Natural Selection

What does the Hardy-Weinberg equilibrium predict?

HW predicts genotype frequencies in a population that is NOT evolving

When real population frequencies don’t match HW predictions, it indicates some evolutionary force is acting 

When Hardy Weinberg is in place, do the allele frequencies constant or change?

They are constant

What are the assumptions of Hardy Weinberg?

• Diploid organisms

• Non-overlapping generations

• Sexual reproduction

• No evolution 

What are the 5 Mechanisms of Evolution which are not acting in Hardy Weinberg?

1. No selection

2. No drift (infinite population size)

3. No mutation

4. No migration

5. Random Mating 

4 Postulates of Natural Selection 

1. Variation exists in a population

2. Variation is genetic (heritable) 

3. Limited resources in population for differential reproduction 

4. Who reproduces is not random 

If 4 Postulates are not met, population will NOT evolve by natural selection but can evolve by a different mechanism

Give example of scenario which shows 4 postulates

Mice with different colored coats: light and dark

• Population shows variation of different colored coats

• Population shows genetic inheritance of offspring showing the light and dark coats

• Differential reproduction as not everyone in population reproduces equally as light contribute more offspring to next generation than dark as they are able to blend in 

• Who reproduces is not random as certain phenotypes there is some who have a better chance of surviving 

Example of a population evolving in real time through natural selection

Finches on Galapagos Island in increase in beak depth over generation

Postulate #1: Variation

• Beak depth size ranges from 5-14 mm

Postulate #2: Variation is heritable

• See if there is correlation between parents and offspring phenotype

• Can measure heritability by plotting mid parent and mid offspring values to see relationship

Postulate #3: Differential Reproduction

• Population crash due to drought year in 1977 —> shows not everybody survived and reproduced as a lot of finches were killed off

Postulate #4: Nonrandom reproduction

• Look at population before drought (N=751) vs survivors after drought in 1978 (N=90)

• Saw that survivors tended to have beaks were a bit bigger and beak depth averaged shifted between the 2 years

• Can also plot survivorship as a function of bill depth to see bigger bills are more likely to survive

What does plot of mid parent and mid offspring values show heritability relationship?

If no relationship = heritability is 0

If some heritability, but a lot of variation = heritability between 0 and 0.99

If little variation and all heritability = heritability is 1

What can we do to see population has evolve for finches? 

• look at finches hatched in 1978 and observe them grown up to measure bill depth 

• If we see change in bill depth that means population has evolved as there is change in allele frequencies in a population overtime  

Why did finches with deeper beaks survive?

• Foraging and Food

• Fewer seeds available during drought year and seeds that were available were large and hard

• Birds with larger beak depth were able to eat the larger seeds for increased survival

Why is heritability important to natural selection?

• If beak size was not heritable, offspring would have mix of big and small beaks for population to not evolve

• If beak size is heritable, big beaked parents give rise to big beaked offspring in order to see shift in beak size change overtime

Narrow Sense Heritability 

• How much offspring resemble their parents 

• What evolutionary biologists care about when talking about how populations evolve 

• Only considers additive variability 

Broad Sense Heritability

• Takes into account of dominance (ex. epistasis -masking of genes) and additive variability

Additive Variability

Variability due to additive effects of genes, result of lots of loci in genome that are making up the phenotype

Dominance variability

Variance due to interactions such as dominance

Why does only additive variation contributes to change? 

Alleles stack together to produce continue range of phenotypes and selection event can select for alleles to see shift in mean population

Ex. Cross between 2 medium sized plants —> produce continuous range of heights in F2 population

• If have selective event where only tall individuals survive and only tall individuals reproduce each other —> next generation will be taller

• See shift in mean population 

Why does overdominance not contribute to change?

Effects of alleles are dependent on each other

Ex. Tall extreme phenotype is heterozygote (Aa) and homozygote is short (AA or aa) 

• When selection event happens, you kill all short plants and leave only tall heterozygotes (Aa) 

• Breed heterozygotes together Aa x Aa to get 25% AA 50% Aa 25% aa for 50% short and 50% tall 

• Population did not change and still looks like original population

• You cannot select recessive alleles if they hide in heterozygotes and evolution cannot use variation effectively  

What is Fisher’s Breeder’s equation?

