Download Population Genetics - Napa Valley College

Biology 240 – General Zoology
Lecture 10 Outline
Population Genetics
- study of the genetic makeup of populations
- quantitative analysis of genetic variation
- mathematical model to analyze evolution of populations
microevolution - changes in allele frequencies in a population over successive generations
The Modern Synthesis of evolutionary biology (1930s-40s)
- combined Darwinian evolutionary theory with Mendelian genetics and mathematical analysis
Sources of genetic variation
mutation - nucleotide changes in DNA (point mutations, additions, deletions, meiosis errors)
gene duplication - can expand and alter the genome on a large scale
sexual reproduction - meiosis and sex “reshuffle” alleles producing new, unique combinations
Mendelian genetics predicts genotype ratios (or frequencies) resulting from a particular cross
e.g., Monohybrid Cross Aa x Aa
alleles: 50% A, 50% a
Expected genotype ratio 1 AA : 2 Aa : 1 aa F1 genotype frequency: 25% AA, 50% Aa, 25% aa
Populations have different allele and genotype frequencies than in controlled crosses, not 1:2:1
Hardy-Weinberg Equilibrium
- mathematical model of allele and genotype frequencies for a population in genetic equilibrium
- theoretical baseline to analyze changes in allele frequencies at a given locus over time
Variables:
1. allele frequency
2. genotype frequency
convert percentages to decimal fractions to do the math
3. phenotype frequency
Given: - a gene with two alleles: B and b
- allele frequency of B = p; frequency of b = q
- sum of allele frequencies, p + q = 1
Hardy-Weinberg equilibrium predicts genotype frequencies in the population by the formula:
p2 + 2pq + q2 = 1
where p2 = BB, 2pq = Bb, q2 = bb
Example
Population with 2 alleles for hair color: B - black hair, b - blonde hair
allele frequency: B = 0.6, b = 0.4
Hardy-Weinberg equilibrium, after one generation of random mating
genotype frequency: BB = (0.6)2 = 0.36; Bb = 2(0.6)(0.4) = 0.48; bb = (0.4)2 = 0.16
phenotype frequency: black hair = BB + Bb = (0.36 + 0.48) = 0.84 = 84%
blonde = bb = 0.16 = 16%
Allele frequencies in the population stay the same:
B alleles = BB + ½(Bb) = (0.36 + 0.24) = 0.6
b alleles = bb + ½(Bb) = (0.16 + 0.24) = 0.4
A population in equilibrium maintains constant allele and genotype frequencies over successive generations.
Conditions for genetic equilibrium:
 random mating (no mating preference for either phenotype)
 large population size (no genetic drift)
 no mutation
 no natural selection
 no gene flow (immigration or emigration)
A population that is not in genetic equilibrium suggests evolution - change in allele frequencies over time.
Biology 240, Lecture 10
Agents of Evolutionary Change
1. Genetic drift
2. Gene flow
3. Natural selection
Genetic drift - random changes in allele frequencies due to small population size
founder effect - small subgroup with different allele frequencies splits off from a source pop.
bottleneck effect - small number of survivors from a population, similar to founder effect
Gene flow - movement of individuals or gametes from one population to another
- immigration adds new alleles to a population; emigration removes alleles
Natural selection - differential reproductive success based on phenotypic variation
- the only agent that produces adaptive evolutionary change
directional selection favors one end of a phenotypic range, causes evolutionary change in population
disruptive selection favors opposite ends of a phenotypic range, acts against intermediate types
stabilizing selection favors intermediate phenotypes, helps maintain status-quo
Hardy-Weinberg principle can be used to analyze the effect of natural selection on a population.
Example
Population with 2 alleles for hair color: B - black hair, b - blonde hair
allele frequency: B = 0.9, b = 0.1
Hardy-Weinberg equilibrium, after one generation of random mating
genotype frequency: BB = (0.9)2 = 0.81; Bb = 2(0.9)(0.1) = 0.18; bb = (0.1)2 = 0.01
phenotype frequency: black hair (BB + Bb) = (0.81 + 0.18) = 99%;
blonde (bb) = 1%
Now natural selection eliminates all blonde-haired individuals before they can mate
genotype frequencies of survivors: BB = (0.81/0.99) = 0.82; Bb = (0.18/0.99) = 0.18; bb = 0
allele frequencies of survivors: B alleles = BB + ½(Bb) = (0.82 + 0.09) = 0.91; b alleles = ½(Bb) = 0.09
Next the survivors mate randomly. In the following generation alleles are redistributed among genotypes
according to Hardy-Weinberg formula:
genotype frequencies: BB = (0.91)2 = 0.828; Bb = 2(0.91)(0.09) = 0.164; bb = (0.09)2 = 0.008
phenotype frequencies: black hair (BB + Bb) = 0.828 + 0.164 = 99.2%;
blonde (bb) = 0.8%
Conclusion: it is hard to eliminate rare recessive alleles from a population. Why?
Most of the recessive alleles are carried by heterozygotes (carriers) that have the dominant phenotype.
Study Questions
1. Apply the Hardy-Weinberg formula to calculate the frequencies of alleles, genotypes, and
phenotypes of a population in genetic equilibrium. For example, starting with a population that has
allele frequencies at a given locus of p = 0.8 and q = 0.2.
2. List the conditions required for a population to be in genetic equilibrium.
3. Understand how population genetics theory relates to evolutionary theory.
4. Define genetic drift and explain the founder effect.
5. Red short-horned cattle are homozygous for the red allele, white cattle are homozygous for the white
allele, and roan cattle are heterozygotes. A population of cattle consists of 36% red, 16% white, and
48% roan cattle. What are the allele frequencies?
6. Cystic fibrosis, an inherited disease caused by a recessive allele, afflicts about 1 in 2500 children born in
Northern Europe. What is the frequency of cystic fibrosis alleles in the population? (Hint: q2 = 0.0004.)
What is the frequency of cystic fibrosis carriers in the population, assuming H-W equilibrium?
7. What did the natural selection field experiment demonstrate about the evolution of a population
subjected to strong predation pressure? What type of selection was demonstrated in this case?
Would you expect the white alleles to be completely eliminated from the population over many
generations? Why or why not?