Download DQ handout

Biology 258 Phenotypic Plasticity
Week 4: Pigliucci Ch 3
Discussion Leader: Jon Allen
References:
Bradshaw, A.D. 1965. Evolutionary significance of phenotypic plasticity in plants. Advances in Genetics 13:115-155.
Dodson, S. 1989. Predator-induced reaction norms. Bioscience. 447-452.
Johannsen, W. 1911. The genotype concept of heredity. American Naturalist 45: 129-159.
Lewontin, R.C. 1974. The analysis of variance and the analysis of causes. American Journal of Human Genetics.
26:400-411.
Via, S. and R. Lande. 1985. Genotype-environment interaction and the evolution of phenotypic plasticity. Evolution.
39:505-522.
Discussion Topics
Pigliucci
Johannsen and Woltereck
No questions, but Woltereck’s misinterpretation of his results must be one of the great
ironies in biology. Pg 50 (Jon)
Schmalhausen
-We should consider internal (developmental constraints) and external (natural selection) factors
to be equivalent sources of evolutionary changes, yet given the limitation on controlling internal
factors are we limited in our ability to understand the plasticity we observe? Pg. 51 (Marty)
-How can we conclude that “developmental mechanisms are more conserved across species than
plasticity” since it’s so difficult to quantify the two in comparable ways? Pg. 55 (Willow)
“plasticity is a higher order phenomenon (i.e. plasticity can be the result of developmental
mechanisms) and so you can’t compare the two. Pigliucci is an $%#@&!” (Margaret)
Bradshaw
-Could Bradshaw’s description of how stabilizing selection can mask genotypic variation be used
to explain some of the conflict between Johannsen and Woltereck’s debate about the relationship
of genotype to phenotype? Pg. 58 (Amber)
-Can we study situations where plasticity facilitates directional selection in the field or in
the lab? Pg. 57-58 (Will)
-How does Bradshaw (or Pigliucci) define ‘genotype’ and what definition is necessary to
make Bradshaw’s conclusions valid? (Willow)
Lewontin
-If ANOVA and reaction norms are so ‘tightly linked’ then how can Lewontin suggest
that the ANOVA is flawed while the reaction norm should be a central tool for plasticity
studies? Pg. 61 (Marty)
-Is Lewontin’s criticism of ANOVA still valid today? Or do most researchers understand the
limitations? (Greg)
-Can Lewontin’s arguments be applied to any study that looks for patterns (not processes) and if
so, is this just a restatement of ‘correlation is not causation’? (Will)
Via and Lande
When does 1985 become ‘historical’? For cars, it takes 25 years to be considered an
‘antique’. (Jon)
What is the difference between hard and soft selection? Pg. 63 (Amy)
“soft”-populations are regulated independently in each environment and each niche
contributes a constant fraction to the total population
“hard”=contribution of each niche is weighted by its mean fitness and therefore the
population is regulated globally (Via and Lande, pg 508)
Dodson
-What possible constraints may be acting on terrestrial organisms (and not aquatic ones) that
limit the evolution of cyclomorphosis? (Justin/Willow)
-If cyclomorphosis is predictable then could a predator take advantage of this and reduce the
benefits of plasticity such that less plastic phenotypes become advantageous? (Amber)
-When maternal exposure to predators causes changes in offspring traits (as in Daphnia, pg 448)
is that phenotypic plasticity? (Justin)
-If Daphnia were exposed to multiple predator types then would a generic cue be more
beneficial for detecting predators then the Chaoborus-specific cue? Are they exposed to
multiple predators? Pg 448 (Greg)
Full text DQ’s
Tara
1) pg 61-62 - When looking at additivity you seem to be left with the classic argument of nature vs nurture. What
are some practical methods of determining if additivity can be explained by environmental factors (nurture) or if it is
just a specific genotypes (nature)?
2) pg 63- Concerning Via and Lande's model- Do you think most organisms reach optimum phenotype in each
environment, or do the exceptions occur more often then optimal conditions?
Marty
1) Schmalhausen gives equal importance to the effects of both internal and external environments (p 51). Pigliucci
further enforces the importance of this idea later on in the paragraph as a major component of current studies of
plasticity. Given the difficulty of controlling an internal environment for experimental purposes, how does this limit
our ability to understand the plasticity we see?
2) Lewontin states that the real object of studies of phenotypic plasticity should be the reaction norm (p 61) as
ANOVA is flawed. However, in Chapter 1, ANOVA and reaction norms are described as 'tightly linked' (p13 and
following discussion). How is it that ANOVA
is a flawed form of analysis while the reaction norm (a visual representation of ANOVA) should be the central tool
in studies of phenotypic plasticity?
Amber
1. The specific type of plasticity that Dodson seems to focus on in his review is cyclomorphosis, in which the
morphology of an organism can vary in cycles. This suggests to me that cyclomorphosis might often be predictable.
