Population Genetics

Evolution depends upon mutation

to create new alleles.

Evolution occurs as a result of allele frequency

changes within/among populations.

What evolutionary forces alter

allele frequencies?

How do allele frequencies changein a population from generationto generation?

Allele frequencies in the gene pool:

A: 12 / 20 = 0.6a: 8 / 20 = 0.4

Alleles Combine to Yield Genotypic Frequencies

Our mice grow-up and generate gametesfor next generations gene pool

Allele frequency across generations: A General Single Locus, 2 Allele Model

Freq A1 = pFreq A2 = q

Genotypic frequencies are givenby probability theory

One locus, 2 Allele Model

Genotype A1A1 A1A2 A2A2

Frequency of allele A1 = pFrequency of allele A2 = 1 - p = q

In a diploid organism, there are two alleles for each locus.Therefore there are three possible genotypes:

Given:

Then:Genotype A1A1 A1A2 A2A2

Frequency p2 2pq q2

A population that maintains such frequencies is said to be at Hardy-Weinberg Equilibrium

Hardy-Weinberg Principle

(1) Allele frequencies in a population will not change, generation after generation.

(2) If allele frequencies are given by p and q, the genotype frequencies will be given by p2, 2pq, and q2

When none of the evolutionary forces (selection, mutation, drift, migration, non-random mating) are operative:

Hardy-Weinberg Principle Depends Upon the Following Assumptions

1. There is no selection

2. There is no mutation

3. There is no migration

4. There are no chance events

5. Individuals choose their mates at random

The Outcome of Natural Selection Depends Upon:

(1) Relationship between phenotype and fitness.

(2) Relationship between phenotype and genotype.

These determine the relationship between fitness and genotype.

Outcome determines if there is evolution

12.2 Growth of 2 genotypes in an asexually reproducing population w/ nonoverlapping generations

% survival to reproduction:

A = 0.05B = 0.10

Fecundity (eggs produced):

A = 60B = 40

Fitness A = 0.05 x 60 = 3Fitness B = 0.01 x 40 = 4

R = Per Capita Growth Rate = Represents Absolute Fitness

The rate of genetic change in a populations depends upon relative fitness:

Relative Fitness of A = Absolute fitness AHighest Absolute Fitness

WA = 3/4 = 0.75

Often by convention, fitness is expressed relative to the genotype with highest absolute fitness.

Thus,WB = 4/4 = 1.0

The fitness of a genotype is the average lifetimecontribution of individuals of that genotype to thepopulation after one or more generations, measuredat the same stage in the life history.

12.3 Components of natural selection that may affect the fitness of a sexually reproducing organism

12.1(2) Modes of selection on a polymorphism consisting of two alleles at one locus

12.1(1) Modes of selection on a heritable quantitative character

Genotype A1A1 A1A2 A2A2

Frequency p2 2pq q2

Fitness w11 w12 w22

Individuals may differ in fitness because of their underlying genotype

Incorporating Selection

Average fitness of the whole population:

p2w11 + 2pqw12 + q2w22w =

Given variable fitness, frequencies after selection:

Genotype A1A1 A1A2 A2A2

Freq p2 w11 2pq w12 q2 w22

w w w

New Frequency of A1

New allele frequencies after mating:

New Frequency of A2

p2 w11

w

pq w12 pq w12

w

q2w22+ +

Fitness: Probability that one’s genes will be represented in future generations.

Hard to measure. Often, fitness is indirectly measured:(e.g. survival probability given a particular genotype)

WAA WAa Waa

1 1 1 + s

Selection coefficientFitness is often stated in relative terms gives the selection differential

Persistent Selection Changes Allele Frequencies

Strength of selection is given by themagnitude of the selection differential

Selection Experiments Show Changes in Allele Frequencies

Cavener and Clegg (1981)

Food spiked with ethanol

HW

Selection can drive genotype frequenciesaway from Hardy Weinberg Expectations

Predicted change in allele

frequencies at CCR5

High frequency (Europe)High selection/transmisson (Africa)

High frequency (Europe)Low selection/transmisson (Europe)

Low frequency (Europe)High selection/transmisson (Africa)

pt + 1 =

What is the frequency of A1 in the next generation?

p2w11 + pqw12

p2w11 + 2pqw12 + q2w22

What is the change in frequency of A1 per generation?

p = pt + 1 - pt = p / w (pw11 + qw12 - w )

With this equation we can substitute values for relative fitness and analyze various cases of selection.

A1A1 A1A2 A2A2

1+s 1 + s 1

A1A1 A1A2 A2A2

1+s 1 1

A1A1 A1A2 A2A2

1 + s 1 1 + t

A1A1 A1A2 A2A2

1 + s 1 1 + t

Dominance

Recessivity

Overdominance

Underdominance

Fitness RelationshipGene Action

Genotype A1A1 A1A2 A2A2

Fitness 1 + s 1 + s 1

Dominance

S = 0.01

A1

Genotype A1A1 A1A2 A2A2

Fitness 1 + s 1 1

Recessive

S = 0.01

A1

Evolution in lab

populations of

flour beetles

support theoretical

predictions.

Dawson (1970)

Genotype A1A1 A1A2 A2A2

Fitness 1 + s 1 1 + t

Overdominance/Heterozygote Superiority

S = - 0.02 t = - 0.04

A1

Stable equilibriumis reached

Genetic diversityis maintained

Mukai and Burdick 1958

Viable allele did not fixin the population

Underdominance

S = 0.01 t = 0.02

Genotype A1A1 A1A2 A2A2

Fitness 1 + s 1 1 + t

A1

Unstable equilibrium

A1 maybe fixed orlost from thepopulation

Frequency-Dependent Selection

Allele frequencies in a population remain near an equilibriumbecause selection favors the rarer allele.

As a result, both alleles are maintained in the population.

Frequency-Dependent Selection

Perissodus

Incorporating Mutation Mutation alone is a weak evolutionary force

However, mutation and selection acting in concertare a powerful evolutionary force