AP Biology: Microevolution – Mutation, Gene Flow, Genetic Drift (Bottleneck, Founder Effect), Selection

Microevolution – Mutation, Gene Flow, Genetic Drift (Bottleneck, Founder Effect), Selection

Concept Summary

• Mutation: Random change in DNA sequence; ultimate source of new alleles and genetic variation.
• Gene flow: Movement of alleles between populations via migration; homogenizes allele frequencies.
• Genetic drift: Random fluctuations in allele frequencies due to chance events; stronger in small populations.
• Bottleneck effect: Drastic population size reduction; reduces genetic diversity and can fix harmful alleles.
• Founder effect: New population established by few individuals; allele frequencies differ from source population.
• Selection (natural/sexual): Differential survival/reproduction based on heritable traits; drives adaptive evolution.

Core Questions

WHAT (definitional)

Q: What is a mutation? A: A permanent, heritable change in the nucleotide sequence of DNA. Trap/Clarification: Mutations are random and not inherently "good" or "bad"—effects depend on environment.

Q: What is genetic drift? A: Random changes in allele frequencies due to sampling error in small populations. Trap/Clarification: Drift is not adaptive—it can fix neutral or even deleterious alleles by chance.

WHY (causal/explanatory)

Q: Why is gene flow important? A: It introduces new alleles, increasing genetic diversity and counteracting drift/divergence. Trap/Clarification: Gene flow can reduce local adaptation if maladaptive alleles enter a population.

Q: Why does the bottleneck effect reduce genetic diversity? A: A severe population reduction randomly eliminates alleles, often disproportionately rare ones. Trap/Clarification: Post-bottleneck recovery does not restore lost alleles—diversity remains low.

HOW (process/application)

Q: How do you calculate allele frequency changes due to drift? A: Use the formula: p’ = p ± ?(p(1-p)/2N), where p = initial frequency, N = population size. Trap/Clarification: Drift’s impact scales with 1/2N—smaller N = larger fluctuations.

Q: How does selection alter allele frequencies? A: Alleles conferring higher fitness increase in frequency via differential survival/reproduction. Trap/Clarification: Selection acts on phenotypes, not directly on alleles (e.g., dominance can mask recessive alleles).

CAN (conditions/possibilities)

Q: Can genetic drift lead to adaptation? A: No—drift is random and can fix neutral or harmful alleles independent of fitness. Trap/Clarification: Drift may appear adaptive if a neutral allele hitchhikes with a beneficial one.

Q: Under what conditions does gene flow prevent speciation? A: When migration rates are high enough to homogenize allele frequencies between populations. Trap/Clarification: Gene flow can promote speciation if hybrids have reduced fitness (reinforcement).

Quick Facts & Traps

• Fact: Mutations are the only source of new alleles—all other mechanisms shuffle existing variation.
• Trap: "Drift only matters in tiny populations."-Reality: Drift is always present but dominates in small populations.
• Fact: Founder effect-bottleneck—founder events involve colonization, not population collapse.
• Trap: "Selection always increases fitness."-Reality: Selection can favor traits that are locally adaptive but harmful in other contexts (e.g., sickle-cell allele).
• Fact: Gene flow can be asymmetric (e.g., pollen vs. seed dispersal in plants).
• Trap: "Bottlenecks always reduce fitness."-Reality: They reduce genetic diversity, but fitness depends on which alleles are lost.

Rapid-Fire True/False

• Statement: Mutations occur in response to environmental pressures. Answer: FALSE Why the common mistake happens: Lamarckian misconception—mutations are random, not directed.

• Statement: A population with 100% heterozygotes (Aa) can experience genetic drift. Answer: TRUE Why the common mistake happens: Drift affects allele frequencies, not just genotype frequencies (e.g., A vs. a can still change).

• Statement: The founder effect always reduces genetic diversity. Answer: TRUE Why the common mistake happens: Some assume rare alleles might be overrepresented, but total diversity still decreases.