Evolution__3rd_Edition

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Certain non-linear relations can
produce the bimodal response
A polygenic, polymorphic character,
subject to stabilizing selection ...
relatively short veins. Initially, there is some variation in the population for genotype,
and an associated normal distribution of vein lengths. Artificial selection produces flies
with a higher concentration of the vein-inducing substance and with correspondingly
longer wing veins. This process can continue until the population reaches a vein length
of about 60–80. At that point, the normal (unimodal) variation for the amount of
vein-inducing substance generates a bimodal distribution of vein lengths. The bimodal
distribution will later disappear, as the population passes beyond the jump in the
genotype–phenotype mapping function. Hence the observed response to selection.
The relation of genotype and phenotype for vein length in Figure 9.13 is a hypothesis
only; but it does show how in theory a bimodal response to selection could arise.
The main points are that when the genotype–phenotype relation has the linear form
of Figure 9.12a, there is a simple response to artificial selection. The population changes
until the genetic variation is used up. However, we have no reason to think that this is
the typical genotype–phenotype relation. When the relation is more complex, the
response to artificial selection can be interestingly different, as the bimodal response to
selection on wing vein length in fruitflies illustrates.
9.10 Stabilizing selection reduces the genetic variability
of a character
We saw earlier that directional selection reduces the amount of genetic variation for a
character, and this can be measured as a decrease in heritability (see Table 9.2). But
what about stabilizing selection? In nature, many (perhaps most) characters are subject
to stabilizing selection, in which the extremes on either side of some optimum are
selected against. (See Section 4.4, p. 78, where Figure 4.4 illustrates how birth weight in
humans is an example of stabilizing selection.)
Stabilizing selection will also tend to reduce heritable variation. Consider a character
that is influenced by a large number of genes. Some of the genes increase the value of
the character and some of them decrease it. Suppose that the character is influenced by
10 loci, and at each two alleles are present. One of the two alleles increases (+) the value
of the character, the other decreases (–) it. An individual’s haplotypes will then each be
a series of alleles, and might for example be symbolized by –++–+––++. Natural selection
favors individuals with an intermediate phenotype, produced by any genotype
made up of half + genes and half − genes. Here are three examples:
++++++++++ +++++––––– +++++–––––
–––––––––– –––––+++++ +++++–––––
(1) (2) (3)
CHAPTER 9 / Quantitative Genetics 245
Four different haplotypes are found in these three individuals. In a population that
contains these four haplotypes, the three genotypes (1), (2), and (3) will all have the
same fitness. We might expect the population to retain considerable genetic variation
as these genotypes interbreed and produce a variety of offspring types. However genotypes
like (3) that breed true have a small advantage. All the offspring of genotype (3)