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