Rucete ✏ Campbell Biology In a Nutshell
Unit 4 MECHANISMS OF EVOLUTION — Concept 23.3 Natural Selection, Genetic Drift, and Gene Flow Can Alter Allele Frequencies in a Population
Evolutionary changes in populations are primarily driven by three main mechanisms: natural selection, genetic drift, and gene flow. Each of these mechanisms alters allele frequencies and thus leads to evolution.
Natural Selection
- Natural selection occurs through differential survival and reproduction:
- Individuals with traits better suited to their environment produce more offspring.
- Favorable traits increase in frequency over generations.
- Example: DDT resistance in fruit flies (Drosophila melanogaster) increased after pesticide use.
Genetic Drift
- Genetic drift describes random changes in allele frequencies, especially in small populations.
- It does not consistently favor any specific allele.
Two forms of genetic drift:
- Founder Effect:
- Occurs when a small group becomes isolated and forms a new population with different allele frequencies.
- Example: High frequency of blindness allele on Tristan da Cunha island.
- Bottleneck Effect:
- Sudden environmental change drastically reduces population size.
- Survivors may not represent original allele frequencies.
- Example: Prairie chickens in Illinois suffered genetic drift, reducing fertility and diversity.
Gene Flow
- Gene flow is the movement of alleles between populations through migration.
- It reduces genetic differences between populations.
- Can enhance adaptation (e.g., insecticide resistance in mosquitoes).
- Can hinder adaptation (e.g., banded snakes in Lake Erie due to mainland gene input).
Effects of Genetic Drift: Summary
- More significant in small populations.
- Allele frequencies change unpredictably.
- Reduces genetic variation.
- May fix harmful alleles, threatening survival.
Comparing Mechanisms
- Natural Selection: Consistently increases adaptive traits.
- Genetic Drift: Random and impactful in small populations.
- Gene Flow: Transfers alleles across populations; may help or hinder local adaptation.
In a Nutshell
Natural selection, genetic drift, and gene flow are major evolutionary forces altering allele frequencies. Natural selection consistently favors adaptive traits, while genetic drift randomly impacts small populations. Gene flow exchanges genetic material among populations, influencing adaptation and genetic diversity.