APBIO - chapter 23

Chapter 23: The Evolution of Populations

Overview of Evolutionary Concepts

  • Misconception: Organisms evolve during their lifetimes.

  • Reality: Natural selection operates on individuals, but populations evolve.

  • Case Study: Medium ground finches on Daphne Major Island evolved during a drought, favoring large beaks for survival.

Microevolution

  • Definition: Change in allele frequencies in a population over generations.

  • Mechanisms that Cause Allele Frequency Changes:

    • Natural selection (only mechanism causing adaptive evolution)

    • Genetic drift

    • Gene flow

Genetic Variation

  • Required for Evolution: Variation in heritable traits.

  • Mendel's Contribution: Established the existence of discrete heritable units (genes).

  • Genetic Variation: Caused by differences in genes/DNA segments.

  • Phenotype: Result of inherited genotype and environmental influence.

Types of Genetic Variation

  • Discrete vs. Quantitative Characters:

    • Discrete characters: Clearly defined categories (e.g., flower color).

    • Quantitative characters: Show a range of phenotypes (e.g., height).

  • Measurement:

    • Gene variability: Average heterozygosity at loci.

    • Nucleotide variability: Comparison of DNA sequences between individuals.

Geographic Variation

  • Definition: Differences in gene pools of separate populations.

  • Example: Isolated populations of mice on Madeira show genetic variation due to drift.

Cline and Geographic Variation

  • Cline: Graded change in traits across geographic gradients.

  • Example: Mummichog fish show genetic variation along temperature gradients due to natural selection.

Sources of Genetic Variation

  • Mutation: Changes in DNA sequence occur, especially in gametes.

    • Point mutations: Affect one base in a gene; can be neutral, harmful, or beneficial.

  • Gene Duplication: Can lead to increased genome size and new traits.

Sexual Reproduction and Genetic Variation

  • Shuffling of Alleles: Results in new genetic combinations; more important than mutation for evolution.

Hardy-Weinberg Principle

  • Definition: Conditions for a population to remain in equilibrium (not evolving).

  • Conditions:

    • No mutations

    • Random mating

    • No natural selection

    • Very large population size

    • No gene flow

  • Calculating Allele Frequencies: Uses the formula p² + 2pq + q² = 1.

Parameters of Hardy-Weinberg Equilibrium

  • Example Calculation: For allele frequencies in a population of wildflowers (320 red, 160 pink, 20 white).

  • Purpose: Helps assess if evolutionary forces are acting on the population.

Factors Altering Allele Frequencies

  • Natural Selection: Differential reproductive success leading to increased allele frequency.

  • Genetic Drift: Random changes in allele frequencies; more pronounced in small populations.

  • Founder Effect: Occurs when a small number of individuals colonize a new area, leading to different allele frequencies.

  • Bottleneck Effect: Sudden reduction in population size, altering the gene pool and variability.

Gene Flow

  • Definition: Movement of alleles between populations, which can counteract adaptation.

  • Example: Impact of immigration on fitness in the great tit (Parus major).

  • Pros & Cons: Can increase or decrease population fitness depending on the traits introduced.

Natural Selection vs. Other Mechanisms

  • Natural Selection: Leads to adaptive evolution through favoring beneficial traits.

  • Genetic Drift and Gene Flow: Do not consistently lead to adaptation; can reduce variation.

  • Sexual Selection: A specific type of natural selection focused on mate preferences.

    • Intrasexual Selection: Competition within the same sex.

    • Intersexual Selection: Mate choice based on traits that signal good health or genetics.

Preservation of Genetic Variation

  • Neutral Variation: Genetic diversity that does not confer an advantage.

  • Diploidy: Maintains recessive alleles, preserving variation not acted upon by selection.

  • Balancing Selection: Maintains stable allele frequencies (includes heterozygote advantage and frequency-dependent selection).

Conclusion

  • Evolution is a complex interplay of chance, natural selection, and environmental factors, emphasizing the significance of genetic variation and the mechanisms that drive evolutionary change.