PHENOTYPIC PLASTICITY AND NORMS OF REACTION
Phenotypic plasticity is the ability of individuals to alter its physiology,
morphology and/or behavior in response to a change in the environmental
conditions. This is clearly demonstrated by the appearance of plants grown
at different densities: crowded plants look spindly and lanky, uncrowded
plants look healthy and robust. In the context of evolution, phenotypic
plasticity demonstrates the two meanings of adaptation: the plastic
response is itself an example of a physiological adaptation and
it is widely held that the ability to be plastic is adaptive
in the sense of increasing fitness.
In thinking about phenotypic plasticity as a evolutionary adaptation
it is important to separate the trait in question from the
plasticity for that trait. For example: growing taller in response
to plant crowding is adaptive in the sense that it increases an individual's
competitive ability for sunlight (lower fitness when shaded by other plants).
The "normal" height for a plant (lets assume there is such a
thing) may have evolved in response to pressures to allocate resources
to growth versus reproduction in a particular way. Thus there is a genetic
basis for plasticity of plant height, and a genetic basis for plant
height itself. The point is that different genes probably
control these processes so the trait and its plasticity can (as
opposed to must) evolve independently.
Now consider the environment: certain physical properties of the environment
can be described by the mean (average) value or the range
of values (highest - lowest). Which aspect of an organism (the trait itself
or the plasticity for that trait) will evolve in response to which measure?
It may be that the plasticity for a trait will evolve in response to the
range of values the environment throws at an organism (e.g., coldest -
hottest, driest-wettest days), whereas the trait itself (e.g., thickness
of fur) will evolve in response to the mean. This is not a rule!
but would be an interesting thing to test and/or think about.
The idea of plasticity is interwoven with the notion of canalization.
In light of the ball rolling down the trough of a developmental pathway
(previous lecture), one can consider the width of the trough as an indication
of the amount of plasticity "tolerated" in the organism in question.
A highly canalized organism (or developmental program) would have
low plasticity.
Another variant form of the plasticity issue is that some organisms
may exhibit threshold effects where there is not a clear gradual
transition between forms, but a stepwise change of phenotype in response
to a gradual environmental change. See fig. 9.11, pg. 242, but note that
these graphs do not have an environmental axis, so a distinct from a norm
of reaction. One example of this are plants that have distinctly different
growth forms in different environments. Question: is there an "environment"
that is half way in between air and water?, and if so would these plants
exhibit a graded response to such an environmental gradient?
A concept that places phenotypic plasticity in the context of a genotype-specific response is the norm of reaction. A norm of reaction is an array of phenotypes that will be developed by a genotype over an array of environments. The quantification of a norm of reaction is conceptually quite simple: one obtains a number of different genotypes (clonal pants are great for this) and grows each one in a variety of different environments (e.g., different nutrient, light, water conditions). After a period of growth one measures the desired trait(s) from each individual and plots the data out as shown in figure below; this case for Drosophila bristles. Each line represents the data for a different genotype. If all lines are perfectly horizontal and on top of one another there is no effect of environment (E) or genotype (G) in case 1 below (each genotype is x, y or z). If all lines are not horizontal but on top of each other there is an environmental effect, but no genotype effect (case 2). If all lines are horizontal but at different positions there is no effect of environment but there is an effect of genotype (case 3 below). If lines not horizontal but are parallel there is an effect of environment and genotype, but there is no genotype x environment interaction (figure and case 4 below). If the lines are anything other than horizontal, there is an effect of environment. If the lines are neither horizontal nor parallel there is an effect of I) environment (nonhorizontality), ii) genotype (lines not on top of each other) and iii) genotype x environment interaction (not parallel; case 5 below).
The interesting case comes when the norms of reaction lines cross. Then there is a range of environments where genotype 1 is "bigger" than genotype 2, where both genotypes are about the same and where genotype 2 is "bigger" than the genotype 1 (see figure below). Thus determining what is the "best" genotype, or the "fittest" genotype depends on the environment.