MicroevolutionMicroevolution

The evolution of local populations or demes.

Individuals are selected

But only populations evolve

Three Levels of Evolutionary Change

• Microevolution: evolution of local populations

• Speciation: Origin of new species (life forms)

• Macroevolution: long term consequences of microevolution and speciation …patterns of evolution

The Modern Synthesis

• By 1900, biologists expected Darwin to be vindicated …evolution by natural selection would be verified

• But no one understood how reproduction occurred

• 1900 - Mendelism rediscovered

At first, Mendelism was considered to refute Darwinism!

• Darwinism– requires pools of genetic variability upon which natural

selection could act, screening variants

• Early Mendelian geneticists – relied on genes that they could detect because of big

mutations that created large phenotypic changes

– these mutations were invariably difficult to maintain

– concluded that natural selection is a cleansing force, not a creative force

– Therefore, the creative force in evolution would be mutation pressure, the origin of rare advantageous mutations

– By the 1920s Biology textbooks were being written that ignored Darwin …alluded to the Mutation Theory of Evolution

The Modern Synthesis:The Synthetic Theory of Evolution

Darwinism & Mendelism Reconciled1930s through 1950

• But hidden variation was sought and demonstrated, 1920s to 1960s

• And the new discipline of population genetics provided the methods to study populations, 1930s to 1950

Genetic variability is the currency of evolutionary change

constantenvironment

changingenvironment

genetically variable population

genetically invariant population

Premise: organisms are so complicated and dependent on specific environmental conditions that most genotypes will fail in a new environment

Genetic variability is the currency of evolutionary change

constantenvironment

changingenvironment

genetically variable population

genetically invariant populationyes

maybeno

yesmaybe

no

yesmaybe

no

yesmaybe

no

Premise: organisms are so complicated and dependent on specific environmental conditions that most genotypes will fail in a new environment

Lottery Model

Winning the lottery is analogous to a genotype being well suited to new environmental conditions

Which is a better chance of winning a lottery?

o Buying one ticket and copying it a thousand times?

o Buying a thousand tickets?

Microevolution

Evolution is a change in the genetic composition of a population

Mutation

SexualRecombination

Gene Flow (migration)

Sources of Genetic Variability (VG) Population-Environment Interactions

VGgenepool

Evolution

adaptive

non-adaptiveGenetic

Drift

NaturalSelection

Changes in VG

Microevolution's 5 Systemic Forces

Sources of Genetic Variability

1. Mutation (source of new alleles & genes)

2. Gene Flow (migration, cohesion mechanism)

3. Sexual Recombination (new gene combinations)

Population Environment Interactions

4. Genetic Drift (random differential reproduction)

5. Natural Selection (non-random differential reprod.)

Are Mutations

Adaptive

or

Adaptively Ambiguous?

Are advantageous mutations more likely to be produced in environments where they will be useful or

are they produced at random?

Fluctuation Test

Subdivide growing bacterial colonies

Two hypotheses:

• Advantageous mutations will be more likely when the environment changes

• Mutations occur constantly and are not more likely when they will be useful

Observed:

Replica Plating

Replica Platingcan transfer a plate full of colonies without

altering their relative positions

source plate with colonies"a hospitable environment"

empty target"a hostile environment"

the replica stamp transfers colonies without altering their relative positions

Two Hypotheses

adaptive mutation hypothesis

adaptively ambiguous mutation hypothesis

replicaplating

replicaplating

Two Hypotheses

adaptive mutation hypothesis

adaptively ambiguous mutation hypothesis

replicaplating

replicaplating

Two Hypotheses

adaptive mutation hypothesis

adaptively ambiguous mutation hypothesis

replicaplating

replicaplating

observed

Migration - Gene Flow

1 migrant per 10,000 residents will prevent populations from diverging except under the influence of strong selection

Mating

Mating

Small populationscan cause Genetic Drift

Industrial MelanismIndustrial Melanism

Kinds of Natural Selection

What is selected?

