Transcript Lecture PPT - Carol Lee Lab

Adaptation
vs
Plasticity
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The Problem:
• People often wish to jump to the conclusion
that a trait change they see is the result of
adaptation
• However, that is not always the case. There
are other mechanisms that could cause
phenotypic variation
• This is what Stephen Jay Gould called the
“Adaptationist Paradigm”
The Problem:
• Adaptations are ubiquitous,
but demonstrating that a
particular trait is an adaptation
is not always easy
Critique of the
“Adaptationist Programme”
Gould & Lewontin 1979. The Spandrels of San Marco and the Panglossian
Paradigm: A Critique of the Adaptationist Programme.
• One of the most important papers in Evolutionary
Biology
• They critique the “Adaptationist” and “Panglossian
Programme” that assumes that a phenotypic change is
the result of adaptation
• Gould & Lewontin point out that not all phenotypic
variation or phenotypic evolution is the result of
adaptation
Gould & Lewontin:
The spandrels of San Marco
San Marco Cathedral, Venice
Gould & Lewontin on
Physical Constraint:
The spandrels of San Marco
might not have been created
for a reason, but might
simply be a by product due
to the creation of arches
San Marco Cathedral, Venice
Physical Constraint
Developmental constraint
Constraint in Body Plan
If body size increases, brain size has to increase
If a larger eye evolves, need a bigger socket (the
socket itself is not the target of selection)
Analogy: the Spandrels of San Marco
Gould & Lewontin
Other potential causes of phenotypic variation
that is NOT Adaptation:
(A) Plasticity: phenotypic change without evolution
(B) Nonadaptive Evolutionary Forces:
Genetic Drift
Genetic Constraint (Linkage, Pleiotropy)
(C) Physical Constraint (allometry, Mechanical
compensation)
Not All Phenotypic Variation is due to
Adaptation
Phenotypic change and variation could have
other causes:
– Changes that are not due to genetic changes, but due to
changes in gene expression: Phenotypic Plasticity
– Changes that are Genetic, but NOT adaptive:
• Genetic Drift: random chance
• Linkage and Genetic Hitchhiking: Genetic changes that
occur because the gene was right next to another gene on a
chromosome that was under selection
• Constraint: physical or structural (like the Spandrels)
Adaptation
Requires Natural Selection
Requires polymorphism in a population
MUST have an effect on Fitness
Is a frequency (%) change in a population
There must be a Selective Force
How can you tell if a trait evolved as a
result of adaptation?
(1) The trait must be heritable
(2) The differences between populations are
genetically based differences rather than inducible
differences (plasticity)
(3) The trait has fitness consequences (promotes
survival, performance, and number of offspring)
(If a trait evolved due to genetic drift, linkage or
pleiotropy, the change is genetic, but may confer
no fitness advantage)
Measuring Heritable Variation
• Quantitative traits are controlled by many loci, many of
which with small effects. For quantitative traits, we depict
the sources of variation as follows:
• Phenotypic variation is a result of variation that is due to
genetic effects (VG), variation due to environmental
factors (VE) and their interaction.
VP = VG + VE + VGxE
• So, some of the variation could be due to genetic
causes, but some might be induced by the environment
(as a result of gene expression).
Conceptual Confusions
Trait variation is often assumed to be
due to Adaptation, when the differences
might be due to Phenotypic Plasticity or
nonadaptive genetic causes
Phenotypic Plasticity
Definition:
• Differences in phenotype that a genotype
exhibits across a range of environments
• Some traits with a plastic component:
intelligence, height, temperature tolerance,
salinity tolerance, muscle mass…
Acclimation (≠ Adaptation)
1) Result of Phenotypic Plasticity
2) Not heritable
3) Short term or developmental response within
a single generation
4) Arises through differential gene expression
or other regulatory mechanism rather than
natural selection
Nature vs Nurture
• Both environment and genetics affect many
traits, but need to experimentally or statistically
separate these factors
• How?
• Example: Common-garden experiment
• Having appropriate controls
• Statistically assessing the effect of environment
This is a general problem
• This type of problem is a factor in all studies that
attempt to associate a gene with a trait
• You need to account for the effects of environment
• For example, problems arise when different labs attempt
to associate a gene with a disease using laboratory
mice that have been reared under different conditions
Types of Plasticity
• Short-term reversible
• Development acclimation:
generally irreversible
Genotype --> Development --> Phenotype
• Within normal tolerance range
• In response to Stress
Plasticity can be depicted
graphically as a Reaction Norm
Response
Environment
Reaction Norm: the function which describes the plastic response
Response
Environment
• In the case of plasticity, the different phenotypes in
different environments are NOT the result of Adaptation…
• The Genotype(s) in the environments are NOT changing
• The differences between them are due to differences in
response (such as gene expression) in different
environments
Dodson, SI. 1989. Predator induced reaction norms. BioScience 39:447–452
Predator induced
formation of helmets
in Daphnia
Hebert and Grewe, 1985
Genotype x Environment
Interaction
• Changes in rank or level of performance
among genotypes when tested in different
environments
• Reveals genetic variation for plasticity
• Could reflect tradeoffs between fitness of
different genotypes in different environments
When lines cross, the implication is that different
environments will select for different phenotypes
Response
Environment
Trade-offs in different environments
Select for this reaction
norm in cold
environments
big
Size
Select for this reaction
norm in hot
environments
small
cold
hot
Temperature
Could get selection for different reaction norms
(different plasticity) in different environments
Genetic variation for plasticity can be determined by
examining the significance of the interaction term from an
Analysis of Variance (ANOVA)
Genetic Variation for Plasticity
No Genetic Variation for
Plasticity
Response
Environment
Environment
Most Importantly,
• Must distinguish plasticity from adaptations to
understand heritable (and permanent) vs
inducible differences, in order to interpret
experiments properly
• Many experiments fail to do this
• Examples: drug response, hormone
replacement therapy
How to distinguish between genetically based
traits vs. phenotypic plasticity?
