Hardy-Weinberg and Microevolution

Hardy-Weinberg Principles

Overview of Key Concepts

• Hardy-Weinberg (HW) theory relates to the equilibrium state of allele frequencies in populations and plays a critical role in studying microevolution.

• Evolution at the Population Level

• Definition of Population: A population consists of all individuals of the same species found in a specified geographical area. This interaction allows for interbreeding and production of fertile offspring.

• Microevolution: Defined as changes in allele frequencies in a population across generations. This concept is rooted in:

  • The inheritance of alleles which affect phenotypes, based on foundational work by Mendel and Morgan.

  • Variation in phenotypes leading to differential success, drawing from Darwinian principles of natural selection.

Mechanisms of Microevolution

• Microevolution is driven by four main mechanisms:

  1. Selection: Natural selection is the only mechanism that can cause adaptive evolution.

  2. Genetic Drift: Changes in allele frequency due to random sampling effects, particularly relevant in small populations.

  3. Gene Flow: Corresponds to the movement of alleles between populations due to migration.

  4. Mutation: Random changes in DNA that introduce new alleles into a population.

Role of Phenotypic Variation in Microevolution

• Source of Variation: Mutations serve as the ultimate source of genetic variation within populations.

  • Types of mutations:

  • Neutral: No effect on phenotype.

  • Deleterious: Harmful impact on phenotype.

  • Beneficial: Positive impact on phenotypes and potential survival.

• Genetic variation can further be enhanced by:

  • Sexual Reproduction: Introducing genetic diversity through recombination.

  • Migration: Facilitating gene flow and introduction of new alleles into the gene pool.

Gene Pool and Allele Frequencies

• Gene Pool: The complete set of alleles present in a population.

  • For diploid organisms, each individual carries two alleles for each gene.

  • An allele is termed fixed if all individuals in a population are homozygous for that allele.

Hardy-Weinberg Equilibrium: Definitions and Calculations

• Hardy-Weinberg Theorem: This theorem provides a mathematical model for calculating expected allele frequencies in the absence of evolution. The equation is defined as:
  $p^2 + 2pq + q^2 = 1$ where:

  • p: frequency of dominant allele

  • q: frequency of recessive allele

• Example Calculations:

  • p = 0.80 -> 80% chance of inheriting p allele

  • q = 0.20 -> 20% chance of inheriting q allele

  • p² = 0.80² = 0.64 (64% of next gen are pp)

  • q² = 0.20² = 0.04 (4% of next gen are qq)

  • 2pq = 2(0.8)(0.2) = 0.32 (32% of next gen are pq)

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  • Null Hypothesis: The calculations form a null hypothesis asserting that if no evolution occurs, allele frequencies (p and q) should remain constant across generations.

Assumptions for HW Equilibrium

• Changes in allele frequency violate any of the following five assumptions of HW equilibrium:

  1. No Selection: No differences in survival and reproduction.

  2. No Mutation: No new alleles introduced to the gene pool.

  3. No Migration (gene flow): No new individuals or alleles can enter or exit the population, affecting allele frequencies.

  4. Large Population Size: Reduces the impact of genetic drift.

  5. Random Mating: No preferences for specific genotypes over others, ensuring allele frequencies are maintained.

Small Populations and Evolution

• Evolution can happen randomly in small populations due to genetic drift. This concept is illustrated through hypothetical examples:

  • Generational Example:

  • Generation 1: (p = 0.7), (q = 0.3)

  • Generation 2: (p = 0.5), (q = 0.5)

  • Generation 3: (p = 1.0), (q = 0.0)

  • This example exemplifies how chance alone can influence genetic outcome in small populations.

Bottleneck Effect

• Bottlenecks: Occur when a significant reduction in population size takes place, typically due to random events (e.g., natural disasters).

  • The resultant smaller population leads to a loss of genetic diversity, which in turn influences allele frequencies in unforeseen ways, centered on the alleles of the remaining individuals.

Founder Effect

• Founder Effect: Describes the phenomenon when a small subgroup of a population establishes a new population, often leading to reduced genetic variation compared to the original population.

  • This is particularly relevant in island biogeography where small groups may become isolated, thus reflecting the genetic makeup of just a few founding individuals rather than the larger population.