AP Biology Population Evolution

19.1 Population Evolution

Key Concepts

1. Population Genetics Synthesis: Integrates Mendelian inheritance with Darwinian evolution.

   • Combines natural selection with genetic principles.

   • Focuses on microevolution (changes in allele frequencies) and its link to macroevolution (speciation).

2. Modern Synthesis:

   • Microevolution: Small changes in allele frequencies within a population.

   • Macroevolution: Large-scale changes leading to new species or taxonomic groups.

3. Hardy-Weinberg Principle:

   • Describes non-evolving populations: allele frequencies remain constant if five conditions are met:

     • No mutations.

     • Random mating.

     • No natural selection.

     • Extremely large population size (no genetic drift).

     • No gene flow (immigration/emigration).

   • Equation: p² + 2pq + q² = 1, where:

     • p = dominant allele frequency.

     • q = recessive allele frequency.

   • Deviations from equilibrium indicate evolution (e.g., pesticide resistance in mosquitoes).

Mechanisms of Evolution

1. Natural Selection: Favors advantageous alleles, increasing their frequency.

2. Genetic Drift:

   • Random changes in allele frequencies, especially in small populations.

   • Examples: Founder Effect (small group isolation) and Bottleneck Effect (population reduction).

3. Gene Flow: Movement of alleles between populations.

4. Mutation: Source of new genetic variation.

19.2 Population Genetics

Genetic Variation

1. Sources of Variation:

   • Polymorphisms: Different alleles lead to varied traits (e.g., blood types).

   • Environmental Influence: Traits like alligator sex determination depend on incubation temperature.

2. Heritable vs. Non-Heritable Traits:

   • Only genetic traits are subject to evolution.

Evolutionary Forces

1. Natural Selection: Acts on phenotypes to favor beneficial traits.

2. Genetic Drift:

   • Bottleneck Effect: Random reduction in population size.

   • Founder Effect: Isolated populations develop unique allele frequencies.

3. Gene Flow: Introduction or removal of alleles due to migration.

4. Non-Random Mating: Influences genotype frequencies (e.g., assortative mating).

Hardy-Weinberg Equilibrium Applications

1. Predict allele frequencies using the equation.

2. Deviations reveal selective pressures or other evolutionary forces.

19.3 Adaptive Evolution

Selection Types

1. Stabilizing Selection: Favors average phenotypes (e.g., mouse coat matching forest floor).

2. Directional Selection: Favors one extreme phenotype (e.g., darker moths during industrialization).

3. Diversifying Selection: Favors extreme phenotypes (e.g., light and dark mice in variable habitats).

4. Frequency-Dependent Selection:

   • Positive: Favors common traits.

   • Negative: Favors rare traits.

5. Sexual Selection:

   • Leads to sexual dimorphism (e.g., peacock tails).

   • Handicap Principle: Traits signaling high fitness despite survival costs.

Limits of Natural Selection

1. Acts only on existing variation; cannot create new traits.

2. Linked genes and trade-offs can constrain adaptation.