Plasticity has a genetic basis!!!????
ONE
Plasticity is a natural character that is under genetic control. Thus natural
selection is acting on the genetic variation for phenotypic plasticity.
Three genetic bases of plastic responses:
Overdominance: increased homozygosity increases plasticity
Pleiotropy: one set of genes gives lots of phenotypic results
Epistasis: multiple sets of genes work together
(these are phenomenological: evidence somewhat shaky)
TWO
One can study constraints on 3 levels:
Within population variation:
Try to estimate amount of genetic variation: Shows heritability of plasticity
Generalizations:
Lots of genetic variation for plasticity to respond to selection
Variation is found in all organisms
One trait can be variable while another is not
An organism can have genetic variation for plasticity to one but not necessarily
another set of environments.
There are no masters-of-all (Darwinian monsters)
Trade-offs exist.
Among population variation:
The mean value of plasticity can be considered a species attribute
Generalizations:
There is an increase in variation for plasticity among different populations
Populations living in different environments may not indicate they have more
plastic reaction norms.
One can link environment to pattern of plasticity but you must know the history.
Multivariate phenotype matrices may be complicated by genetic differentiation
and phenotypic plasticity.
Populations with different reaction norms may not result from selection: check
history.
Different populations with similar reaction norms in similar environments may
not be due to selection: once again, check history
Among species variation:
Useful to see if phenotypic plasticity has an impact on macroevolution. These
studies are rare because: Phenotypic plasticity is an individual character (this point is
criticized), the experiments are tough and big, and it is not known if plasticity is
important to evolution (also criticized).
Generalizations:
Close species may have high differences in plasticity: evolves quickly
Close species may have identical plasticities (that may be due to similar selective
pressures or constraints)
Plasticity facilitates evolution of specialists
1
You can identify different types of plasticity evolving in closely related species
that exist in the same environment
Closely related species may develop with same reaction norms and arrive at
different phenotypes and plasticities
Cautiously comparing unrelated taxa with similar and different ecologies results
in good info on adaptive plasticity
Adaptive plasticity can evolve in correlated response to different environmental
cues.
THREE
Genetic constraints on and around phenotypic plasticity:
Constraints of simple (1 or 2) reaction norms
The relation between constraints and plasticity is varied and complex
Interenvironment correlations are usually high when measuring one character
across different environments: low correlations indicate interesting phenomena.
It is tough to differentiate between constraints and stabilizing selection
Constraints concerning groups of reaction norms
Plasticity integration: Some plasticities of distinct characters but of the same
environments are correlated.
There are no generalizations yet.
FOUR
Relationships between plasticity and other epigenetic/genetic phenomena
Heterozygosity and plasticity are found to may or may not be related: they should
not be: environmental buffering.
Developmental noise and plasticity. There shouldn’t be a link: noise is reduced
homeostasis in development and plasticity is a response to macroevolutionary variation.
Thus the only link may be: noise and non adaptive plasticity.
Canalization and homeostasis and plasticity: Homeostasis is the physical result of
the developmental phenomenon of canalization (development that tends to limit final
phenotype). Generally, increased canalized genotypes give rise to homeostatic
phenotypes.
FIVE
How is genetic variation for plasticity is maintained in natural populations?
More heterozygous genotypes should be capable of yielding the same phenotype
regardless of environmental disturbance (-ve relationship between plasticity and genetic
variation). This is a bad theory: not very generalized.
Plasticity cannot maintain variation. Also bad theory because, a) many types of
plasticity, b) types of plasticity interact and c) we don’t know the genetic basis yet
Genetic variation persists but forget about the plasticity. Dump it on a variable
known as “evolutionary load”.
What about the relationship between plasticity and ecotypic variation? Ecotype:
genetically specialized to specific environment.
The amount of measurable genetic variation for quantitative traits is variable.
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Amber
1. In the section on "Constraints Concerning Groups of Reaction Norms," Pigliucci discusses the Witte et
al. (1990) study on cichlid fishes (p. 87-88). Apparently, the skulls and jaws of the cichlids are plastic
when the fish are exposed to alternative diets. The authors described two opposite types of constraints: 1)
the anatomical plasticity is constrained by a lack of plasticity in surrounding structures and 2) the
plasticity of the anatomical feature may force changes in the surrounding structures so that the surrounding
structures are plastic because of the "internal" environment and not the external. How would a researcher
distinguish between these two types of constraints experimentally?
