THE GENETiCAl THEoRy of NATuRAl SElECTioN 117
heterozygotes and homozygotes for the Ho+ allele
grow normal horns. These alleles have strong pleio-
tropic effects on fitness components [23]. Vestigial
horns decrease mating success but increase survival.
As a result of this trade-off, heterozygous males have
the highest overall fitness. We will explore the evolu-
tionary outcome of this situation shortly.Hitchhiking: When one allele goes for a
ride with another
You saw in Chapter 4 that an allele at one locus is
sometimes found together with an allele at a second
locus more often than expected by chance. When that
happens, we say that the two loci are in linkage dis-
equilibrium. Recall from Chapter 4 that linkage dis-
equilibrium is a statistical property of the population,
like an allele frequency, and can occur whether or not
the loci are physically linked.
An important consequence of linkage disequilib-
rium is hitchhiking. This happens when an allele at
one locus spreads by natural selection acting on a
second locus that is in linkage disequilibrium with
the first. FIGURE 5.14 shows a hypothetical popula-
tion of the grove snail (Cepaea nemoralis), which has
separate loci that affect color and banding pattern
on its shell. There is linkage disequilibrium in this
population. The allele for an unbanded shell is cor-
related with the allele for a pink shell: all of the yellow
shells are banded, but only half of the pink shells are.
If all the banded snails die (for example, because birds
prey on them), all the survivors will be pink as well
as unbanded. Thus the frequencies of the shell color
alleles changed because of selection on the banding
alleles. While it sounds paradoxical, one implicationFutuyma Kirkpatrick Evolution, 4e
Sinauer Associates
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Evolution4e_05.13.ai Date 12-29-2016(A) (C)(B)0.10.40.71.01.3Relative mating sucess1.01.52.02.53.0Relative survivalHo+Ho+ Ho+HoP HoPHoP0.91.01.11.2Relative tnessFIGURE 5.13 Genetic trade-offs between survival and mating success
maintain a dramatic polymorphism in the size of horns in male Soay sheep.
(A) A homozygous Ho+Ho+ male with normal horns. (B) A homozygous
HoPHoP male with vestigial horns. (C) HoPHoP males have the lowest mat-
ing success, while Ho+Ho+ males have lowest survival. The net result is that
Ho+HoP heterozygotes have the highest overall fitness. (A, B from [23]; C
after [23].)Futuyma Kirkpatrick Evolution, 4e
Sinauer Associates
Troutt Visual Services
Evolution4e_05.14.ai Date 12-29-2016 01-06-17SelectionFIGURE 5.14 An allele can spread by hitchhiking
if it is associated with another allele that is a target
of selection. In this hypothetical population of the
grove snail (Cepaea nemoralis), alleles for band-
ing of the shell are in linkage disequilibrium with
alleles for shell color. All yellow shells are banded,
but only half of the pink shells are. If all snails with
banded shells die (for example, as the result of
bird predation), the survivors will be unbanded
as well as pink. The frequency of shell color has
changed even though selection acted only on
banding. Note that linkage disequilibrium can
occur between two loci even if they are not physi-
cally linked to each other.05_EVOL4E_CH05.indd 117 3/23/17 9:01 AM