Evolution, 4th Edition

PHEnoTyPiC EvoluTion 137

the distribution of phenotypes in a population. The three rows show the situa-

tion when different numbers of loci contribute variation to the trait. The left-hand

panels show traits that have small amounts of environmental variation, while the

right-hand panels show traits for which the genetic and environmental sources of

variance are about equally large (a situation typical of many traits). When just a

single locus affects the trait, the phenotypic distribution shows distinct categories

if environmental variation is small, but the distribution is smooth if there is more

environmental variation. When ten loci contribute variation to the trait, the distri-

bution is very smooth even when there is little environmental variation. The com-

bination of a moderate number of loci with some environmental variation explains

why so many familiar traits such as body height vary in a smooth, continuous way.

Figure 6.3 shows another feature common to quantitative traits: phenotypes often

follow a normal distribution, which is also known as a Gaussian or bell-shaped

distribution.

The mean value of the trait in the population evolves when allele frequencies

at the loci change. The change in the mean can be so large that the range of trait

values in the population falls entirely outside the range that was present in the

Futuyma Kirkpatrick Evolution, 4e

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Frequency

86 10 12 14

Trait value

More environmental variance

More loci

86 10 12 14

Trait value

10 loci

4 loci

1 locus

Frequency

86 10 12 14

Trait value

86 10 12 14

Trait value

86 10 12 14

Trait value

Frequency

86 10 12 14

Trait value

FIGURE 6.3 A phenotypic distribution is

determined by genetic and environmental

factors. The vertical blue bars show the phe-

notypes that would be produced without

any environmental influence. The green

curves show the phenotypic distributions

for each of the genotypes that result with

the addition of environmental variance. The

red curve shows the phenotypic distribu-

tion for the entire population. Each locus

has two alleles with frequencies 1/2. The loci

are in linkage equilibrium, and alleles have

equal and additive effects on the trait (there

is no dominance or epistasis). The pheno-

typic distributions become smoother with

larger numbers of loci. Left panels: Small

environmental variance. Right panels: Large

environmental variance.

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