adaptation

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.

Evolution is the change in allele frequency at each generation in a population

One way in which evolution occurs is through Natural Selection

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Darwin (1809-1882)

Darwin’s Contribution:

Natural Selection

“Of course, long before you mature, most of you will be

eaten”

• Too many offspring are produced • Limited resources and competition• Variation in a population• Better adapted individuals survive• 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”)

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

Adaptation

(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 & LewontinOther potential causes of phenotypic variation

that is NOT Adaptation:

(A) Plasticity: phenotypic change without evolution

(B) Nonadaptive Evolutionary Forces:Genetic DriftGenetic 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)

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Adaptation vs

Plasticity

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

Dodson, SI. 1989. Predator induced reaction norms. BioScience 39:447–452

Developmental plasticity

Hebert and Grewe, 1985

Predator induced formation of helmets

in Daphnia

Plasticity can be depicted graphically as a Reaction Norm

Response

Environment

Reaction Norm: the function which describes the plastic response

Reaction Norms

P= G + EVP=VG+VE

• 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

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 common-garden 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

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?

Common Garden Experiment

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

N = 100pA= 0.5T = 100 generations

Futuyma (2009)

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

• 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

Definition: The tendency for certain alleles to be inherited together due to their physical proximity on the chromosome

Human linkage map

Linkage

• 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

Definition: The tendency for certain alleles to be inherited together due to their physical proximity on the chromosome

Human linkage map

Linkage

“Selection toy”Selection for ball size or for ball color?

(After Futuyma 2009, Sober 1984)

PleiotropyDefinition: 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 PlanIf body size increases, brain size has to increaseIf 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)

• 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

Examples: Adaptation or not?

EPIGENETICS

WHAT IS EPIGENETICS?• Epigenetics – gene regulation changes

that does not involve a change in DNA sequence

• Epigenetic changes can be INHERITED!!

Common mechanisms may include but not limited to:

-DNA methylation

-Histone modifications (De)Acetylation (De)Methyaltion, Ubiquitination, Phosphorylation

-Regulatory non-coding RNAs

EPIGENETICS

DNA methylation