Individuals …mostly

Levels of Selection

Darwinian Selection

• Genic

• Individual Selection

• Group

• Kin

• Species

Levels of Selection

• Genic

• Individual Selection

• Group

• Kin

• Species

Levels of Selection

problem with problem with individual selection:individual selection:

recombination destroys recombination destroys fit genotypesfit genotypesimmediatelyimmediately

problem with problem with individual selection:individual selection:

recombination destroys recombination destroys fit genotypesfit genotypesimmediatelyimmediately

• Genic Selection

• Individual

• Group

• Kin

• Species

so what replicates so what replicates with fidelity?with fidelity?

small sections of DNAsmall sections of DNAcontaining few genescontaining few genes

so what replicates so what replicates with fidelity?with fidelity?

small sections of DNAsmall sections of DNAcontaining few genescontaining few genes

Levels of Selection

• Genic

• Individual

• Group Selection

• Kin

• Species

Traits that seem to conflict with Traits that seem to conflict with individual selection:individual selection:

short-term advantagesshort-term advantages(individual selection)(individual selection)

versusversus

long-term advantageslong-term advantages(group selection)(group selection)

Traits that seem to conflict with Traits that seem to conflict with individual selection:individual selection:

short-term advantagesshort-term advantages(individual selection)(individual selection)

versusversus

long-term advantageslong-term advantages(group selection)(group selection)

e.g.e.g., paradoxes:, paradoxes:

alarm callsalarm callsaltruismaltruism

Problem: birth & death of Problem: birth & death of populations is slow compared to populations is slow compared to

individual selectionindividual selection

e.g.e.g., paradoxes:, paradoxes:

alarm callsalarm callsaltruismaltruism

Problem: birth & death of Problem: birth & death of populations is slow compared to populations is slow compared to

individual selectionindividual selection

Levels of Selection

• Genic

• Individual

• Group

• Kin Selection

• Species

When does genetic sacrifice pay off?When does genetic sacrifice pay off?

1. reciprocal altruism ...rare1. reciprocal altruism ...rare

2. the beneficiaries of your sacrifice2. the beneficiaries of your sacrifice

carry your genes, carry your genes, i.e.i.e., they are kin, they are kin

Levels of Selection

• Genic

• Individual

• Group

• Kin

• Species Selection

low speciation & extinction rates

high speciation & extinction rates

time

Other Kinds of Selection

• Sexual Selection ...already discussed

• Balancing selection

• Heterozygote selection

• Frequency-dependent Selection ”the advantage of being rare”

◊ Frequency-dependent Mating ...rare mate advantage

◊ Apostatic selection ...predator/pathogen pressure

◊ Selection for Ecological Combining Ability ...competition

Evolution

A

E

B

C

D

Grandom

∆Vg

Hnon-random

∆Vg

F

I

J

Microevolution

Evolution is a change in the genetic composition of a population

Mutation

SexualRecombination

Gene Flow (migration)

Sources of Genetic Variability (VG) Population-Environment Interactions

VGgenepool

Evolution

adaptive

non-adaptiveGenetic

Drift

NaturalSelection

Changes in VG

Calculating Hardy-Weinberg Equilibrium

Calculating Hardy-Weinberg Equilibrium

• Count the number of each diploid genotype

• Calculate allele frequencies

• Calculate Hardy-Weinberg Equilibrium

• Compare observed and predicted values

• Departures indicate evolution occurring

Assumptions of HWE:Microevolution's 5 Systemic Forces Not Active

Sources of Genetic Variability

1. Mutation (source of new alleles & genes)

2. Gene Flow (migration, cohesion mechanism)

3. Sexual Recombination (new gene combinations)

Population Environment Interactions

4. Genetic Drift (random differential reproduction)

5. Natural Selection (non-random differential reprod.)

Isozyme Surveys

current currentGenotype

FF

FS

SS

MonomericEnzymes

DimericEnzymes

Genotype

FF, FS, or SS

FF, FS, or SS

FF, FS, or SS

Calculate allele frequency and HWE expectations

• Count the number of each diploid genotype

• Calculate allele frequencies

p = (2*D + H) / 2N; q = (2*R + H) / 2N; note that p + q = 1

• Calculate Hardy-Weinberg Equilibrium

DHWE = p2; HHWE = 2pq; RHWE = q2

• Compare observed and predicted values

How will inbreeding affect the expected number of genotypes, compared to

Hardy-Weinberg Equilibrium?

