The Evolution of Populations — Microevolution, Hardy-Weinberg, and Selection Explained | Chapter 23 of Campbell Biology

Welcome to Last Minute Lecture! This post breaks down microevolution and population genetics, as covered in Chapter 23 of Campbell Biology. Learn how allele frequencies change over generations, how genetic drift and gene flow operate, and how Hardy-Weinberg equilibrium serves as a benchmark for evolution in real-world populations. Watch the full video summary below and subscribe for more clear, chapter-by-chapter guides to evolutionary biology!

Introduction: Microevolution in Populations

Evolution acts on populations, not individuals. Microevolution describes the change in allele frequencies within a population from one generation to the next, driven by mechanisms such as natural selection, genetic drift, and gene flow. Classic studies, such as the finches of the Galápagos Islands, illustrate how environmental changes and heritable traits (like beak size) drive evolutionary adaptation.

Hardy-Weinberg Equilibrium: Theoretical Baseline for Evolution

• The Hardy-Weinberg equation (p² + 2pq + q² = 1) models how allele and genotype frequencies remain constant in a non-evolving, ideal population.
• Assumptions: no mutations, random mating, no natural selection, large population size, no gene flow.
• In reality, these conditions are never fully met—evolution is always at work in natural populations.

Sources and Types of Genetic Variation

• Genetic Variation: Differences in DNA sequence among individuals; provides the raw material for evolution.
• Arises through mutations, genetic recombination during sexual reproduction, and gene flow between populations.

Mechanisms of Evolution

• Natural Selection: Individuals with traits best suited to their environment survive and reproduce more successfully, increasing those alleles in the population.
• Genetic Drift: Random changes in allele frequencies, especially impactful in small populations.
  • Founder Effect: A small group establishes a new population, reducing genetic diversity.
  • Bottleneck Effect: Sudden population reduction (e.g., by disaster) leads to loss of genetic variation.
• Gene Flow: Movement of alleles between populations via migration, reducing genetic differences between groups.

Types of Natural Selection

• Directional Selection: Favors one extreme phenotype, shifting the population mean.
• Disruptive Selection: Favors both extreme phenotypes over intermediates, increasing variation.
• Stabilizing Selection: Favors intermediate phenotypes, reducing variation.
• Sexual Selection: Traits increasing mating success become more common, often leading to sexual dimorphism (distinct male and female traits).

Adaptive Evolution and Genetic Diversity

• Adaptive Evolution: Natural selection consistently leads to traits that enhance survival and reproductive success (fitness).
• Heterozygote Advantage: Heterozygous individuals have higher fitness (e.g., sickle-cell allele and malaria resistance).

Key Terms and Glossary

• Microevolution: Changes in allele frequencies within a population.
• Hardy-Weinberg Equilibrium: Theoretical state where allele frequencies don’t change.
• Genetic Drift: Random fluctuations in allele frequencies.
• Founder Effect: Loss of genetic diversity when a new population is founded by a small group.
• Bottleneck Effect: Drastic reduction in population size and genetic diversity.
• Gene Flow: Movement of alleles between populations.
• Adaptive Evolution: Evolution that results in a better fit between organisms and their environment.
• Directional/Disruptive/Stabilizing Selection: Modes of selection shaping population traits.
• Sexual Selection: Selection for traits that increase mating success.
• Heterozygote Advantage: When heterozygotes have higher fitness than homozygotes.

Conclusion: Evolution’s Dynamic Process in Populations

Chapter 23 of Campbell Biology reveals how microevolution shapes populations through complex interactions between selection, drift, and gene flow. Understanding these forces explains both the stability and diversity of life on Earth. For a detailed walk-through, watch our full video summary and subscribe to Last Minute Lecture for more evolutionary biology guides.

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