• The response to selection (Δz) is influenced by the selection differential (S) on a trait and its heritability (h²)

• h² is heritability and measures how much a trait’s variation is due to genetics (ranges from 0 to 1)

• S (selection differential) is difference between mean of survivors and mean of entire population

• Δz is change in population from one generation to the next

Finch Beak Sample with Fisher’s Breeder Equation to calculate S, Δz, h² ? 

• Original population mean beak depth = 9.5 mm

• Survivor Finches mean = 6.5 mm

• Mean of offspring of survivors = 8 mm

• Selection differential (S) = 9.5 - 6.5 - 3.0 mm

• Δz = 9.5 - 8 mm = 1.5 mm 

• h² = 1.5/3 = 0.5

If the mean of offspring and mean of average population is the same, what would be heritability?

Heritability would be 0 and population would not evolve

Directional Selection

• Individuals with extreme trait favored

• Mean of population changes

• Phenotypic variance decreases

• Seen in variable environments

Stabilizing Selection 

• Individuals with mean trait favored

• Mean does not change

• Variance decreases

• Seen in stable environments 

• Ex. Eurosta flies make galls which are attacked by wasps and birds, if too small attacked by wasps and too big are attacked by birds —> so flies make average sized gall to survive better 

Disruptive Selection

• Individuals with either extreme trait favored

• Mean does not change

• Variance is maintained

• Can cause speciation

• Ex. If small and large seeds are only available, large and small beak depth sizes will survive best

These all show stabilizing selection, which shows the strongest example of stabilizing selection?

• A: shows survival rate drops very fast once get away from the mean for intense selection

Frequency Dependent Selection 

• Which trait is favored depends on frequency of trait in a population

• Positive frequency dependent selection: as certain trait gets more common, it becomes more favorable 

  • Mean changes and shifts toward whichever phenotype becomes common

  • Variance decreases

• Negative frequency dependent selection: as certain trait gets more rare, it becomes more favorable 

  • Mean does not change

  • Variance maintained, does not change

Negative Frequency Dependent Selection

• Maintains phenotypic/genetic diversity

• Variance is maintained as population never reaches a stable state where one phenotype is optimal over another

• A type of balancing selection

• Ex: Scale eater example where fish develop hard scales on side that is getting eaten, but cichlids will adapt to which side is developing harder scales for back and forth phenotype

How can we quantify selection?

1. S = selection differential

2. Selection Gradients (β)

What is selection gradient (β)?

• β is way to measure the strength of selection based on quantitative phenotype

• Measures how much your fitness changes when your phenotype changes

• Second form of Breeder’s equation: △z=Gβ

Absolute fitness

# offspring an individual produces (ranges from 0 to infinity)

Relative fitness

# offspring standardized to max # produced in population (ranges from 0 to 1)

Inclusive fitness

takes into account your genes in other people’s bodies

Relative Fitness

B1B1=1

B1B2=0.75

B2B2=0.5 

What do these values mean? 

All individuals of B1B1 are going to survive and reproduce 

¾ individuals with a B1B2 are going to be survive and reproduce 

½ individuals with a B1B2 are going to survive and reproduce

Selection coefficients 

s = measure of the strength of selection on an allele  (ranges from -1 to 1)

• Positive # = beneficial 

• Negative $ = deleterious

Fraction that fitness is changed compared to a reference for each allele copy 

How alleles change in frequency over time?

• △p=spq

• s = measure of strength of selection of an allele

• p = frequency of allele 1

• q= frequency of allele 2 

• Assuming heterozygotes have intermediate fitness

When is allele change in frequency (△p) the fastest?

• When s is big

• When variation is maximized (when P and Q are both frequent)

  • Maximized when P and Q is 0.5

How will the frequency of A1 change, given the following genotype fitness?