2. On page 83 in the section on genetic constraints, Pigliucci summarizes a study by Dahlhoff and Somero
(1993) on the effect of temperature on respiration in five species of abalone. He concludes this paragraph
by saying "Overall, it seems that the reaction norms of distinct species can be more or less constrained
depending on the corresponding species' ecology and, therefore, on past natural selection. This implies a
different potential response to selection on plasticity by species with distinct ecological amplitudes." Is it
just me, or is this fairly obvious?
3. On page 77, Pigliucci states that research on variation in plasticity among species has led to a number
of generalizations. One of them is "3) There is some indication that plasticity can facilitate the evolution of
specialized ecotypes, starting from a generalist strategy." Could this explain adaptive radiations, like in the
cichlid fishes?
Greg
1) On the bottom of page 86 Pigliucci talks about disentangling genetic constraint from the effects of
stabilizing selection. Any genetic constraint implies a lack of genetic variation in certain directions;
whether or not this lack of variation is produced by stabilizing selection, Isn't this still a genetic
constraint?
2) On pages 105-106 Pigliucci presents some examples of changes in heritability with changes in the
environment. Two of the examples demonstrate that heritability decreases under stressful conditions, but
intuitively it seems equally likely that stressful conditions could increase additive genetic (and not just
dominance and epistatic) variance. For example, couldn't the production of inducible heat shock proteins
"release" genetic variation for various responses to heat stress not seen at lower temperatures where only
constitutive forms of HSP's are present?
Willow
1. On p. 75, Pigliucci puts forth the opinion that it is valid to discuss mean plasticities of populations, just
as you would for morphometric traits. Can plasticity be considered an independent trait, like any other? It
seems to me that it is a component of other traits, rather than a trait in its own right.
2. In discussing the concept of plasticity integration (p. 87-88), Pigliucci mentions that there are not many
generalizations to be made from studies that have been conducted in this area so far. This is unfortunate,
because this seems like it would be useful to understand adaptation of several traits, in concert, to an
environment and that this could help us to understand the mechanism of how selective pressures work
better than looking at traits that have evolved separately. How would it be possible to discern whether
traits with similar reaction norms in relation to environment display that similarity because they are really
integrated, versus having similar reaction norms just due to coincidence?
Martin
1)Pigliucci asks (on pg 68-69) is Plasticity a natural character? I'm a bit confused about his subsequent
defenition of what a natural character is. As long as a trait is not neutral (and costs of plasticity seem to be
assumed as far back as chapter 2) it seems that selection would act on in in response to environments. I'm
confused as well as to the alternatives of being a Natural Character (both definitions and theories- if
selection does not shape you, then you are not a natural character?)
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2) On page 88, Pigliuuci talks about limits to plasticity presented by unplastic anatomical features (i.e. the
skull). He then presents an alternative- that of traits which are not plastic to an external environmental
factor, but rather are plastic to internal environmental factors in response to external factors. Isn't this
making the story more complex than needed? It seems that such characters would then be plastic in
response to the initial external environmental factor with some sort of intermediate step. How then do we
define those characters that are plastic to an internal environment as different than those responding to
external environments (which seem to often have an internal component to their response -i.e. hormones)?
Tara
1) pg 87&88- Witte et al. discuss limits to environmentally induced flexibility. They state that some
anatomical features must be plastic in a coordinated fashion to produce plasticity. It intuitively makes
sense that the degree of plasticity of an anatomical feature can be limited by surrounding features that are
not plastic. For example If you assume that a mammals head size is plastic and in this particular
case it is beneficial to have a big head. However the mammals head can only reach a certain size where the
neck can still support it. Therefore how plastic the size of the mammals head can be is limited by
the strength of the neck. Witte then goes on to conclude that features not plastic from the environmental
stimuli can become plastic to adjust to the features that are plastic from the environment. So in the
example I've used the mammals neck would then become stronger to support the larger head. These
cascades of plasticity, where the external environment causes plasticity which then causes internal
plasticity, seems to bring a new dimension to our discussion of if it matters what's in the black box. As
people have argued in class, you need to understand what is in the black box if you are going to fully
understand the phenotypic plasticity that is being expressed. However by just looking at what is in the
black box you may be missing larger phenotypic changes that are affecting the black box. So knowing this
can you still just study what is in the black box without knowing the larger picture, and inversely can you
study the larger picture when it may be a cascade of internal and external responses for any given
phenotypic plasticity.
