature is that if an allele’s
frequency is below a threshold value, then selection tends
to drive it out of the population. However, if the allele fre-
quency is above the threshold, selection favors it to increase
further. The value of the threshold is determined by the rela-
tive fitnesses of the two homozygotes.^2
If some chromosome rearrangements are underdominant,
how did they become fixed in different species? Since selec-
tion is clearly not the answer, something else must be respon-
sible. We will return to this mystery when we discuss random
genetic drift (see Chapter 7) and meiotic drive (see Chapter 12).Positive frequency-dependent selection
Earlier we discussed negative frequency-dependent selec-
tion, which preserves variation by favoring an allele when
it is rare. Frequency-dependent selection can also favor the
most common allele, a situation called positive frequency
dependence. The butterfly Heliconius erato is a single species
with many geographic races that differ dramatically in their
coloration (FIGURE 5.24). This species is distasteful, and
birds that eat a butterfly quickly learn to avoid it. In field
experiments, butterflies were marked and released with
two color patterns: the local pattern and a pattern from
one of the neighboring races [31]. Butterflies with the local
pattern survive more than twice as well as those with the
“wrong” color pattern, which generates a selection coeffi-
cient of about s = 0.17 at each of the three major loci that
control the differences in color pattern among the races.
Other data show that these survival differences result from
bird predation: the birds avoid the color pattern they know,
but attack butterflies that have unfamiliar colors. As a
result, strong selection favors whatever color is locally common. Within a pop-
ulation there is typically little variation for color: positive frequency-dependent
selection eliminates polymorphism. The same form of selection occurs on the
closely related Heliconius melpomene, which mimics and co-occurs with H. erato.The Evolution of a Population’s Mean fitness
Many of our questions about evolution concern adaptations such as the feathers of
birds and the brains of humans. But we can also ask: How does fitness itself evolve?
The mean fitness of a population, which is abbreviated with the symbol w—, is
simply the average of the fitnesses of the individuals in it. We can easily calcu-
late w— if we know the fitnesses and the frequencies of the three genotypes at a
locus. (Mean fitness can be calculated using either relative or absolute fitnesses,
but in either case the symbol w— is used.) As selection causes the allele frequency
p to evolve, the mean fitness w— evolves. The evolution of w— follows simple but
important rules when fitnesses are constant in time and other evolutionary fac-
tors (such as mutation) are weak relative to selection. These principles were dis-
covered by R. A. Fisher and Sewall Wright, two of the founders of population
genetics (see Chapter 1).(^2) This type of threshold is called an unstable equilibrium. If the allele frequency lies exactly at
this equilibrium, it will not change, but the slightest deviation will cause the allele frequency to
evolve either to 0 or to 1. In contrast, overdominance produces a stable equilibrium toward which
allele frequencies converge.
Futuyma Kirkpatrick Evolution, 4e
Sinauer Associates
Troutt Visual Services
Evolution4e_05.24.ai Date 12-28-2016 01-06-17
In each pair of butter ies,
H. melpomene is shown
on top, with H. erato below.
FIGURE 5.24 Positive frequency-dependent selection favors
whatever color pattern is locally most common in populations
of poisonous butterflies. The butterflies Heliconius melpomene
and H. erato each show extraordinary geographical variation
in coloration, and their colors vary in parallel. Both species gain
a fitness advantage by resembling the other because birds are
more likely to associate their coloration with distastefulness and
so avoid attacking them. The birds learn to avoid a color pattern
more quickly when it is common. This results in positive frequen-
cy-dependent selection, and eliminates variation in color within
populations of both species. (Photos courtesy of Andrew Brower.)
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