Dodson also emphasizes the usefulness of cyclomorphosis in
avoiding predation. However, how useful would it be if it's predictable? Wouldn't this lead to coevolution or an
evolutionary arms race between the predator and the prey? I suppose this would be the case whether the plasticity
was predictable or not. So, in predator-prey interactions, could a single organism that had an unusually small amount
(in comparison to the rest of the population) of
plasticity for a given trait (i.e. Daphnia's shape) actually be selected for because it doesn't really enter the race? Not
to mention that it doesn't have the extra cost of maintaining the plastic
response....?
2. On page 58 of the Pigliucci, stabilizing selection and its affect on the role of plasticity is discussed as part of the
summary of Bradshaw's contribution to the field. Bradshaw apparently pointed out that under stabilizing selection,
plasticity could actually cause different genotypes to result in the same phenotype. Is this idea hinted at somewhat
by the debate (page 50) between Johannsen and Woltereck? Apparently, this situation in Daphnia was part of what
caused Woltereck's confusion about the relationship between genotype and phenotype. How often might a similar
situation come up in modern science? Could a situation in which the plasticity of two different genotypes converges
on one level of expression be passed over altogether, and the plasticity not even noticed?
Willow
1. In the Pigliucci chapter, organisms are often referred to as having a particular genotype. Given that most of the
papers discussed were published before molecular techniques were much in use, what does Pigliucci mean by
genotype? I assume that the genotype is something they are inferring from mating crosses, or some other indirect
technique, but it does not seem to follow that much about an organism's total genetic makeup can be inferred by
these methods. On page 56, it
is said that Bradshaw came to the conclusion that "plasticity of a given trait can be completely independent of the
plasticity of another trait for the same genotype." When using techniques for genotyping which only give you
information about a few loci, how can he know whether the
individuals have the same overall genetic makeup, and does this not make the above conclusion somewhat invalid?
2. On page 55 of Pigliucci, he states that "developmental mechanisms are clearly more conserved across species
than plasticity." This seems to me to be a bias imposed by the fact that it was written by someone who specialized
in the field of phenotypic plasticity. Given the incredible diversity of developmental mechanisms in nature how can
he quantify the degree of conservation of either development or plasticity in a way that makes them comparable?
3. In the Dodson paper, he mentions that an aquatic habitat appears to be a requirement for a cyclomorphic response
to occur in response to predators (in animals, at least.) Even though transmission of chemical signals is not as
efficient through air as through water, it does occur.
Are there any other reasons that we would not see this type of adaptation in terrestrial animals?
Justin
1. At the end of the second column on page 447, Dodson states "Curiosly, the great majority of examples of
cyclomorphosis occur in aquatic or intertidal habitats." What is special about these
types of environments that this would be the case? Is it perhaps due to great heterogeneity within these
environments? I thought this might be a good topic for discussion.
2. At the end of the second column on page 448, Dodson describes a study he performed in 1989. "In a survey of
seven Daphnia species exposed to three predators, [he] found that when mothers were exposed to predation the
offspring often showed changes as neonates and adults
in body length, helmet length, tail spine length, egg length, and clutch size." I found that to be a particularly
interesting result. Basically, mothers can detect environmental conditions and pass that
information on to offspring. Is this phenotypic plasticity as we've defined it? Or is this an example of GxE? I'll
have more to say about this one in class.
Greg
1) On page 448 Dodson discusses the chemical cues that induce morphological changes in Daphnia. These chemical
cues seem to be very specific; are the cues Dodson refers to only produced by the C. americanus predator? Having
no knowledge of the daphnia system and what has and has not been worked out, it seems like daphnia could
potentially be exposed to many different taxa of predators, in which case having a less specific cue (chemical or
otherwise)
would be much more advantageous.
2) On page 60 to 62 (Pigliucci), is Lewontin's criticism of ANOVA still valid today? I think most researchers
understand the limits of ANOVA (i.e.interactions limit interpretations of main effects) and that it can be used to
identify interactions but not to characterize them.
Will
1. The discussion of Bradshaw's thoughts on how different types of selection should affect plasticity was interesting
(p. 57-58). This question has two parts. First, I have forgotten the exact definition of "genetic assimilation," but I
don't think it's too important for the rest
of the question.
The theory that plasticity might be allow a population to persist in an environment where directional selection is
acting in the same direction as the plasticity is interesting. Is this one of the areas in which plasticity could play a
role in evolution, and is it even possible to study this situation in the field or even the lab?
2. Regarding Lewontin's argument that gene by environment interactions invalidate the ANOVA as a technique for
analyzing how genes affect phenotypes (p. 59-63), can't this type of argument be applied to any study that looks for
patterns (not processes) and, if so, isn't it just a
restatement that correlation is not causation?
Isn't it better to use the ANOVA first, figure out what the variance is,and investigate all potential sources that violate
the assumptions of ANOVA?
If some of the assumptions of quantitative genetics and ANOVA are violated in a system, is the partitioning of
variance totally invalidated or can the model be corrected to take these violations
into account?
Amy
1. On page 56, P briefly mentions some "long standing debate" about heterozygosity and plasticity. What is this all
about? I am not sure I see the connection at all.
2. Hard selection and soft selection....difference not defined (probably means I should know this already!), but it
obviously makes a difference. How, and why is this difference important?