• Animal Model Analyses: Determine how much of a trait is due
to additive, dominance, genetic variance etc (quantitative
genetic methods) – not cover here
• Common-garden experiment: rear different populations in a
common environment to remove the effects of environmental
plasticity, and determine how much variation is remaining
(and due to genetic effects).
• Look at selection response in nature (R= h2S, breeder’s
equation)
• Selection Experiments (Experimental Evolution): Impose
selection on a population, then examine evolutionary shift
• Molecular Genetic Approaches: transgenic or gene knockout
studies, to determine the impacts of particular genes
What is a common-garden
Experiment?
• An Experiment in which individuals from
different populations or species are reared
under identical conditions (can be over a
range of conditions)
• Remove differences due to environmental
plasticity
Example:
Different Populations
A saltwater population and a freshwater population of
a small crustacean (copepod) show differences in
salinity tolerance.
Are the differences due to simply being reared at
different salinities, or are the differences due to
genetically based differences?
Common Garden
Experiment
Different Populations
Rear under common conditions
To determine the differences
when the environment is held
constant
Common Garden
Experiment
If the populations still differ
under common-garden
conditions, the differences are
genetically based.
But are these genetic
differences the result of
adaptation? (or some other
genetic cause)
Different Populations
Laboratory Selection Experiments
• But is salinity really the factor causing the
evolutionary physiological change?
• Perform selection experiments to test whether the
evolutionary change happens in response to salinity
alone.
Control
saline ancestors
Selection for several generations
ancestor
5
freshwater selected lines
5
acclimate at same salinity
5
Selection in
the Lab
Take the saline population
and then imposed selection
for freshwater tolerance
Compare the populations
before and after selection
Do the selection lines show
the same evolutionary shift
to fresh water as the wild
population?
0
5
15
Common garden experiment at the end of the selection
How do we detect
Evolutionary Adaptations?
• Transgenic and gene knockout studies
• Is that gene causing the trait, and does it
have fitness consequences?
• Generally use model systems, such as
mice, fruit flies, etc.
Example: Human IQ Data
• Data: Many studies use survey data on human
populations in the US (not a common-garden
experiment, where environment is held
constant)
• Did not statistically account for differences in
environment
• A correlation is associative, and not necessarily
causative
Problems with data and
statistical analysis:
• Several reanalyses have found that
socio-economic status (and historical
factors) was a stronger determinant of
IQ scores than race.
• Socio-economic status could reflect
nutrition, access to education, etc.
Impact of environment must
be accounted for:
• There is an IQ gap between blacks and whites in America,
Japanese and Koreans in Japan, Ashkenazi and
Sephardic Jews in Israel, and Protestants and Catholics in
Northern Ireland. As economic conditions improve for the
subordinated groups, the gaps are reduced
• A common-garden experiment has never been
performed on humans with respect to IQ scores to
determine actual genetic differences with environmental
effects removed
• Do not know of any study that has effectively controlled for
socio-economic differences
Examples: Adaptation
or not?
• A plant grows taller to obtain more
sunlight
• Weeds in cornfields (corn is tall) are on
average taller than weeds of the same
species in soybean fields in order to
obtain more sunlight
Examples: Adaptation
or not?
• A plant grows taller to obtain more
sunlight
Plasticity
• Weeds in cornfields (corn is tall) are on
average taller than weeds of the same
species in soybean fields in order to
obtain more sunlight
Not enough information
Examples: Adaptation
or not?
• Weeds in a cornfield have been found to
grow taller than those in soybean fields
when both populations are reared in
common-garden conditions
• Taller weeds in the cornfields survive and
a greater rate and leave more offspring
Examples: Adaptation
or not?
• Weeds in a cornfield have been found to grow
taller than those in soybean fields when both
populations are reared in common-garden
conditions
They are genetically different, but not know for
sure if it is adaptation (could be linkage, genetic
drift)
• Taller weeds in the cornfields survive and a
greater rate and leave more offspring
Adaptation