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Week 5

Chapter 4: Forces of Evolution

Learning Objectives

  • Outline a 21st-century perspective of the Modern Synthesis.

  • Define populations and population genetics, along with methods used to study them.

  • Identify the forces of evolution and provide examples for each.

  • Discuss the evolutionary significance of mutation, genetic drift, gene flow, and natural selection.

  • Explain how allele frequencies can be used to study evolution in real-time.

  • Differentiate between micro- and macroevolution.

Historical Context

  • Origins of Life: Life originated approximately 3.8 billion years ago, likely from a single-celled organism.

  • Phylogenetic Tree: Genetic analyses help trace the lineage of all living organisms via common ancestry.

The Modern Synthesis

  • Combines Darwin's natural selection with Mendelian genetics, facilitating the development of population genetics.

  • Involves mathematical models of evolutionary change and elucidates variation in observable traits. Key contributions include:

    • Fisher (1919) and Haldane (1924): Mathematical models enhancing population genetics.

    • Dobzhansky (1937) and Wright (1932): Identification of chromosomes carrying genes.

Population Genetics

Defining Populations

  • A population is a group of individuals of the same species that can interbreed and reproduce.

  • Populations are analyzed by their gene pool, which contains all the alleles for every gene in the population.

Forces of Evolution

  1. Mutation

    • Source of all genetic variation, occurring during DNA replication.

    • Can be neutral, harmful, or beneficial; crucial for evolutionary processes.

    • Types include point mutations, insertions, deletions, and chromosomal alterations.

    • Example: Beneficial mutations providing adaptive advantages, such as thicker fur in certain dog breeds.

  2. Genetic Drift

    • Random changes in allele frequencies, significantly affecting small populations.

    • Bottleneck Effect: Reduction in population size due to events (e.g., disasters).

    • Founder Effect: A small number of individuals start a new population, potentially carrying limited genetic variation.

  3. Gene Flow (Migration)

    • Movement of alleles between populations, increasing genetic diversity.

    • Often involves migration or admixture between populations, which can counteract genetic drift.

    • Nonrandom mating patterns can influence gene flow (assortative mating).

  4. Natural Selection

    • Process whereby individuals with advantageous traits reproduce more successfully.

    • Can take different forms:

      • Directional Selection: Favors one extreme phenotype (e.g., dark peppered moths in polluted areas).

      • Stabilizing Selection: Favors intermediate traits (e.g., average birth weight in humans).

      • Disruptive Selection: Favors extremes and can lead to speciation (e.g., Darwin’s finches).

Hardy-Weinberg Equilibrium

  • A theoretical framework to determine allele frequency stability in a population.

  • Conditions for equilibrium are rarely met in reality but serve as a baseline for comparison:

    1. No mutation

    2. Random mating

    3. No natural selection

    4. Large population size

    5. No gene flow

Relationship between Sickle Cell Allele and Malaria

  • Sickle Cell Trait: A heterozygous condition (HbA/HbS) provides malaria resistance in regions where malaria is prevalent, illustrating balancing selection.

Speciation: Formation of New Species

Concept of a Species

  • Defined as a group of interbreeding natural populations that can produce fertile offspring.

  • Recognizes existence of fertile hybrids between closely related species.

Process of Speciation

  1. Reproductive Isolation: Breaks originally continuous gene flow.

    • Prezygotic Barriers: Prevent mating (e.g., temporal, behavioral, habitat isolation).

    • Postzygotic Barriers: Allow mating, but result in sterile offspring (e.g., mules).

  2. Genetic Divergence: Accumulation of genetic differences over time through mutation and natural selection, leading to reproductive isolation.

Types of Speciation

  • Allopatric Speciation: Geographic barriers isolate populations, leading to divergence.

  • Sympatric Speciation: Occurs without physical barriers, often through genetic changes like polyploidy in plants.

Adaptive Radiation and Extinction

  • Adaptive Radiation: Rapid diversification into various ecological niches, often following mass extinctions (e.g., mammal diversification post-dinosaur extinction).

  • Extinction: Failure to adapt leads to species loss, with human activity possibly instigating a sixth mass extinction.

Key Takeaways

  • Evolution is defined as changes in allele frequencies over generations through mutation, gene flow, genetic drift, and natural selection.

  • The Hardy-Weinberg model serves as a foundational tool for understanding population evolution.

  • Speciation results from isolation and genetic divergence, with adaptive radiation showcasing the rapid evolution of species.