Biology 3: Fundamentals of Biology

Biology 3: Fundamentals of Biology

The Evolution of Populations

The Hardy-Weinberg Principle

  • Definition: The Hardy-Weinberg principle states that allele/genotype frequencies will stay the same in a population from generation to generation if specific conditions are met:

    1. The population is very large.

    2. The population is isolated from other populations.

    3. There are no mutations.

    4. There is random mating.

    5. There is no natural selection (all individuals have equal reproductive success).

Historical Background

  • Founders: The principle was independently proposed by G.H. Hardy and Wilhelm Weinberg in 1908.

Conditions of the Hardy-Weinberg Principle
Large Population

  • Explanation:

    • The term “large” is relative.

    • Some texts describe it as an “infinite” population,

    • Emphasis on the theoretical nature of the Hardy-Weinberg principle, akin to a perfect closed system for the laws of thermodynamics.

Population Isolation

  • Explanation:

    • Isolated populations do not receive outside alleles, maintaining unique gene pools.

    • Example: “Uncontacted peoples”, tribes living without contact with civilization, e.g., an estimated 100 such tribes primarily in the Amazon.

No Mutations

  • Explanation:

    • Mutations introduce new genes or alleles, disrupting Hardy-Weinberg equilibrium.

    • Key Point: For equilibrium, there must be NO mutations; the composition of the gene pool remains constant across generations.

Random Mating & No Natural Selection

  • Explanation:

    • Random mating ensures each individual has an equal probability of mating, allowing for the equal transmission of genes.

    • No natural selection means all individuals contribute equally to the next generation, irrespective of their phenotypes.

The Hardy-Weinberg Mathematics

Allele Frequency Calculation

  • General Notation:

    • Let A and a be the two alleles in the population,

    • p = frequency of allele “A”

    • q = frequency of allele “a”

    • Relation: p + q = 1

Example of Hardy-Weinberg Application
Parent Generation Example

  • Initial Alleles:

    • Parent generation: Yy and Yy.

    • Allele summary: Allele Y = 2, Allele y = 2

    • Ratio: Y:y = 1:1

Offspring Generation

  • Genotypes: YY, Yy, Yy, yy.

  • Allele Count: Allele Y = 4, Allele y = 4.

  • Ratio Maintained: Y:y = 1:1.

  • Conclusion: Allele frequencies remain constant across generations.

Detailed Calculation Example

  1. Total Number of Alleles Calculation:

    • Given 12 cats (3 BB, 6 Bb, 3 bb).

    • Total alleles within population = 24 (2 alleles per cat).

  2. Frequency of Alleles:

    • Count B alleles:

      • From BB: 6 (3 * 2) + Bb: 6 + bb: 0 = 12.

    • Count b alleles:

      • From BB: 0 (3 * 0) + Bb: 6 + bb: 6 = 12.

  3. P and Q Frequencies:

    • p = rac{B ext{ alleles}}{ ext{total alleles}} = rac{18}{30} = 0.6

    • q = rac{b ext{ alleles}}{ ext{total alleles}} = rac{12}{30} = 0.4

    • Validating: p + q = 0.6 + 0.4 = 1

Genotype Frequency Calculation

  • Equations:

    • (p + q)^2 = p^2 + 2pq + q^2

    • Where:

    • p^2 = homozygous dominant,

    • 2pq = heterozygous,

    • q^2 = homozygous recessive.

    • Total must equal 1:

    • p^2 + 2pq + q^2 = 1

Example Genotype Frequencies

  • Given frequencies of p = 0.5, q = 0.5:

    • p^2 = 0.25

    • 2pq = 0.5

    • q^2 = 0.25

Real-Life Applications of the Hardy-Weinberg Principle

Violation of the Hardy-Weinberg Conditions

  • The principle states that allele/genotype frequencies will stay constant if the following conditions hold true:

    1. The population is very large.

    2. The population is isolated from other populations.

    3. There are no mutations.

    4. There is random mating.

    5. There is no natural selection.

    • However, in reality:

    1. Genetic Drift can occur.

    2. Gene Flow can mix alleles from different populations.

    3. Mutations can occur, introducing new alleles.

    4. Nonrandom mating may happen based on preferences.

    5. Natural selection can favor certain phenotypes.

Genetic Drift

  • Definition:

    • Refers to changes in allele frequencies due to random sampling effects, significantly impacting small populations more than large ones.

    • Result: Loss of genetic diversity within a population.

  • Special Cases:

    1. Population Bottleneck: A significant reduction in population size due to catastrophic events, leading to a decrease in genetic variability.

    • Example: Cheetahs during the last ice age, where they nearly went extinct, resulting in a limited number of alleles.

    1. The Founder Effect: Establishment of a new population from a few individuals leading to reduced genetic variation.

    • Example: The Amish community, where a higher incidence of polydactyly exists due to the small founding population carrying that gene.

Gene Flow

  • Definition: Transfer of alleles from one population to another, often via migration.

  • Impact Example: Introduction of Sika deer in Western Europe leads to gene flow with native red deer, causing the extinction of the pure red deer gene pool.

Mutation

  • Definition: Mutations introduce new alleles into the gene pool, which persist if advantageous.

  • Example: Sickle cell anemia mutation, which persists due to conferring resistance to malaria in heterozygotes.

Nonrandom Mating

  • Definition: Many organisms do not mate randomly; they often prefer mates with similar phenotypes, leading to assortative mating.

  • Consequences: Can lead to inbreeding, reducing genetic diversity, exemplified by King Charles of Spain whose lineage suffered from inbreeding due to selective mating practices.

Natural Selection

  • Definition: A process where organisms better adapted to their environment tend to survive and reproduce more, changing allele frequencies over generations.

  • Types of Selection:

    1. Stabilizing Selection: Where average traits are favored; extremes are selected against.

    • Example: Robins favor laying four eggs as an optimal number for survival rates.

    1. Directional Selection: Favors one extreme trait over the mean; typical with changes in environment.

    • Example: Color change in peppered moths during the Industrial Revolution due to changes in their environment.

    1. Diversifying Selection: Opposite of stabilizing, where extremes are favored over the average trait.

    • Example: In certain environments, gray and white rabbits survive better than solely white rabbits.

Conclusions on Natural Selection

  • Key Insights:

    1. Natural selection shapes populations but acts on individuals.

    2. Only heritable traits are subject to natural selection.

    3. It operates without a predetermined goal—results depend on current environmental conditions.

    4. Adaptations are context-dependent; advantageous traits can become maladaptive if environmental conditions change.

Antibiotic Resistance Reporting
CDC Findings

  • Reported that in 2013, over 2 million people in the United States contracted illnesses from drug-resistant microorganisms, causing over 23,000 deaths. Identified 15 microorganisms of urgent concern to public health.

Final Notes on the Hardy-Weinberg Principle

  • The cornerstone for understanding genetic variation in populations, it emphasizes that deviations from equilibrium (like genetic drift, gene flow, mutations, nonrandom mating, and natural selection) are crucial for understanding real-world population genetics.

Questions?

  • Topics discussed include Genetic Drift, Natural Selection, with an emphasis on real-life applications of biological principles.