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Population Genetics and Evolution. Darwin’s Observations (review) Galapagos Islands Many similar species had slight differences Favorable variations allow.

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Presentation on theme: "Population Genetics and Evolution. Darwin’s Observations (review) Galapagos Islands Many similar species had slight differences Favorable variations allow."— Presentation transcript:

1 Population Genetics and Evolution

2 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

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

4 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

5 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

6 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

7 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

8 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

9 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

10 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 = 1 q = 1 -.85 q =.15

11 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


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