• A1A1=1, A1A2=0.99, A2A2=0.98

• A1A1=0.98, A1A2=0.99, A2A2=1

• A1A1=1, A1A2=0.75, A2A2=0.5

How will the frequency of A1 change, given the following genotype fitness?

1. A1A1=1, A1A2=0.99, A2A2=0.98

• A1 will increase at slowish rate since s is small (not big difference between A1A1 and A1A2 

• Hit maximum when P and Q are 0.5 and then level off 

2. A1A1=0.98, A1A2=0.99, A2A2=1

• A1 will decrease and level off

3. A1A1=1, A1A2=0.75, A2A2=0.5

• Linear 

• A1A1 is really beneficial compared to A2A2 so slope will increase 

Ultimate fate of an allele in a population if selection is only thing acting in a population

• Allele that is beneficial will go to fixation (frequency = 1), which means will rise until everyone in population will have it

• Allele that is deleterious will go to loss (frequency = 0)

• Change in allele frequencies is maximized so slope is biggest when allele frequencies are 0.5

Why does strength of selection matters? 

High selection coefficient (s) = frequency of alleles is going to rise very fast

Low selection coefficient (s) = frequency of alleles change very slowly 

Codominant inheritance

• Heterozygotes expresses both alleles

• Ex: Relative fitness AA=1, AB=0.75, BB=0.5

• Intermediate fitness level

Dominant Inheritance

• Dominant allele is favored

• Heterozygote takes on phenotype of one of the homozygotes

• Ex. Relative fitness AA=1 Aa=1 aa=0.5

  • A = beneficial and dominant

• Observe steep increase in frequency of dominant allele and level off 

• Takes a long time to go to fixation because deleterious allele preserved in heterozygotes

Recessive Inheritance

• Recessive allele favored

• Ex. Relative fitness AA=0.4 Aa =0.4 aa=1

  • a is recessive and beneficial 

• Takes long time to kickstart because beneficial allele get hidden in heterozygotes

In what order will allele go to fixation the quickest to slowest?

Codominance, Dominant allele favored, Recessive allele favored 

Heterozygote advantage

• Heterozygote favored (overdominance)

• Type of balancing selection

• Both alleles maintained in population —> maintain diversity

• Alleles reach equilibrium frequency which depends on fitness of homozygotes

Ex: Relative fitness VV=0.735, VL=1, and LL=0

• Since VV is higher than LL fitness, equilibrium frequency shifted to be higher around 0.8

• If both were equal, equilibrium frequency would be at 0.5

Heterozygote Disadvantage

• Homozygote favored (underdominance)

• Which allele goes to fixation depends on starting frequency AND strength of selection

• Ex: Relative fitness: C2C2=0.25, C2N2=0, N2N2=1

• Threshold starting frequency which depends on difference between 2 homozygotes

• N2 is more fit than C2 so for C2 to go to fixation it has to start at high frequency, if start at low then will be lost 

Adaptive Landscape

how fitness changes based on frequency of alleles in population

Codominance adaptive landscape

• If allele beneficial, shows straight line and positive slope as when allele goes to fixation, population has highest fitness

• If allele is deleterious, shows straight line and negative slope as when allele is loss, population has highest fitness

Overdominance adaptive landscape

• Cannot get rid of deleterious alleles and going to be maintained in population

• Hump shaped

• If start at low frequency and once reach peak (equilibrium frequency) will not change

• If start at high frequency, will decrease until reach equilibrium point

• Can never reach its theoretical maximal fitness

Underdominance Adaptive Landscape

• U shaped

• If allele starts as common, increase in frequency until it goes to fixation

• If allele starts as rare, decrease in frequency until it is lost

For selection what matters to effecting allele frequency?

1. Strength of selection for rate of change

2. Mode of inheritance

3. Starting Frequency

4. Heterozygote fitness

Natural selection generally decreases variation, except for

types of balancing selection which can maintain diversity

• Negative dependent frequency selection

• Overdominance (heterozygote favored/advantage)