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• ExamplesGreen 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% G 50%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 alleleq = frequency of the recessive allele

• Equationsp+q = 1p2 + 2pq + q2 = 1• p2 = frequency of homozygous dominant individuals• 2pq = frequency of heterozygous individuals• q2 = 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 = 1.85 + q = 1q = 1 - .85q = .15

In a population of birds, 25 % show the recessive trait for short wing feathers (q2). What is the frequency (q) of the gene for short wing feathers?q2 = .25q = .5

What is the frequency (p) for long wing feathers?p + q = 1p + .5 = 1p = .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 (p2)?(.5)2 = .25