2) pg 92,93&94- These pages deal with plasticity versus developmental noise. After a string of studies
with very ambiguous results Pigliucci makes the statement that we do not expect to see a relationship
between plasticity and developmental noise. He states that the fundamental difference is that
developmental noise represents a phenotype induced by microenvironmental variation, and plasticity is a
response to macroenvironmental variation. Could you please explain why plasticity can only occur on the
macroenvironmental level? He also seems to argue that nonadaptive plasticity can be a result of
developmental noise but the same doesn't apply for adaptive plasticity, why?
Justin
1) On page 76, Pigliucci discusses a study by Blanckenhorn (1991) as an example of among-population
genetic variation for plasticity. In this study, B utilized a common garden experiment to test whether water
striders from cold water populations vs. warm water populations displayed genetic differentiation. His
results showed significant genetic differentiation between the two populations in the mean expression of
life history traits (development time, body size, fecundity, etc.). Two items struck me from this study:
First, the null hypothesis was that any differences in life history characters between populations would be
due entirely to differences in microhabitat, which would be compensated for via phenotypic plasticity, and
that would disappear upon development in a common environment. This hypothesis was not supported by
the data, but he did find that the populations were also genetically distinct for their phenotypic plasticities
to temperature. What is unclear to me at the writing of this question, is whether there is a distinct plasticity
associated with every particular phenotypic trait, or whether there is an overarching plasticity trait that may
affect different phenotypic traits differently? Secondly, these two populations occur 6 km apart in streams
that differ about 5 C in temperature on average, yet there was the aforementioned distinct genetic variation
for plasticity. How do spatial and/or temporal scales affect the evolution/occurrence of plasticity in a
population? What does this imply about the predictive power of studies that have measured plasticity only
in one or limited few populations of a particular organism?
2) My second question is less specific, but focuses on the section re: genetic constraints. Other than
pleiotropy and physical linkage, what other aspects of genetic function can be considered a constraint?
basically looking for a more detailed understanding/definition of what exactly is a genetic constraint.
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Jon
1) What is the difference between 'biological' traits (pg 68) and non-'biological' traits? I'm confused by his
distinction between the two. Similarly I'm not sure I understand his use of the term 'natural character'. It
seems to me that all traits of organisms are biological by definition and they are natural as well.
2) On pg. 81 he confuses me again with his arguments about phylogenetic relatedness and independent
evolution. How does he define 'phylogenetically unrelated'? He seems to use this as evidence that
parallel evolution is the result of selection, but it doesn't seem necessary for his arguments. Are cases of
selection on phylogenetically constrained reacion norms uninteresting or somehow invalid?
3) I'm still unsatisfied with our discussions of 'plasticity for fitness'. He returns to this topic several times
in this chapter (an example is on pg. 89). he makes the point that 'plasticity in fitness is, by definition,
deleterious'. But aren't all organisms plastic for fitness? That is, they have different levels of fitness in
different environments. If this is the case then isn't it the relative fitness of genotypes that is important to
measure?
4) I think I disagree with his distinction between plasticity and developmental noise. It seems to me that
plasticity may be a response to micro or macroenvironmental variation. Developmental noise may be hard
to quantify beacuse it is very sensitive to environmental variation, but that doesn't mean it's unimportant for
the evolution of phenotypic plasticity. I also am not sure that his evidence for the lack of a relationship
between plasticity and noise is all that convincing (pg93-94). It may be true that there is no plastic
response to temperature changes in arabidopsis, but there may be lots of plasticity in response to other
environmental parameters and those may be tightly linked to the level of developmental noise present. As
he points out in other parts of this chapter, plasticity in one trait is often not correlated with plasticity
in other traits.
Will
1. How long can a chapter be before it is considered too long?
2. On page 77 he argues that there is a lack of studies across species about phenotypic plasticity. Isn't it
rediculous to expect we can uncover the process from the pattern in these studies because we know so little
about the development of 99.99% of species?
3. On page 84 he talks about the genetic constraints on and around phenotypic plasticity. Is it valid to talk
about fitness and response to selection in the lab study on Brassica rapa with the two predators at different
(and arbitrary) levels? What exactly does he mean by fitness when discussing this study?
You can talk about a reaction norm, but . . .
* You can't make it drink?
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