AP Biology Unit 7: Natural Selection and Evolution

Introduction

AP Biology Unit 7 focuses on natural selection and evolution, foundational concepts in biology that explain how species change over time. This unit covers Darwin’s theory, mechanisms of evolution, population genetics, and speciation. Understanding these principles will help you grasp how life diversifies and adapts to different environments.

Key Topics in Unit 7

• Introduction to Natural Selection
• Natural Selection
• Artificial Selection
• Population Genetics
• Hardy-Weinberg Equilibrium
• Evidence of Evolution details
• Common Ancestry
• Continuing Evolution
• Phylogeny
• Speciation
• Extinction
• Variations in Populations
• Origin of Life on Earth

1. Natural Selection: Darwin’s Theory

• Proposed by Charles Darwin in On the Origin of Species (1859).
• Describes how organisms with advantageous traits survive and reproduce more successfully.
• Key principles:
  • Variation: Differences in traits among individuals.
  • Overproduction: More offspring are produced than can survive.
  • Adaptation: Beneficial traits increase survival.
  • Descent with Modification: Favorable traits accumulate over generations.

2. Mechanisms of Evolution

Evolution occurs through multiple mechanisms beyond natural selection:

• Genetic Drift: Random changes in allele frequencies, significant in small populations.
  • Bottleneck Effect: A sharp population reduction alters allele frequency.
  • Founder Effect: A new population is started by a small group with limited genetic variation.
• Gene Flow: Movement of alleles between populations.
• Mutations: Random changes in DNA that introduce genetic variation.
• Non-Random Mating: Selective breeding affects allele distribution.

3. Hardy-Weinberg Equilibrium

A model that predicts allele frequencies in a non-evolving population. The five conditions required for Hardy-Weinberg equilibrium are:

1. Large population size (no genetic drift)
2. No gene flow (migration)
3. No mutations
4. Random mating
5. No natural selection

The Hardy-Weinberg equation:
p² + 2pq + q² = 1 (where p and q represent allele frequencies).

4. Speciation and Phylogenetics

Speciation is the process by which new species form.

• Allopatric Speciation: Geographic isolation leads to divergence.
• Sympatric Speciation: Speciation without physical separation (e.g., polyploidy in plants).
• Reproductive Isolation: Barriers prevent interbreeding (prezygotic and postzygotic).

Phylogenetics studies evolutionary relationships using phylogenetic trees and cladograms.

• Homologous structures suggest common ancestry.
• Analogous structures result from convergent evolution.

5. Evidence for Evolution

Multiple lines of evidence support evolution:

• Fossil Record: Shows gradual changes over time.
• Comparative Anatomy: Homologous and vestigial structures.
• Molecular Biology: DNA and protein similarities.
• Embryology: Similar embryonic development in related species.
• Biogeography: Geographic distribution of species.

AP Biology Exam Tips for Unit 7

1. Understand Key Terms: Know definitions and examples of natural selection, genetic drift, and speciation.
2. Practice Hardy-Weinberg Problems: Solve allele frequency questions efficiently.
3. Analyze Phylogenetic Trees: Interpret evolutionary relationships.
4. Use Real-World Examples: Connect concepts to antibiotic resistance, finch beak variations, and industrial melanism.
5. Practice FRQs: Free-response questions often test evolutionary mechanisms and evidence.

Conclusion

Unit 7 in AP Biology is essential for understanding how species evolve and adapt. Mastering these concepts will strengthen your grasp of evolutionary biology and help you succeed on the AP exam. Keep practicing, use real-world examples, and review key formulas like Hardy-Weinberg to excel in this unit.

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