Microevolution

AP Biology Notes — Microevolution

I. Definition of Microevolution

  • Microevolution: change in allele (gene) frequencies in a population across generations.

  • Occurs over short time spans (years to decades).

  • Observable directly.

  • Example: Kauai crickets evolving silence in ~15 years due to predation.

II. Classic Example: Kauai Crickets

  • Male crickets normally chirp to attract females.

  • Ormia ochracea invades Kauai.

  • Fly locates chirping males and deposits larvae → host death.

  • A mutation caused some males to have noiseless wings.

  • Silent males survived and reproduced.

  • Over ~20 generations, silent trait became dominant.

  • Demonstrates natural selection + mutation → microevolution.

III. Hardy–Weinberg Principle (No Evolution Conditions)

A population does not evolve if:

  1. No natural selection

  2. Random mating

  3. No mutation

  4. Very large population

  5. No migration (gene flow)

➡ Since these conditions never fully exist, evolution always occurs.

IV. Mechanisms of Microevolution

1. Natural Selection

  • Individuals with traits that increase survival and reproduction leave more offspring.

  • Favorable alleles increase in frequency.

Types of Selection

  • Stabilizing selection

    • Favors average phenotype

    • Reduces extremes

    • Example: medium-sized sled dogs

  • Directional selection

    • Favors one extreme

    • Example: faster horses

  • Disruptive selection

    • Favors both extremes

    • Eliminates average

    • Example: small + large fish survive nets

2. Nonrandom Mating (Sexual Selection)

  • Mate choice is selective, not random.

  • Traits that increase mating success become more common.

  • Examples:

    • Peacock tails

    • Frog calls

    • Antlers in moose

  • Changes genotype frequencies, contributes to evolution.

3. Mutation

  • Mutation = change in DNA sequence.

  • Only source of new alleles.

  • Mutations are random.

  • Effects:

    • Beneficial

    • Harmful

    • Neutral

  • Only mutations in reproductive (germ) cells affect evolution.

  • Example: mutation causing silent cricket wings.

4. Genetic Drift

  • Change in allele frequencies due to chance, not fitness.

  • Strongest in small populations.

  • Reduces genetic variation.

Types

  • Bottleneck effect

    • Population drastically reduced by random event.

    • Survivors’ genes ≠ original population.

  • Founder effect

    • Small group colonizes new area.

    • New population reflects founders’ genes.

    • Example: high Huntington’s disease frequency in Lake Maracaibo region.

5. Migration (Gene Flow)

  • Movement of individuals between populations.

  • Introduces new alleles into gene pool.

  • Increases genetic variation.

  • Makes populations more genetically similar over time.

  • Common in humans due to modern mobility.

V. Key Takeaways (AP Exam Focus)

  • Microevolution = allele frequency change, not individual change.

  • Natural selection acts on phenotypes, but evolution occurs in populations.

  • Mutation creates variation; selection and drift change frequencies.

  • Genetic drift is random; natural selection is nonrandom.

  • Hardy–Weinberg describes a theoretical non-evolving population.