Adaptation
What is Adaptation?
• Not Phenotypic Plasticity
• Not Genetic Drift
• Must be targets of selection
(not Pleiotropy, not linkage)
What is Adaptation?
Genetic change in a population resulting from
natural selection, whereby the average state of a
character becomes improved with respect to a
specific function, allowing an organism to achieve
greater fitness in its environment.
Darwin (1809-1882)
Evolution is the change in allele
frequency at each generation
in a population
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One way in which evolution
occurs is through
Natural Selection
Darwin’s Contribution:
Natural Selection
• Too many offspring are produced
• Limited resources and competition
• Variation in a population
• Better adapted individuals survive
“Of course, long before you mature,
most of you will be eaten”
• Survivors leave more offspring (“fitness)
• Thus, the average composition of the population is altered
• Natural selection leads to adaptation
“Population speciation through
Natural Selection”
Mutation
This mutation
happens to be
beneficial
Individuals with this mutation
happen to leave more offspring
(greater “fitness”)
Adaptation
Requires Natural Selection
Requires heritable variation (polymorphism)
in a population
MUST have an effect on Fitness
there is some selective force
Is a allele frequency change in a population
(2) Critique of the “Adaptationist Programme”
Gould & Lewontin 1979
• 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:
The spandrels of San Marco
might not have been created
for a reason, but might
simply be a byproduct due to
the creation of arches
San Marco Cathedral, Venice
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, mechanically
forced correlation)
Outline
(1) What is Adaptation?
• Not Phenotypic Plasticity
• Not Genetic Drift
• Must be targets of selection
(not Pleiotropy, not linkage)
Adaptation
vs
Plasticity
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Definition - Plasticity
• Differences in phenotype that a genotype exhibits
across a range of environments
• Some traits with a plastic component:
height, temperature tolerance, salinity tolerance,
muscle mass…
Acclima(tiza)tion (≠ 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
Types of Plasticity
• Short-term reversible
• Development acclimation:
generally irreversible
Genotype --> Development --> Phenotype
• Within normal tolerance range
• In response to Stress
Developmental plasticity
Dodson, SI. 1989. Predator induced reaction norms. BioScience 39:447–452
Predator induced
formation of helmets
in Daphnia
Hebert and Grewe, 1985
Plasticity can be depicted
graphically as a Reaction Norm
Response
Environment
Reaction Norm: the function which describes the plastic response
Reaction Norms
P= G + E
VP=VG+VE
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
Most Importantly,
• Must distinguish plasticity from adaptations to
understand heritable (and permanent) vs
inducible differences, in order to interpret
experiments properly
How control for plasticity in
Experiments?
• If you want to determine whether a trait
is the result of plasticity or genetically
based, you need to perform a commongarden experiment to remove the
effects of environmental plasticity.
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
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?
Different Populations
Outline
(1) What is Adaptation?
• Not Phenotypic Plasticity
• Not Genetic Drift
• Must be targets of selection
(not Pleiotropy, not linkage)
Lecture 2: genetic mechanisms of adaptation
how to detect and analyze adaptation
Genetic Drift
• Definition: Changes in
allele frequency from one generation to the next
simply due to chance (sampling error)
• The change in genetic composition (evolution)
depends on Who leaves Offspring, the # of
Offspring, and Which Offspring happen to survive
(which gametes, which alleles)
Genetic drift
Futuyma (2009)
N = 100
pA= 0.5
T = 100 generations
Outline
(1) What is Adaptation?
• Not Phenotypic Plasticity
• Not Genetic Drift
• Must be targets of selection
(not Pleiotropy, not linkage)
Lecture 2: genetic mechanisms of adaptation
how to detect and analyze adaptation
Linkage
Definition: The tendency
for certain alleles to be
inherited together due to
their physical proximity on
the chromosome
Human linkage map
• Phenotypic evolution could arise due to linkage
(≠adaptation): Genes might experience an
evolutionary shift because another gene closely
linked on the chromosome is under selection
(selective sweep)
• This is a genetic mechanism of change that is NOT
adaptive
Linkage
Definition: The tendency
for certain alleles to be
inherited together due to
their physical proximity on
the chromosome
Human linkage map
• Consequence: Selection at a locus (gene)
might cause selection at many other genes
closely linked on a chromosome, even if there
is no reason for those other genes to evolve
Linkage
“Selection toy”
Selection for ball size
or for ball color?
(After Futuyma 2009, Sober 1984)
Pleiotropy
Definition: The phenomenon
where a gene affects several
different traits
• If a gene controls the expression of many traits,
selection at that gene will affect those many traits
• Consequence: selection at a gene due to the
importance of evolution of one trait could affect
many other traits that are affected by that gene
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 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
• Adaptations are ubiquitous,
but demonstrating that a
particular trait is an adaptation
is not always easy
How can you tell if a trait evolved as a
result of adaptation?
(1) The trait must be heritable
The differences between populations are
genetically based differences rather than inducible
differences (plasticity)
(2) The trait has fitness consequences (promotes
survival, performance, and number of offspring)
(If a trait evolved due to genetic drift, linkage or
pleiotropy it may confer no fitness advantage)
How do we detect
Evolutionary Adaptations?
• Transgenic and gene knockout studies
• Is that gene causing the trait, and does it
have fitness consequences?
• That’s why we use mice to understand
human genetics
Examples: Adaptation
or not?
• After high altitude training athletes have
increased number of red blood cells
(RBC)
• Tibetans and Sherpas have higher RBC
than lowland (<2000 m) people (Yi et al.
2010, Science 329:75-78)
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 at a
greater rate and leave more offspring
EPIGENETICS
WHAT IS EPIGENETICS?
• Epigenetics – gene regulation changes
that does not involve a change in DNA
sequence
• Epigenetic changes can be INHERITED!!
EPIGENETICS
Common mechanisms may include but not limited to:
-DNA methylation
-Histone modifications
(De)Acetylation (De)Methyaltion,
Ubiquitination, Phosphorylation
-Regulatory non-coding RNAs
DNA methylation