adaptation vs plasticity. the problem: people often wish to jump to the conclusion that a trait...
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Adaptation vs
Plasticity
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”
• Adaptations are ubiquitous, but demonstrating that a particular trait is an adaptation is not always easy
The Problem:
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 & LewontinOther 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)
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
Adaptation
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
• 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…
Definition:
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
• 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
Response
Environment
Dodson, SI. 1989. Predator induced reaction norms. BioScience 39:447–452
Hebert and Grewe, 1985
Predator induced formation of helmets
in Daphnia
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
Size
Temperature
Could get selection for different reaction norms (different plasticity) in different environments
cold hot
big
small Select for this reaction norm in hot environments
Select for this reaction norm in cold environments
Genetic variation for plasticity can be determined by examining the significance of the interaction term from an Analysis of Variance (ANOVA)
Response
No Genetic Variation for Plasticity
Genetic Variation for Plasticity
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
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?
Example:
Different Populations
Rear under common conditionsTo determine the differenceswhen the environment is held constant
Common Garden Experiment
Different PopulationsIf 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)
Common Garden Experiment
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.
Selection in the Lab
Selection for several generations
Control
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?
Common garden experiment at the end of the selection 0 5 15
ancestor freshwater selected lines
saline ancestors
5 5 5acclimate at same salinity
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 sunlightPlasticity
• 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
• 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
Examples: Adaptation or not?