Biology Reference
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FIGURE 12.3
Two genotypes reared in two environments.
(A) Genotypes differ in their phenotypes but neither responds to
the environmental factor; (B) Both genotypes respond to the envi-
ronmental factor, in parallel directions; (C) One genotype responds
to the environmental factor, the other does not; (D) Both genotypes
respond but in different directions.
multidimensional, we might not be able to depict the array of phenotypes produced by
two genotypes in two environments. Those four shapes might not lie within a plane. It can
therefore be difficult to depict norms of reaction (a subject we return to below), but for
now we will project them onto a plane and show the endpoint of each vector as a shape in
a two-dimensional space (
Figure 12.3
).
Figure 12.3
shows two genotypes in two environ-
ments. In the first case, the two genotypes differ in their phenotypes but neither alters its
phenotype in response to the environmental factor (
Figure 12.3A
). In the second case, both
genotypes alter their phenotypes in response to the environmental factor, in parallel direc-
tions (
Figure 12.3B
). In the third case, one genotype exhibits a large response and the other
does not (
Figure 12.3C
). In the fourth case, both genotypes exhibit a large response but in
different directions (
Figure 12.3D
). What complicates the statistical analysis, which might
otherwise be straightforward, is that a significant interaction term could mean that the
genotypes differ solely in the magnitude of their response or that they differ solely in
the direction of that response or that they differ in both. We therefore need to compare the
phenotypic trajectories of plasticity or, as
Adams and Collyer (2009)
term them, pheno-
typic change vectors. Methods for comparing lengths and directions of ontogenetic trajec-
tories were discussed at length in Chapter 11 and these are the same methods that we use
to compare phenotypic trajectories, in general.
Studies of plasticity often use complex designs, measuring multiple replicates of each
sampling as well as several such units responding to several environmental factors. These
complex, multilevel designs make it possible to measure the overall plasticity of a geno-
type, or of a population or of even higher-level units (e.g. ecotypes, species) over an array
of environmental factors. Overall plasticity would be measured by the variation of a given
genotype over the array of environmental factors. In
Figure 12.4
, Genotype A varies little
in response to all the environmental variables; we can see that all four of its four environ-
ment-specific phenotypes are tightly clustered, occupying a small region of the phenotype
space. Genotype B responds to all the factors more than Genotype A, therefore the four
environment-specific phenotypes occupy a larger area of the phenotype space. Genotype
C responds to one environmental factor but not to the others.
