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Biol 258: Phenotypic Plasticity, fall 2002
Week 3: Pigliucci, Ch. 2; Gotthard and Nylin, 1995
Discussion leader: Justin McAlister
Summary of Pigliucci Ch. 2:
Empirical approaches to studying phenotypic plasticity
1) Environmental manipulations - “classical” plasticity experiments
 typically employ some type of factorial design
 factors include temperature & food availability for animals; water, light, and nutrients for plants
 common garden experiments (i.e., in a single environment)
 results presented in terms of reaction norms or in environment vs. environment plots
 statistically analyzed by ANOVA or interenvironment genetic correlations
2) Transplant experiments
 Reciprocally transplant organisms between field sites
- Ecotype: genetically specialized population that has evolved specific adaptations to cope with a
particular environmental range
- Specialist vs. generalist
3) Phenotypic manipulation: phenocopies and mutants
 Organism is “manipulated” to test specific adaptive hypotheses concerning the presence or absence of a
given plastic response
 One approach: “tricking” the organism into displaying the response when conditions are not appropriate
 Another approach: using single-gene mutations that alter the phenotype in specific ways
4) Artificial selection
 Selection to alter the population mean of a trait, thereby demonstrating that it has a genetic basis.
 Comparing reaction norms of different lines obtained in response to selection under constant (but distinct)
environmental conditions, thereby addressing the evolution of plasticity as a correlated response.
5) QTL mapping
 Two lines showing distinct phenotypic differences are crossed and the F1, F2 and later generations are
screened for the frequency of recombination between the phenotype under study and available markers. A
statistically significant association of the phenotype with a series of markers on the same chromosome
indicates that at least one gene affecting the trait is likely to be physically linked with those markers, and
therefore found on that chromosome.
How many types of plasticity?
 Phenotypic plasticity, behavior, and physiological reactions – is time scale an appropriate qualifier?
 Dependent morphogenesis and autoregulatory morphogenesis
 Phenotypic modulation versus developmental conversion
 Allelic sensitivity and regulatory plasticity
 Heterochrony: alteration in the relative timing of developmental events
One generalization (p. 45) drawn from studies of regulatory plasticity: “It certainly is the result of a historical
process of adaption, though it may not be currently adaptive.”
Summary of Gotthard and Nylin, 1995
What is “adaptive,” and what is an “adaptation” in the case of plasticity; are they the same? Should the origin and
maintenance of an “adaptive” feature be distinguished?
2 Sections: First – reaction norms and a discussion of phenotypic plasticity as a putative adaptation
Second – a review of empirical studies of possibly adaptive plasticity
Reaction Norms – the set of phenotypes expressed by a single genotype across a range of environmental conditions
Adaptive plasticity and plasticity as an adaptation
Adaptive:
Adaptation:
A summary of methods – from Table 1.

Adaptive plasticity
Type of evidence:
1. Optimality models predicting the reaction norm, followed by tests
2. Reciprocal experiments

Adaptation within species
Type of evidence:
1. Hypothesis based on optimality, predicting variations among populations, followed by tests
2. Reciprocal transplantations

Adaptations at or above species level
Type of evidence
1. Phylogenetic methods
2. Species comparisons
3. Design criterion
Empirical review of studies that have utilized the above methods for demonstrating adaptive plasticity and plasticity
as an adaptation.
Themes for discussion
1) What is our definition of plasticity?

(tara 1) Why should physiological acclimation and behavior be excluded from what is considered
phenotypic plasticity?

(amber 3) Gotthard and Nylin describe plastic traits as "traits that only appear in response to a stimulus in a
situation where they increase fitness (but are not expressed or even reversed when the stimulus is not
present or removed)." Does that fit our working definition of plasticity?

(marty 1) Is how plasticity works more important or informative then why it occurs? Is it enough to look at
the plastic response without concern for the underlying mechanism?
2) Experimental methods

(willow 1) Given the plethora of variables in a natural ecosystem, what useful information can be gained
from field experiments without combining them with lab experiments?

(amy 1) What understanding of plasticity does QTL mapping really give us?
3) Adaptive vs. adaptation

(ted 1) What should we take home as the definition of the term "adaptive"? If we accept Gotthard and
Nylin's definitions of adaptive (beneficial for a function) and adaptation (designed for a function and origin
linked to this function), how do they make our understanding of the concept of adaptive plasticity any less
difficult to understand?

(amy 2) How is the adaptive vs. adaptation differentiation useful for dealing especially with plasticity?

(marty 2) Do such classifications (adaptive vs. adaptation) give us a better understanding of the plastic
responses we are looking at?

(jon 1) Is there an objective way to invoke the 'design criterion'? Gotthard and Nylin present convincing
cases, but it seems that the argument is basically: if there's some obvious function for the plasticity then it's
adaptive. What about in cases where the function is less obvious or is controversial? It seems like this
'criterion' is pretty subjective and perhaps not always useful.
4) Reaction norms

(greg 2) Is it really important whether a reaction norm originally arose through adaptation, “exaptation”, or
as a by-product of constraint? Isn't whether or not a reaction norm has recently evolved or is currently
evolving due to natural selection the most relevant question to answer in order to investigate the evolution
of plasticity?

