Hong 7.1_7.2 Natural Selection

Unit 7: Natural Selection

Topic Overview

  • Natural Selection (EVO-1): Refers to the process where organisms with favorable traits are more likely to survive and reproduce.

  • Key Concept: Evolution is a change in the genetic makeup of a population over time, supported by multiple lines of evidence.

Darwin’s Finch Collection

  • Beak Variability:

    • All finches are similar in size but vary in beak shape and size.

    • These variations are crucial for adapting to different food sources.

Observations of Darwin’s Finches

  • Beak Differences:

    • Different beaks are associated with the consumption of various food types.

    • Survival and reproduction are linked to beneficial adaptations that arise from these food sources on the islands.

Correlation of Species to Food Source

  • Key Finch Species and Their Food Sources:

    • Medium Ground Finch: Seed eaters

    • Cactus Ground Finch: Cactus flour eater

    • Small Tree Finch: Insect eater

    • Woodpecker Finch: Insect eater

    • Large Ground Finch: Seed eater

  • Genus Classification:

    • Geospiza: Ground finches

    • Camarhynchus: Tree finches

    • Certhidea: Warbler finches

  • Evolutionary Origin: Common ancestor believed to come from the South American mainland.

Adaptations

  • Definition: Enhancements that increase an organism's ability to survive and reproduce in particular environments.

  • Examples:

    • Desert fox: Large ears for heat dissipation

    • Arctic fox: Small ears for heat conservation

  • Natural Selection Mechanism: Adaptations arise through natural selection based on environmental conditions and existing heritable variation.

Understanding Natural Selection

  • Competition for Resources: Limits resources lead to differential survival among individuals.

  • Phenotypic Advantage: Individuals with more favorable traits (phenotypes) are more likely to survive and reproduce, thus passing those traits to future generations.

Environmental Changes and Selective Pressure

  • Example: Peppered moth evolution reflecting adaptation to pollution.

  • Differential Reproduction: Environmental pressures affect reproduction rates based on various traits.

Evolutionary Fitness

  • Definition: Measured by reproductive success—an organism's ability to survive and bear fertile offspring.

  • Heritability: Adaptations must be transferable to successive generations for evolution to occur.

  • Population Dynamics: Traits that lead to greater reproductive success become more prevalent in the population over time.

Types of Natural Selection

  1. Directional Selection: Favors one extreme phenotype, shifting the population’s traits.

  2. Disruptive Selection: Favors both extreme phenotypes over average phenotypes.

  3. Stabilizing Selection: Favors average phenotypes, reducing variation within a population.

Case Study: American Tree Sparrows

  • Mass Distribution Study: Examines changes in body mass distribution pre- and post-snowstorm.

    • Key Question: How did the snowstorm affect average body weight?

    • Potential Outcomes:

      • A: Decrease in average body weight.

      • B: No change in total population.

      • C: Increase in average body weight.

      • D: No effect on average body weight.

Summary of Key Points

  1. Natural Selection Process: Organisms with beneficial adaptations have greater survival odds.

  2. Influencing Factors: Environmental stability, genetic variation, adaptations, and reproductive fitness play significant roles.

  3. Phenotypic Variation: Can result in shifts in the prevalence of certain traits within the population over time.