Evolution__3rd_Edition

240 PART 2 / Evolutionary Genetics
The quantitative genetic model has
broad application
The strength of selection has been
measured in Darwin’s finches ...
the population suddenly started to respond again after an interval (in generations
14–17). The renewed bout of change is attributed to a rare recombinant or mutation
that reinjected new genetic variation into the population.
The relation between response to selection (R), heritability, and selection differential
(S) enables us to calculate any one of the three variables if the other two have been measured.
For example, we saw in Chapter 4 that fishing has selected for small size in
salmon, because larger fish are selectively taken in the nets. The selection differential S
can be estimated from three measurements: the average size of salmon caught in the
nets, the average size of salmon in the population at the mouth of the river (before they
are fished), and the proportion of the population that is taken by fishing. All three have
been measured and lead to the estimates that the salmon who survive to spawn are
about 0.4 lb (0.18 kg) smaller than the population average. Figure 4.3 shows that
response (R) a the average size of the salmon a decreased by about 0.1 lb between each
2-year generation. We can therefore estimate the heritability, h 2 = 0.1/0.4 = 0.25.
9.8 Strength of selection has been estimated in many
studies of natural populations
A character such as beak size may be experiencing directional selection in a bird population.
We can estimate the response to selection (R) by measuring the average size over
a number of years. Standard quantitative genetic techniques can be used to estimate
heritability. We can then use the two numbers to estimate the selection differential.
The selection differential expresses how strongly selection is acting (in the case of directional
selection, but not stabilizing selection). If the successful individuals are very
different from the average individuals in the population, selection is strong, and the
selection differential (S) will be large. If selection is weak, the successful individuals will
be more like a random sample from the population as a whole and S will be small.
In Darwin’s finches, Gibbs & Grant (1987) measured the response to selection (R),
and heritability, for several characters related to body size, and used these to estimate
selection differentials. We saw that in Geospiza fortis heritability of beak size is about
80%; and after the bout of selection for large size in 1976–77, the finches were about 4%
larger. We can estimate the selection differential as S = 0.04/0.8 =+5%. The results for
several characters in three periods is shown in Figure 9.9. As the direction of selection
reversed from favoring larger beaks between 1976 and 1978 and smaller beaks between
1983 and 1985, the selection differentials reversed from positive values in 1976–77 to
negative values in 1984–85. The next El Niño event came along after Gibbs and Grant’s
paper. This time the changed weather had little effect on the seed size distribution, and
the selection differential was round about zero (Grant & Grant 1995).
In Darwin’s finches the measured relations between the selection differential,
heritability, and response to selection all fit with the predictions of quantitative genetic
theory. Any two of the three can be measured, and the third accurately predicted
(Grant & Grant 1995). However, Section 9.12 below will look at some more puzzling
cases a in which a character is subject to directional selection (the value of S is nonzero),
and has been shown to be genetically heritable, but shows no evolutionary
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