(jon 2) Can we use correlations between observed and predicted reaction norms as evidence for the
adaptive nature of plasticity?
5) Character means

(willow 2) If plasticity is determined by measuring the response of an organism for a given trait/traits to
different environments, and character mean is the average expression of a given trait in an environment,
how could these two measures not be related, since they are both determined by measuring traits in relation
to environment?

(greg 1) Wouldn't trait means in different environments always be the direct targets of selection, producing
reaction norms as a consequence (as suggested by Via and Lande)? Why would you need to invoke
selection on the "capacity for plasticity" as a separate process?
6) Allelic sensitivity

(tara 2) How is heterochrony related to allelic sensitivity (p. 44 - 45)?

(amber 2) How would you distinguish between plasticity and allelic sensitivity in an experiment?
7) Ecotypes

(amber 1) Pigliucci states that ecotypes are specialists (p. 34), and that while ecotypes can be plastic, the
plasticity is either incidental or the residual of previous history. But, couldn't ecotypes be plastic for many
other traits?
8) Clarifications and definitions

(willow 3) What does Pigliucci mean by "coherence of the phenotype?" (p. 35, 2nd paragraph) Does this
mean that a number of characters exist within an organism and evolve together, rather than as separate
traits, to some extent?

(ted 2) What is a “spandrel?” From what I understand of the Gotthard and Nylin paper, it is the unavoidable
endpoint resulting from a constraint, thus should it be considered an adaptation, plasticity or simply an
effect?
Full Discussion Questions
Tara Fitzhenry
1) pg 42- Pigliucci talks about what is plasticity and what is not plasticity on this page. In the first sentence in Chapter 1 states: "Phenotypic
plasticity is the property of a given genotype to produce different phenotypes in response to distinct environmental conditions." In chapter 2 he
makes the argument that phenotypic plasticity should not encompass physiological acclimation or behaviors, but rather phenotypic plasticity
should refer to morphological plasticity. Since physiological acclimations and behaviors can be phenotypically regulated and thus change with
different environmental factors. I don't understand why these two groups should be excluded from what is considered phenotypic plasticity?
2) pg 44 & 45- I am a little confused by these two pages, especially how heterochrony is relating to allelic sensitivity?
Amber Rice
1. In the Pigliucci book on page 34, he discusses ecotypes, specialists, and generalists. He says that populations that are ecotypes are specialists,
and that while ecotypes can be plastic, the plasticity is either incidental or the residual of previous history. But, couldn't ecotypes be plastic for
many other traits?
2. On page 43, Pigliucci refers to allelic sensitivity (which he had mentioned in the previous chapter). He defines allelic sensitivity as when "the
same set of alleles controlling a given character can trigger a distinct response of that character to different environments" (p. 19). It seems to me
that this would appear an awful lot like plasticity. How would you distinguish between plasticity and allelic sensitivity in an experiment?
3. On page 6 of the Gotthard and Nylin paper, they describe plastic traits as "traits that only appear in response to a stimulus in a situation where
they increase fitness (but are not expressed or even reversed when the stimulus is not present or removed)." Does that fit our working definition
of plasticity?
Willow Gabriel
1. On p. 12 of the Gotthard and Nylin article (under "Adaptations within species - reciprocal transplantations"), the authors describe a study in
which populations of lizards were transplanted between field sites in New Jersey and Nebraska. They state that field experiments such as this are
"more likely to reveal evolutionary and ecologically important differences between populations" than lab experiments, but they also mention that
only laboratory experiments can identify the specific components of an environment which are affecting the species reaction.
Given the plethora of variables in a natural ecosystem, what useful information can be gained from field experiments without combining them
with lab experiments? (e.g. The authors of the lizard study make the assumption that the difference in photoperiod between the NJ and NE sites
was responsible for the results of their experiments. If this is the best explanation which can be given for the results of a field study, would it not
be better to run experiments in the tightly controlled environment of a lab, and only use field experiments for confirmation of lab results?)
2. On p. 39, 1st paragraph, Pigliucci describes a study in which the authors attempted to determine if plasticity is genetically independent of
character mean. If plasticity is determined by measuring the response of an organism for a given trait/traits to different environments, and
character mean is the average expression of a given trait in an environment, how could these two measures not be related, since they are both
determined by measuring traits in relation to environment? (Maybe my understanding of the terms is still a bit fuzzy?)
3. What does Pigliucci mean by "coherence of the phenotype?" (p. 35, 2nd paragraph) Does this mean that a number of characters exist within an
organism and evolve together, rather than as separate traits, to some extent? I would have liked Pigliucci to expand on the core idea of this
paragraph, because it seems to provide an interesting frame of reference for thinking about evolutionary mechanisms.
Ted Uyeno
Why have we selected a paper on the adaptive value of plasticity for this week? I think it may have been better to leave this paper until we read
the ecology chapter (10), where Pigliucci introduces the concept of phenotypic plasticity as an adaptive strategy. Anyhow, since we opened the
box, my big question is what should we take home as the definition of the term "adaptive"? Gotthard and Nylin attempt to clarify what is meant
