116 CHAPTER 5where s is the selection coefficient that gives the relative fitness advantage of the
mutant heterozygote relative to the homozygote with the original allele.^1
To clarify these ideas, consider a situation in which A 1 A 1 individuals have a rela-
tive fitness of w 11 = 1. A new mutation A 2 appears that gives heterozygotes a fitness
w 12 = 1.01, and so s = 0.01. Beneficial mutations that increase fitness this much are
rare—most have even smaller effects. Nevertheless, the mutation has only a 2 per-
cent chance of becoming established. The conclusion is that even an allele with a
large fitness advantage has an overwhelming chance of being lost by chance when
it first appears. If it is lucky enough to survive the first few generations, however,
then it is much more likely to survive and spread throughout the population.
The conclusion is that even when a mutation increases fitness, it is not certain
that natural selection will cause it to spread to fixation. For a type of mutation
that recurs again and again, chance will have little impact in the long run because
eventually one of the mutations will survive and spread. But some kinds of muta-
tions are exceedingly rare, and may occur only once in very long periods of evo-
lutionary time. In those cases, the chance loss of beneficial mutations is a major
limitation to adaptation.Evolutionary Side Effects
Natural selection often has side effects. These result from genetic correlations,
which occur when two traits tend to be inherited together. One cause of genetic
correlations is pleiotropy (see Chapter 4). If you are taller than average, you are
also likely to have feet that are larger than average. Height and foot size are genet-
ically correlated because alleles that make individuals large for one trait tend also
to make them large for the other.
You can see immediately how this kind of genetic correlation will cause evo-
lutionary side effects. Imagine, for example, that natural selection favors shorter
individuals. As alleles for short height spread, height will decrease—and so will
foot size, which might or might not be advantageous. Early in the evolution of
mammals, selection fixed alleles that enabled females to nurse their young. Among
them were alleles for nipples that are used to deliver the milk. Those alleles, how-
ever, are also expressed in males. Nipples in males evolved as a pleiotropic side
effect of natural selection acting on females.
An allele that increases fitness through its effect on one trait sometimes
decreases fitness because of its effect on another trait. We then say there is an
evolutionary trade-off. When there is a trade-off, natural selection favors the allele
that has the highest fitness overall. As that allele spreads, it will increase some
fitness components (early reproduction, for example) but have negative effects on
others (survivorship). Recall that the alleles for light skin color that are common in
northern populations of humans are beneficial (they increase vitamin D produc-
tion) but also have tradeoffs (they make skin cancer more likely). You will see in
Chapter 11 that trade-offs are an important cause of senescence in humans and
other species.
Soay sheep are a primitive breed that lives wild on a remote island off the coast
of Scotland. Most males have large, curled horns that are important to their mat-
ing success. About 13 percent of males, though, develop tiny vestigial horns (FIG-
URE 5.13). This variation results from polymorphism at a single locus. Many of the
males that are homozygous for an allele called HoP grow vestigial horns, while(^1) Equation 5.5 is an approximation that is very accurate when s is smaller than 0.1 and the size of
the population is much larger than 1/s individuals. Chapter 7 discusses what happens when the
population size is smaller than that.
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