Biology Reference
In-Depth Information
CHAPTER
12
Evolutionary Developmental
Biology
(2): Variational P
roperties
This chapter discusses methods for analyzing variational properties, including pheno-
typic plasticity, canalization, developmental stability, morphological integration and the
related property of modularity. What all of them have in common is that they determine
by how much, and in what combinations of traits, phenotypes
can
vary. We emphasize
can
vary rather than
do
vary, for two reasons. First, phenotypes can vary in more than they
do
each phenotype is a single instance of what a genotype can produce and each popu-
lation is a single case of the phenotypes that could be produced by that population's geno-
types. Second, the variation within a population depends not only on the (intrinsic)
variational properties of the genotypes
it also depends on the processes that determine
the mix of genotypes within a population as well as on the environments they encounter
during development. A population could be nearly invariant phenotypically for a variety
of reasons, including that it is genetically homogeneous, that the environment is uniform,
or that deviants die young. None of these are attributes of the genotypes within the popu-
lation
none will travel with the individuals or be transmitted to their offspring. But a
population could be nearly invariant because of developmental mechanisms that suppress
the expression of variation. These mechanisms
can
travel with the individual and
can
be
transmitted to its offspring. The distinction between the observed, realized variation
within a population and the variation that could be produced motivates the distinction
between “variation”, i.e. the realized, observable variation in a population; and “variabil-
ity”, i.e. the propensity to vary (
Wagner and Altenberg, 1996
). Although we cannot
directly observe propensities, the distinction between variation and variability is nonethe-
less useful because it highlights the difference between population-genetic processes that
determine the mix of genotypes within a population and the intrinsic-developmental
mechanisms that regulate the expression of (co)variation.
The variational properties are all conceptually related to each other. All of them can be
regarded as attributes of the genotype
phenotype map, which provides a unifying theme
