Population Genetics and Evolution

Darwin’s Observations (review) Galapagos Islands Many similar species had slight differences Favorable variations allow individuals to survive in their environment and pass these variations on to their offspring Natural selection

How do these variations arise? Changes in genes – Mutations Random or result of recombination during meiosis

What does an accumulation of variations lead to? Evolution – Change in a gene pool of a species over time – Gene Pool All the genes in a population of organisms

Allele Frequency The frequency of an allele within a gene pool Examples Green Peas (G), Yellow Peas (g) In a population of all homozygous yellow peas, what is the frequency of the G and g alleles? 100% g 0% G In a population of heterozygous green peas, what is the frequency of the G and g alleles? 50% G50%g

Population Genetics The study of allele frequencies within populations. Allows us to study evolution Individuals that reproduce pass on their alleles to the gene pool of the next generation

Changing the Equilibrium of a Gene Pool 5 factors Mutation – Introduction of a new allele Migration (gene flow) – Movement into or out of the gene pool Random change in allele Frequency (genetic drift) – Occurs in small isolated populations Selection – Natural or artificial Non random mating – Preference for mating with individuals of a specific phenotype

Predicting Allele Frequencies Hardy-Weinberg Principle – Allows us to mathematically predict allele frequencies in a stable population Following conditions must be met: – Large population – No Selection – No Mutations – Mating must be random – No Migration

Hardy-Weinberg Equations Frequency of two alleles (p &q) p = frequency of the dominant allele q = frequency of the recessive allele Equations p+q = 1 p 2 + 2pq + q 2 = 1 p 2 = frequency of homozygous dominant individuals 2pq = frequency of heterozygous individuals q 2 = frequency of homozygous recessive individuals

Practice In a population of birds, the dominant allele for long wing feathers has a frequency (p) of 85%. What is the allele frequency of the recessive gene (q)? p+q = q = 1 q = q =.15

In a population of birds, 25 % show the recessive trait for short wing feathers (q 2 ). What is the frequency (q) of the gene for short wing feathers? q 2 =.25 q =.5 What is the frequency (p) for long wing feathers? p + q = 1 p +.5 = 1 p =.5 What percent of these birds are heterozygous for long wings (2pq)? 2 (.5) (.5) =.5 What percent of these birds are homozygous for long wings (p 2 )? (.5) 2 =.25