Calculate allele frequency and HWE expectations

• Count the number of each diploid genotype

• Calculate allele frequencies

• Calculate Hardy-Weinberg Equilibrium

• Compare observed and predicted values

Population Structures

• Polymorphic Genes– Transient polymorphisms

– Balanced polymorphisms - equilibrium conditions

• Monomorphic Genes - Fixation for one allele

– Structural constraints may not permit allelic variation

– Fixation will result from selection when one allele has a higher fitness than others

– Fixation will result from genetic drift

Population Size, Selection and Drift

p

generations

A

B

Dpopulation size

Transient polymorphisms

Balanced polymorphisms

Fixation - monomorphisms

C

p

1.0

0A

B

C

Population Size, Selection and Drift

p

generations

A

B

Dpopulation size

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10.00

0.10

0.20

0.30

0.40

0.50

Distribution of population allele frequencies in a sampled geographical region

Allele frequencies per local population

Frequency of populations

exhibiting given allele frequencies

a population system subject to genetic drift

C

Genetic Drift

Q: Assuming that you are studying a population system whose local populations all begin with an allele frequency of about 0.5, what kinds of affects would genetic drift have over time?

a. allele frequecies will tend to fixation of one allele or the otherb. balanced polymorphisms will be commmonc. natural selection will work weekly if at alld. transient polymorphisms will be common at firste. transient polymorphisms will be common later in the population’s historiesf. mutations will tend to be lost rapidly in older populations

Identify examples of:

a. a lineage evolving (∆VG)

b. transient polymorphism

c. balanced polymorphism

d. monomorphism

e. genetic drift dominating

f. natural selection dominating

pTime (generations)ABCTime (generations)DETime (generations)FGTime (generations)HIJ

Species and Speciation

Are Species Real?

• natural selection occurs in demes (local populations)

• species are composed of numerous local populations (population systems)

• over time, all populations accumulate genetic differences from their ancestors (divergence)

• what would cause local populations to evolve in unison? (coherent population systems, preventing divergence of their populations)

The Role of Gene Flow

• Isolated populations inevitably accumulate genetic differences of time (Divergence)

• Even low migration rates can prevent divergence (producing evolutionary coherence of a population system, preventing divergence of populations)

• Species composed of coherent population systems are more than just names, they are cohesive lineages ...real species

Biological Species

Population systems that can interbreed, but which are isolated from other populations.

Crucial Criterion:

Reproductive Isolation

Anagenesis versus Cladogenesis

Anagenesis Cladogenesis

A Speciation Mechanism

A. harrisiA. harrisi

A. leucurusA. leucurus

Grand Canyon Squirrels: AmmospermophilusAllopatric Speciation

Geographical (Allopatric) Speciation

RiverRiver RiverRiver RiverRiver RiverRiver

DivergenceReproductive

IsolationExtensiveGene Flow

T I M E

Deme

Gene Flow

?

What Happens if Mating Opportunities Resume?

• Mating produces viable offspring? Offspring fertile:

– Hybridization & gene flow erase divergence

• Mating produces viable offspring? Offspring not fertile:

– Hybridizing individuals waste reproductive effort, suffer low fitnesses

• Mating produces inviable offspring?

– Hybridizing individuals waste reproductive effort, suffer low fitnesses

• Mating avoided ...no hybridization?

– Already two different species

Reproductive Isolation Reinforcement

• Postzygotic barriers– reduced hybrid viability

– reduced hybrid fertility

– hybrid breakdown

• Prevents fertilization if mating occurs– mechanical isolation

– gametic isolation

• Prevents Mating– Habitat isolation

– Temporal isolation

– Behavioral isolation

Post-Z

Pre-Z

higherfitnesses

becausewastagelessened

higherfitnesses

becausewastagelessened

PostzygoticReproductive Isolation

Horse x DonkeyHorse x Donkey

infertileinfertileMuleMule

Blue BoobiesBlue Boobiesbehavioral isolationbehavioral isolation