What evolutionary forces alter 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 change in a population from generation to generation?

Allele frequencies in the gene pool: A: 12 / 20 = 0.6 a: 8 / 20 = 0.4 Alleles Combine to Yield Genotypic Frequencies

Our mice grow-up and generate gametes for next generations gene pool

Allele frequency across generations: A General Single Locus, 2 Allele Model Freq A1 = p Freq A2 = q Genotypic frequencies are given by probability theory

One locus, 2 Allele Model Genotype A1A1 A1A2 A2A2 Given: In a diploid organism, there are two alleles for each locus. Therefore there are three possible genotypes: Genotype A1A1 A1A2 A2A2 Given: Frequency of allele A1 = p Frequency of allele A2 = 1 - p = q 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 When none of the evolutionary forces (selection, mutation, drift, migration, non-random mating) are operative: Allele frequencies in a population will not change, generation after generation. If allele frequencies are given by p and q, the genotype frequencies will be given by p2, 2pq, and q2

Hardy-Weinberg Principle Depends Upon the Following Assumptions There is no selection There is no mutation There is no migration There are no chance events 5. Individuals choose their mates at random

The Outcome of Natural Selection Depends Upon: 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

% survival to reproduction: A = 0.05 B = 0.10 12.2 Growth of 2 genotypes in an asexually reproducing population w/ nonoverlapping generations % survival to reproduction: A = 0.05 B = 0.10 Fecundity (eggs produced): A = 60 B = 40 C:\Figures\Chapter12\high-res\Evolution-Fig-12-02-0.jpg Fitness A = 0.05 x 60 = 3 Fitness 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 A Highest 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 lifetime contribution of individuals of that genotype to the population after one or more generations, measured at the same stage in the life history.

12.3 Components of natural selection that may affect the fitness of a sexually reproducing organism C:\Figures\Chapter12\high-res\Evolution-Fig-12-03-0.jpg

12.1(2) Modes of selection on a polymorphism consisting of two alleles at one locus C:\Figures\Chapter12\high-res\Evolution-Fig-12-01-2.jpg

12.1(1) Modes of selection on a heritable quantitative character C:\Figures\Chapter12\high-res\Evolution-Fig-12-01-1.jpg

Individuals may differ in fitness because of their underlying genotype Incorporating Selection Individuals may differ in fitness because of their underlying genotype Genotype A1A1 A1A2 A2A2 Frequency p2 2pq q2 Fitness w11 w12 w22 Average fitness of the whole population: w = p2w11 + 2pqw12 + q2w22

Given variable fitness, frequencies after selection: Genotype A1A1 A1A2 A2A2 Freq p2 w11 2pq w12 q2 w22 w w w New allele frequencies after mating: p2 w11 pq w12 pq w12 q2w22 + + w w New Frequency of A1 New Frequency of A2

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 Fitness is often stated in relative terms Selection coefficient gives the selection differential

Persistent Selection Changes Allele Frequencies Strength of selection is given by the magnitude of the selection differential

Selection Experiments Show Changes in Allele Frequencies HW Cavener and Clegg (1981) Food spiked with ethanol

Selection can drive genotype frequencies away from Hardy Weinberg Expectations

Predicted change in allele frequencies at CCR5 High frequency (Europe) High selection/transmisson (Africa) Predicted change in allele frequencies at CCR5 High frequency (Europe) Low selection/transmisson (Europe) Low frequency (Europe) High selection/transmisson (Africa)

What is the frequency of A1 in the next generation? p2w11 + pqw12 pt + 1 = p2w11 + 2pqw12 + q2w22 What is the change in frequency of A1 per generation? Dp = pt + 1 - pt = p / w (pw11 + qw12 - w ) With this equation we can substitute values for relative fitness and analyze various cases of selection.

Gene Action Fitness Relationship A1A1 A1A2 A2A2 1+s 1 + s 1 Dominance Recessivity A1A1 A1A2 A2A2 1 + s 1 1 + t Overdominance A1A1 A1A2 A2A2 1 + s 1 1 + t Underdominance

Dominance Genotype A1A1 A1A2 A2A2 Fitness 1 + s 1 + s 1 S = 0.01 A1

Recessive Genotype A1A1 A1A2 A2A2 Fitness 1 + s 1 1 S = 0.01 A1

Evolution in lab populations of flour beetles support theoretical predictions. Dawson (1970)

Overdominance/Heterozygote Superiority Genotype A1A1 A1A2 A2A2 Fitness 1 + s 1 1 + t S = - 0.02 t = - 0.04 Stable equilibrium is reached A1 Genetic diversity is maintained

Viable allele did not fix in the population Mukai and Burdick 1958

Underdominance Genotype A1A1 A1A2 A2A2 Fitness 1 + s 1 1 + t S = 0.01 t = 0.02 Unstable equilibrium A1 A1 maybe fixed or lost from the population

Frequency-Dependent Selection Allele frequencies in a population remain near an equilibrium because 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 concert are a powerful evolutionary force