Biology 30

Population Genetics

Chapter Outcomes

• Define a gene pool.• Describe the gene pool of a population at genetic

equilibrium.• Summarize the five conditions upon which the

Hardy-Weinberg principle is based.• Describe how the Hardy-Weinberg equation is

used to determine whether a population is undergoing microevolution.

Chapter Outcomes

• Calculate allele and genotype frequencies in a population.

• Outline the conditions required to maintain genetic equilibrium.

• Identify and compare the effects of mutations, gene flow, non-random mating and genetic drift on gene pool diversity.

• Apply the Hardy-Weinberg principle to published data.

Chapter Outcomes

• Distinguish between founder effect and the bottleneck effect on gene pools.

• Explain how the process of natural selection is related to microevolution.

• Explain the cause of heterozygote advantage and how it affects a gene pool.

• Describe strategies used in captive breeding and population management.

• Explain that genetic engineering can have intended and unintended effects on gene pools.

Genetic Diversity in Populations

• Recall that a population is a group of organisms of the same species living in one area

• Within a population, there are many genes• The sum of the genes (and their different alleles)

is known as the gene pool• Gene pools are studied by population geneticists

Genotype, Phenotype & Allele Frequency

• Genotype Frequency: is a measure of the fraction, ration, or percent of the homozygotes and heterozygotes in a population sample for the given variations in a trait

• Phenotype Frequency: is a measure of the fraction, ratio, or percent of the offspring or sample population expressing either the dominant or recessive variations of a trait (could also have intermediate variations)

• Allele Frequency: is a measure of the fraction, ratio, or percent of the one variation occurring in the gametes of a populaiton

The Hardy-Weinberg Principle

• the Hardy-Weinberg principle predicts that if other factors remain constant, the gene pool will maintain a constant composition over many generations

• this is expressed by a mathematical equation:

The Hardy-Weinberg Equations

p2 + 2pq + q2 = 1(genotype frequency where phenotype can be interpreted)

p+q = 1(allele/gamete frequency)

Where:• p is the frequency of the A allele• q is the frequency of the a allele• if the values of p and q are known, we can

calculate the frequency of the alleles AA, Aa, and aa (and vice-versa)

Limits to the Hardy-Weinberg Principle

• Large populations• Random mating• No mutations• No migration• No natural selection against any of the

phenotypesThese are to maintain no significant change in the gene pool and are usually limited to shorter periods of time

Application of the Hardy-Weinberg Principle

• In a population, we know that a dominant trait is present 82% of the time. Determine the percentage of individuals that make up each genotype.

The Hardy-Weinberg & Population Change

• If a gene pool changes over time, one of the 5 conditions it is based on must also have changed

• Therefore, the strength of this principle is to determine whether or not a population is evolving

• The Hardy-Weinberg equation also allows us to determine what percentage of a population are “carriers” of a trait

Evolutionary Change

• gene pools are unstable in that they are constantly responding to both the biotic and abiotic changes in their ecosystems

• Evolutionary change takes time, especially with K-selected populations and involves Agents of Change …

Agents of Change

1. Mutation (changes in the nucleic base sequence causing a change in protein production)

2. Non-random mating (survival of the fittest)3. Non equal viability (struggle to exist with competition)4. Genetic Drift (chance changes in populations –

Founder Effect and Bottleneck Effect5. Gene Flow (migration of gene pools)

The Founder Effect

• New populations are often formed by only a few individuals (Founders)

• The founders will only carry part of the original gene pool from the population

• Therefore, the new gene pool will be limited• Examples:

• Blue Fugates• Philadelphia Amish

The Bottleneck Effect

• Starvation, disease, human activities, or natural disasters can quickly reduce a large population

• The survivors only have a subset of the alleles present before the disaster, and therefore, the gene pool loses diversity

• Gene pool change caused by a rapid decrease in population is known as the bottleneck effect

• Examples:• Northern Elephant Seals• Cheetahs

Natural Selection

• Natural selection is the only process that leads directly to evolutionary adaptation

• Example:• Sexual Selection• Heterozygote Advantage• Lethal Alleles• Alpha Males• Reproductive Isolation• Geographic Isolation

• Recall that natural selection occurs in the following order:• Variation• Overproduction• Struggle to Exist• Survival of the Fittest• Origin of a New Species

Human Activities & Genetic Diversity

• Humans can affect genetic diversity of populations in many ways:

1. Habitat fragmentation2. Unregulated hunting & habitat removal3. Introduction of mutagens4. Introduction of non-native species5. Introduction of new genomes to give one

species an advantage over another

Cloning to Save Species

• Cloning can be one way to preserve ancient gene pools

• Creating clones of endangered species could reverse the threat of extinction

• In 2000, a cloned Asian gaur (a rare ox-like mammal) was born in Iowa to a domestic cow that served as a surrogate mother