Chapter 13: How Populations Evolve - Detailed Notes

Evolution: Key Concepts

• Definition: Heritable changes in populations over generations that lead to Earth's diversity and transformation of life.

• Creationism: Belief that life is created by God, implying species are fixed and unchanging.

• 18th Century Naturalists: Proposed the idea that species evolve and are not permanent.

Historical Figures in Evolution

• Georges Buffon:

  • Suggested fossils could be ancient versions of living species.

• Jean Baptiste Lamarck:

  • Proposed the theory of inheritance of acquired characteristics, where traits acquired during an individual's life could be passed to offspring.

• Charles Lyell:

  • Presented uniformitarianism, suggesting slow geological processes can lead to substantial changes.

Charles Darwin's Contributions

• Main Observations:

  • Similarities between temperate S. American and tropical species.

  • Galapagos flora and fauna resemble S. American mainland characteristics.

• Key Book: "On the Origin of Species"

  • Major points:

  • Evolution as the basis of life's diversity.

  • Natural selection as the mechanism for evolution.

Inferences Leading to Natural Selection

• Key Observations:

  • Overproduction: Organisms reproduce extensively.

  • Individual Variation: Variability exists in every population.

  • Adaptation: Some traits enhance survival in specific environments.

• Inferences:

  • Survival of the Fittest: Those best adapted are likely to reproduce.

  • Gradual Change: Populations evolve over time.

Natural Selection Explained

• Definition: Differential survival and reproduction among individuals.

• Requirements for Natural Selection:

  • Genetic variation within the population.

  • Heritability of variations to offspring.

  • Competition for resources.

  • Differences in reproductive success.

Evidence of Evolution

• Biogeography:

  • Study of species distribution; similarities between island and mainland species.

• Fossil Record:

  • Shows progression from simpler organisms in deep layers to more complex organisms in upper layers.

• Comparative Anatomy:

  • Homologous structures indicate common ancestry despite different functions.

• Molecular Biology:

  • Shared genetic code across species suggests evolutionary relationships.

Addressing Prevailing Beliefs

• Questions Raised:

  • How did species move from creation centers?

  • Purpose of vestigial structures?

  • Evidence from fossil records counters the idea that species are fixed.

The Modern Synthesis

• Definition: Combining genetics with evolutionary theory.

  • Smallest unit that can evolve: population.

  • Natural selection as the key driver of evolution.

  • Long-term changes result from accumulated small changes.

Genetic Variations & Population Genetics

• Types of Genetic Variation:

  • Sexual Recombination: Meiosis and random fertilization increase genetic diversity.

  • Mutations: Introduce new traits; beneficial mutations are favored.

  • Gene Flow: Movement of genes between populations.

• Hardy-Weinberg Principle:

  • Describes allele frequencies in populations at equilibrium:
    p^2 + 2pq + q^2 = 1

  • Conditions for equilibrium:

  1. Large population size.

  2. Isolation from other populations.

  3. No mutations.

  4. Random mating.

  5. Absence of natural selection.

Microevolution

• Definition: Changes in allele frequencies within a population over time, enabling adaptations.

Types of Natural Selection

• Stabilizing Selection: Favors average phenotypes.

• Directional Selection: Favors one extreme phenotype.

• Disruptive Selection: Favors extremes over the average phenotype.

Effects of Mutations and Genetic Drift

• Mutations:

  • Sustains genetic diversity; harmful ones are eliminated.

• Genetic Drift:

  • Changes in allele frequency from random events (bottleneck and founder effect).

Conclusion

• Evolution doesn't aim for perfection: It responds to environmental changes, ensuring survival of responsive species.

• Quote: “It is not the strongest of the species that survive, nor the most intelligent, but the one most responsive to change.” — Charles Darwin