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Chapter 6: Mendelian Genetics in Populations

Key Contributors

• Godfrey Hardy (1877-1947)

• Wilhelm Weinberg (1862-1937)

Modern Synthesis of Evolution and Genetics

• Timeframe: 1930s to 1960s

• Integration of Darwin’s theory of evolution by natural selection with genetics.

• Incorporation of mathematical theory and hypothesis testing into scientific methodology.

Foundations of Population Genetics

• Establishment of the field during the modern synthesis.

• Darwin’s Four Postulates:

  1. Individuals are variable within a species.

  2. Some variations are heritable.

  3. More offspring are produced than can survive.

  4. Favorable variations enhance survival and reproductive success.

• Concept of Natural Selection introduced as non-random survival and reproduction.

Restatement of Darwin's Postulates in Population Genetics

• 1. Allelic variation exists among individuals.

• 2. Alleles are inherited via meiosis and fertilization.

• 3. More offspring produced than can survive.

• 4. Some allelic combinations confer higher fitness leading to greater survival and reproduction.

Concept of Microevolution

• Definition: Change in allele frequencies within a population across generations.

• Microevolution is evolution occurring at the population level, the unit of evolutionary change.

Hardy-Weinberg Equilibrium Principle

• A null model to analyze allele and genotype frequency changes in populations when evolution is not occurring.

• Key Aspects:

  • Alleles behave predictably under specific conditions.

  • Applies predominantly to diploid sexual organisms.

Population Dynamics

• Population Structure: Collection of interbreeding individuals and their descendants.

• Lifecycle Overview:

  • Adults generate gametes.

  • Gametes combine to form zygotes, evolving into the next generation.

• Objective: Monitor the legacy of Mendelian genes across generations to track allele prevalence.

Example of Allele Tracking

• Scenario: Mice with locus A containing two alleles A and a.

• Random mating among adults is assumed for tracking allele frequencies.

Initial and Final Allele Frequencies

• Initial frequencies: A = 0.6, a = 0.4

• Final frequencies through a sample process: A = 0.625, a = 0.375.

Importance of Random Sampling

• Random Genetic Drift (RGD): Variations may occur due to chance during reproduction.

• Example: Genetic sampling errors observed in populations leading to shifts in allele frequencies.

Hardy-Weinberg Calculation Methodology

• When mating occurs:

  • Calculate genotype frequencies using established rules (e.g., A allele in egg and sperm).

  • Must observe that genotype proportions provide expected values equivalent to Hardy-Weinberg equilibrium.

Predictions of Hardy-Weinberg

• If a population is in Hardy-Weinberg equilibrium, allele frequencies will remain stable over generations regardless of initial frequencies.

• Conclusion: Population remains static in terms of evolution if HWE is followed strictly.

General Case

• Mathematical Model:

  • Single locus with alleles A (p) and B (q).

  • Expected genotype frequencies:

    • AA: p²

    • AB: 2pq

    • BB: q²

Significance of Hardy-Weinberg Equilibrium

• Illustrates conditions under which evolution does not occur.

• Serves as a reference model for identifying evolution when assumptions are violated.

Basic Assumptions of Hardy-Weinberg

1. No selection—every individual contributes equally.

2. No mutation—no new alleles introduced.

3. No migration—genetic isolation maintained.

4. Infinitely large population—no genetic drift.

5. Random mating—no mate selection bias.

Importance of HWE:

• Enables forecasting genotype frequencies based on allele frequencies.

• Under random mating, genotype frequencies will quickly match Hardy-Weinberg predictions.

Considerations for Dominance

• Example: Traits like polydactyly in cats, illustrating dominance in phenotypic expression.

• Frequencies of dominant (N) and recessive (n) alleles defined as p and q, respectively.

• The Hardy-Weinberg model applied yields predicted genotype distributions (p², 2pq, q²).

Exploring Allele Frequencies

• Utilizes Hardy-Weinberg predictions to demonstrate changes in allele frequencies due to selection, mutations, or other factors.

• Usage in Case Studies: Specific traits examined for evolutionary trends in different species.

Consequences of Violating HWE Assumptions

• Emergence of evolution through shifts in allele frequencies.

• Chi-square tests assess deviations from expected frequencies under HWE to determine if population is evolving.

Applications in Observable Case Studies

• Examining specific traits under selective pressures (e.g., in Drosophila populations under alcohol stress).

• Discussion of empirical evidence supporting HWE theory through controlled experiments.

Conclusion

• Vital for understanding how underlying genetic mechanisms facilitate evolution and setting conditions for Hardy-Weinberg equilibrium to break down.