180 CHAPTER 7Crossing an adaptive valley by drift
Adaptation can sometimes result when drift acts against selection. Many chromo-
some rearrangements are underdominant: heterozygotes have lower fitness than
both homozygotes. Recall from Chapter 5 that in this situation the adaptive land-
scape has a valley. When a new rearrangement appears by mutation, its frequency
is low and selection will tend to drive it out of the population. If drift is strong, how-
ever, the frequency of the rearrangement can sometimes increase by chance until its
frequency is past the low point in the valley (FIGURE 7.16). Now selection favors the
rearrangement, and it spreads to fixation. The population has crossed the adaptive
valley by drift, an event called a peak shift. Sewall Wright, one of the founders of mod-
ern evolutionary biology (see Chapter 1), believed that peak shifts are important to
adaptation. That view, however, is not shared by many evolutionary biologists today.
The chance that drift will cause a population to cross an adaptive valley is
highly sensitive to the depth of the valley, which depends on the strength of selec-
tion against heterozygotes. A fitness reduction in heterozygotes of even 1 percent
is enough to make this outcome virtually impossible except in extremely small
populations. Chromosome rearrangements, which are often underdominant,
evolve more frequently in weedy plants than in long-lived plants [14]. One hypoth-
esis to explain that pattern is that weeds have frequent population bottlenecks and
often self-fertilize, both of which can drastically reduce Ne.The fate of beneficial mutations in large populations
When a beneficial mutation first appears, it is present as only a single copy. The
individual carrying it may leave no offspring, or if it does, the mutation may not
be passed through meiosis to its offspring. A mutation that increases fitness has a
much greater chance of becoming fixed than does a deleterious mutation, but its
fate is still not certain.
A surprising conclusion from population genetics theory is that a single copy
of a new beneficial mutation is almost certain to be lost by drift, even if it has a
large positive effect on fitness. Say that a mutant heterozygote has a relative fit-
ness advantage of s. Then even in a large population with very weak genetic drift,
the probability that the mutation will become fixed is only 2s. A mutation that
improves survival by 1 percent, which is an unusually large advantage, has a 98
percent chance of being lost in the first few generations after it appears. Some Futuyma Kirkpatrick Evolution, 4e
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Troutt Visual Services
Evolution4e_07.15.ai Date 11-14-2016 01-18-171981 1983 1985 1987 1989 1991 1993 1995 1997 1999Number of snakesYear50101520253035All snakesNew juveniles20 males from outside
populations were
introduced into the
inbred Swedish
population in 1992.The introduced males
were removed in 1996.FIGURE 7.15 Population decline and
rebound in an inbred population of adders
in Sweden. The red line shows the total
number of males found in the population
each year; the blue line shows the number
of juveniles recruited into the population
each year. (After [24].)07_EVOL4E_CH07.indd 180 3/23/17 9:09 AM