Phenotypic Plasticity-A Central Concept?
Implications of phenotypic plasticity
Adaptogenesis—how adaptation can occur in multiple environments
Natural variation and maintenance
Quantitative genetics Heritability is environment dependent
Genetic correlation in multiple environments
Modeling
Character correlation and constraints
Gene regulation-Regulatory genes respond differently in different environments.
Plasticity genes may trigger different pathways in different evirons
Comparative method- new area for plasticity but illuminating for hypothesis testing
(i.e. null hypothesis of adaptation isn’t flat rx norm, but that of the
common ancestor)
Willow: How about comparing degree of plasticity in a comparative context?
Tara: Can plasticity be used to build a tree given its often facultative nature and its
possible “unlinkedness” to speciation?
Microevolution within Populations
Selection VERSUS Constraint
Quantify 2 types of variation—genetic and environmental (type/degree)
Pig says we know tons about the genetic and little of the environmental
Marty: Isn’t it circular to use the organism of study as the indicator of environment for
the organism of study?
So for understanding we need to measure selection under natural conditions (Arabidopsis
leaf # at 2 sites) AND what type of genetic constraints (lack of variation , epistasis,
pleiotropy, dominance)
Greg: Strongly disagrees with P’s use of the Arabadopsis leaf # example as illustrating
differing selection pressures in different environments.
Microevolution among Populations
Genetic correlations indicative of:
1. Selection
2. Constraints
What plasticity adds to the debate-- Considerations of constraints affecting same trait in
different environments
Constraints being plastic (not much of a constraint
then, huh)
Arabidopsis bolting example
Jon: I hate figure 11.2. How can constraints be plastic?
Four approaches to study constraints on plasticity
1. induce mutations and look for mutants that lie outside correlation line.
2. characterize pleiotropic and epistatic effects
3. select on one trait and then look for correlated effect on others (mechanistically
linked?)
4. comparative method—when dealing w/ correlated characters can tell what was
selected first
Greg: How about using genetic correlations at the level of the family? This would free
you from the model organism.
West-Eberhard Interlude
Steps to plasticity to result in speciation:
1. phenotype fixation
2. increased divergence due to above
3. acceleration of reproductive isolation due to above
4. rapid attainment of compatibility in sympatry
Willow: Regarding #1 What constitutes phenotypic fixation? If you never see more than
one environment can you tell if a phenotype is fixed? [See related Greg question below]
Jon: Regarding #1…is evolution slower for more plastic organisms. Does speed of
evolution=effectiveness of selection? How does evolutionary novelty (pg 264) fit into
this? WWPS?
Ted: Regarding #1…how do multiple phenotypes compete for the support of the rest of
the genome?
Evidence that polyphenism contributes to speciation:
1. a monophenic derived from an ancestral polyphenic population
2. the polyphenic population of origin had its phenotypes regulated by a switch
3. phenotype fixation occurred prior to or with reproductive isolation (was not an
effect of isolation)
4. accentuated divergence after fixation
Amber: Are all 4 types of evidence necessary, or is there some subset that is sufficient?
Amber: What constitutes acceptable evidence for 3 & 4?
Ted: Regarding pitcher plant mosquito…could paedomorphosis be a fexation of a
metamorphic polymorphism?
Greg: Butterfly example…Has any divergence taken place here since P. virginiensis still
have the ability to suppress diapause and develop similarly to P. napi? I challenge that
virginiensis has become fixed!
Tara: Aphid example…does the host switch she describes really constitute an
evolutionary step?
Justin: Can you split a population of species based only on plasticity differences? How
would this fit into to a species concept?
Marty: How realistic is it that facultative expression of a trait can allow an
environment/niche switch without hitting the “valley”?
Ted: Do we have to invoke plasticity to explain species swarms?
Macroevolution above the Species Level
Shade avoidance phylogeny example—different shade avoidance traits in Arabidopsis
show similar phylogenetic patterns. WHY?
1. genetic constraints imposed by regulatory gene(s) for these multiple traits
2. convergent evolution since lots of “space” for evolution to work traits that require
multiple steps in production
Macroevolution at Large Scales—Where does plasticity come from?
Justin: What is the relationship between the evolution of a trait and its plasticity? Can
the trait and the plasticity of that trait arise together or must a stable trait evolve
plasticity?
Look for answers with
1. molecular genetics via probing for homologous sequences
2. comparative method
Example: Where does shade avoidance come from?
1. probing for homologous DNA led to similar stretches in green algae….so its old
and may not have originally been for photoreceptors (exaptation)
2. functions do not match phylogeny (don’t know what that tells us)
If time permits…worship Richard Goldschmidt.