Unit Overview
Natural Selection Overview
AP BiologyΒ· 5 min read π 13-20% of total AP Biology exam score
1. Unit at a Glance
This unit moves from core foundational principles of natural selection to broad patterns of evolution across Earth's 4-billion-year history. We start with how natural selection works at the individual and population level, then expand to genetic variation, population genetics, and the Hardy-Weinberg model. Next we cover evidence for evolution, reconstruct evolutionary relationships with phylogenetics, and end with topics on speciation, extinction, and the origins of life itself. Concepts build sequentially, so working through sub-topics in order will help you connect ideas for exam questions.
This unit covers the following sub-topics:
AP Biology Introduction to Natural Selection
Get a foundational overview of the core principles of natural selection.
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AP Biology Natural Selection
Dive deeper into how natural selection drives adaptive evolution.
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AP Biology Artificial Selection
Learn how human selection drives evolution of domesticated organisms.
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AP Biology Variations in Populations
Explore how genetic variation arises and is maintained in populations.
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AP Biology Population Genetics
Study how evolutionary forces change genetic variation at the population level.
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AP Biology Hardy-Weinberg Equilibrium
Master the Hardy-Weinberg model to calculate and predict allele frequencies.
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AP Biology Evidence of Evolution
Review the multiple lines of scientific evidence supporting evolution.
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AP Biology Common Ancestry and Speciation
Explore how new species form and how all life shares common ancestry.
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AP Biology Phylogenetics
Learn to build and interpret phylogenetic trees of evolutionary relationships.
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AP Biology Extinction
Examine the causes of extinction and its role in life's history.
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AP Biology Origins of Life on Earth
Explore leading scientific hypotheses for how life first formed on early Earth.
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2. Common Pitfalls
Wrong move:
Claiming individuals evolve in response to environmental change.
Why:
Individuals do not evolve; natural selection acts on existing variation in populations to change allele frequencies over generations.
Correct move:
Always frame evolution as a population-level change in allele frequency over time.
Wrong move:
Assuming Hardy-Weinberg equilibrium is a model that describes most natural populations.
Why:
Hardy-Weinberg is a null model that requires 5 strict conditions that almost never hold in nature.
Correct move:
Use Hardy-Weinberg to test for evolution by comparing observed allele frequencies to expected equilibrium values.
Wrong move:
Interpreting phylogenetic trees as ranking organisms as 'more evolved' based on their position on the tree.
Why:
All living species have evolved for the same amount of time since their last common ancestor; position does not indicate progress.
Correct move:
Read trees by identifying most recent common ancestors from branching points to determine evolutionary relationships.
3. Quick Reference Cheatsheet
Concept / Formula | Description |
|---|---|
Natural Selection | Differential survival/reproduction of individuals with favorable heritable traits leads to adaptive evolution |
Sum of allele frequencies for 2 alleles at a locus equals 1 (Hardy-Weinberg) | |
Expected genotype frequencies for 2 alleles at Hardy-Weinberg equilibrium | |
5 Hardy-Weinberg Conditions | No mutation, random mating, no gene flow, large population size, no selection |
Speciation | New species form when reproductive isolation stops gene flow between diverging populations |
Most Recent Common Ancestor (MRCA) | Branching point on a phylogenetic tree where two lineages diverged |
Endosymbiosis Theory | Mitochondria and chloroplasts evolved from engulfed prokaryotes in early eukaryotes |
What's Next
Start your study of this unit with the foundational introduction to natural selection, to build a core base for all other concepts in the unit. Concepts build on each other: population genetics and Hardy-Weinberg depend on understanding how natural selection works, while phylogenetics and speciation build on evidence for evolution and common ancestry. After completing all sub-topics in this unit, you will move on to study ecology in AP Biology Unit 8.