GB 20- Population Genetics

Evolution

Changes in allele frequencies in populations over generations 

Phenotypic Variation:

• observable differences between individuals (hair color, height, etc…) 

• Sometimes controlled by single gene 

• Ex: Mendel’s pea plants 

• Sometimes due to 2 or more genes: 

• Causes traits to be continuum ex: human height, skin color 

Genetic Variation:

• Differences among individuals in gene or nucleotide sequence 

• Sources of genetic variation: 

• 1. Mutation- small or large scale 

• 2. Recombination 

• 3. Rapid reproduction: more mutations per unit time 

Population:

a group of potentially sexually interbreeding organisms of same species living in the same area at the same time

Population genetics:

study of genetic variation within populations and evolutionary forces that act upon it 

Gene pool:

all alleles of all genes in a population at any given time

In diploid species:

• each locus present twice in each individual (may be homozygous or heterozygous) 

Allele frequency:

• proportion of a specific allele within a population 

Fixed allele

• allele at a frequency of 1 in a population– the only allele at a locus 

Genetic Equilibrium: 

No change in allele frequency from generation to generation– populations at equilibrium are NOT evolving 

Hardy Weinberg equations: 

• Allows for calculation of allele and genotype frequencies under HW equilibrium

• p= freq of dominant allele in a population 

• q= freq of recessive allele in a population 

Equation for allele frequencies: 

• P + q = 1 

Calculating genotype frequencies

• p^2 + 2pq + q^2 = 1 

• Frequency of homozygous dominant offspring = p^2 

• Frequency of heterozygous offspring = 2pq 

• Frequency of homozygous recessive offspring = q^2 

• Sum of all genotype frequencies = 1

Importance of HW: 

• Describes non- evolving populations 

• Reference point to compare frequencies in evolving populations 

• Determine why populations are evolving 

5 conditions: required for HW equilibrium 

• 1. No mutation 

• 2. Random mating 

• 3. No selection 

• 4. No genetic drift 

• 5. No gene flow 

mechanisms of evolution: 

• 1. Mutations: any change to dna sequence  (mutation increases diversity in population)

• 2. Non random mating: inbreeding (decreases variation because it leads to higher homozygosity)  

• 3. Genetic drift: random changes in allele frequencies (big population shrinks really fast- population bottleneck; big population and little piece leaves- founder effect) (decrease in variation)   

• 4. Gene flow: migration (doesn’t specifically increase or decrease variation but 2 populations experiencing gene flow become more similar overtime)   

• 5. natural selection: non random changes in allele frequencies (decreases diversity in population, Selection acts on phenotype, for selection to affect population, must be at least two phenotypes– polymorphic )