by the term adaptive (to say nothing of defining the term "adaptation" or "spandrel"). On page 5, they summarize the two uses of the term
"adaptive". Either it is a change that is beneficial for a function or it is a beneficial trait that has arisen due to natural selection. Pigliucci notes that
if plasticity has an ecological or evolutionary effect, it is because it has an impact on fitness (p. 160). Thus, he implies that the plasticity can be
adaptive and the adaptation may be selected for by nature. Pigliucci therefore uses the te!rm adaptive as simply the adjective of adaptation with
all its connotations of natural selection. How can an adjective (definition: a word that expresses an attribute of something) describe a concept with
a larger scope than its noun? If we accept Gotthard and Nylin's definitions of adaptive (beneficial for a function) and adaptation (designed for a
function and origin linked to this function), how do they make our understanding of the concept of adaptive plasticity any less difficult to
understand? I like Gotthard and Nylin's suggestion of using the word "beneficial" instead of their definition of "adaptive" and wish they did as
well. This discussion may seem like mere semantics, however I think rigorous use of terms with concrete definitions, at least amongst ourselves,
would help us better learn and share the concepts. Another question I have is, what IS a "spandrel". I've only heard that word mentioned in
literature pertaining to medieval architecture! From what I understand of the Gotthard and Nylin paper, it is the unavoidable endpoint resulting
from a constraint, thus should it be considered an adaptation, plasticity or simply an effect?
Marty Ferris
1) Is how plasticity works more important or informative then why it occurs? Pigliucci on pages 42-43 distinguishes between Behavioral,
Physiological and Phenotypic Plasticities, yet these three avenues of plasticity can arise (and not exclusively) in response to a singular
environmental variable (i.e. in response to a temperature change, behavior, physiology, phenotype or some combination of all three can arise). In
other words, is there a need for such classifications, or is it enough to look at the plastic response without concern for the underlying mechanism?
2) Gotthard and Nylin distinguish between traits that are adaptive and those that are adaptations. While it is fairly easy (according to them) to
determine if a trait is adaptive (beneficial), they point to the difficulty of determining whether a trait is an adaptation. Indeed there are cases
where traits initially thought to be adaptations are later shown not to be (Restriction-Modificaiton Systems in bacteria). Do such classifications
(though sometimes wrong) give us a better understanding of the plastic responses we are looking at?
Amy Skypala
1. QTL mapping (pg40-41). This is one of the ways of studying plasticity that P. gave us. BUT beyond mapping a locus to a phenotype, what
understanding of plasticity does it really give us? I am very underwhelmed here, so please someone convince me of the earthshatteringness of
this.
2. Concerning the first few pages of the supplemental reading...The distinction between adaptive and adaptation is just a rehash of the ole
"exaptation" canard deliniating origin versus maintainance. That wasn't all that helpful (for me) dealing with standard adaptation (if that is a
thing), how is it any different dealing especially with plasticity?
Greg Ragland
1) On page 4 (second column) Gotthard and Nylin discuss a distinction between selection on the "capacity for plasticity" and selection on trait
means in different environments. Wouldn't trait means in different environments always be the direct targets of selection, producing reaction
norms as a consequence (as suggested by Via and Lande)? Why would you need to invoke selection on the "capacity for plasticity" as a separate
process?
2) If a researcher is interested in the evolution of an observed reaction norm, is it really important (other than as another interesting direction of
inquiry) whether that reaction norm originally arose through adaptation, "exaptation", or is a by-product of constraint? Isn't whether or not a
reaction norm has recently evolved or is currently evolving due to natural selection the most relevant question to answer in order to investigate
the evolution of plasticity?
Jon Allen
1a) Gotthard and Nylin: Is there an objective way to invoke the 'design criterion'? I'm mainly referring to their discussion on pg. 9 where they
suggest that ' the design criterion can thus be invoked when the link between environmental and phenotypic change is predictable, but improbable
enough that it must have been shaped by selection.' This seems like a pretty fuzzy way of determining when this criterion can be used or not.
They return to this discussion at the end of the paper (pg. 13) where they give well known examples of adaptive morphological plasticity. These
are convincing cases, but it seems that the argument is basically: if there's some obvious function for the plasticity then it's adaptive. What about
in cases where the function is less obvious or is controversial? It seems like this 'criterion' is pretty subjective and perhaps not always useful.
1b) Another comment on G and N: I disagree with their statement on pg 14 that it is useful to view plasticity as the result of organisms 'choosing'
a phenotype on the basis of environmental input. It seems to be misleading to represent morphological plasticity as a choice. In many cases the
outcome is a predictable response to the environmental input.
2) Both Pigliucci(pg 36-37) and gotthard and Nylin spend some time making arguments about how verbal or mathematical arguments that predict
reaction norms that conform to the observed reaction norms are valuable for confirming adaptive plasticity. Are these arguments circular? Can we
use correlations between observed and predicted reaction norms as evidence for the adaptive nature of plasticity? I think that we probably can,
but I still have a feeling of circularity in these arguments that might be useful to discuss.