# Natural Selection Overview

> AP Biology · Unit 7: Natural Selection
> Source: https://www.owlsprep.com/study/ap-biology-u7-overview/
> Weight: 13-20% of total AP Biology exam score

This unit explores the core mechanisms of evolution, how variation arises in populations, and how natural selection shapes the diversity of life on Earth. It is one of the most heavily tested concepts on the AP Biology exam.

**Prerequisites:** [Foundational knowledge of genetics and inheritance from AP Biology Unit 6](https://www.owlsprep.com/study/ap-biology-u6-overview/)

## Learning objectives

- Explain how natural selection acts on genetic variation to drive adaptive evolution
- Calculate allele and genotype frequencies using the Hardy-Weinberg equilibrium model
- Evaluate multiple lines of scientific evidence that support evolution and common ancestry
- Construct and interpret phylogenetic trees to map evolutionary relationships between organisms
- Describe the processes of speciation, extinction, and leading scientific hypotheses for the origin of life on Earth

## 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](https://www.owlsprep.com/study/ap-biology-u7-introduction-to-natural-selection/) — Get a foundational overview of the core principles of natural selection.
- [AP Biology Natural Selection](https://www.owlsprep.com/study/ap-biology-u7-natural-selection/) — Dive deeper into how natural selection drives adaptive evolution.
- [AP Biology Artificial Selection](https://www.owlsprep.com/study/ap-biology-u7-artificial-selection/) — Learn how human selection drives evolution of domesticated organisms.
- [AP Biology Variations in Populations](https://www.owlsprep.com/study/ap-biology-u7-variations-in-populations/) — Explore how genetic variation arises and is maintained in populations.
- [AP Biology Population Genetics](https://www.owlsprep.com/study/ap-biology-u7-population-genetics/) — Study how evolutionary forces change genetic variation at the population level.
- [AP Biology Hardy-Weinberg Equilibrium](https://www.owlsprep.com/study/ap-biology-u7-hardy-weinberg-equilibrium/) — Master the Hardy-Weinberg model to calculate and predict allele frequencies.
- [AP Biology Evidence of Evolution](https://www.owlsprep.com/study/ap-biology-u7-evidence-of-evolution/) — Review the multiple lines of scientific evidence supporting evolution.
- [AP Biology Common Ancestry and Speciation](https://www.owlsprep.com/study/ap-biology-u7-common-ancestry-and-speciation/) — Explore how new species form and how all life shares common ancestry.
- [AP Biology Phylogenetics](https://www.owlsprep.com/study/ap-biology-u7-phylogenetics/) — Learn to build and interpret phylogenetic trees of evolutionary relationships.
- [AP Biology Extinction](https://www.owlsprep.com/study/ap-biology-u7-extinction/) — Examine the causes of extinction and its role in life's history.
- [AP Biology Origins of Life on Earth](https://www.owlsprep.com/study/ap-biology-u7-origins-of-life-on-earth/) — Explore leading scientific hypotheses for how life first formed on early Earth.

## Common pitfalls

- **Wrong:** Claiming individuals evolve in response to environmental change.
  - Why it fails: Individuals do not evolve; natural selection acts on existing variation in populations to change allele frequencies over generations.
  - Correct: Always frame evolution as a population-level change in allele frequency over time.
- **Wrong:** Assuming Hardy-Weinberg equilibrium is a model that describes most natural populations.
  - Why it fails: Hardy-Weinberg is a null model that requires 5 strict conditions that almost never hold in nature.
  - Correct: Use Hardy-Weinberg to test for evolution by comparing observed allele frequencies to expected equilibrium values.
- **Wrong:** Interpreting phylogenetic trees as ranking organisms as 'more evolved' based on their position on the tree.
  - Why it fails: All living species have evolved for the same amount of time since their last common ancestor; position does not indicate progress.
  - Correct: Read trees by identifying most recent common ancestors from branching points to determine evolutionary relationships.

## Cheatsheet

| Concept / Formula | Description |
| --- | --- |
| Natural Selection | Differential survival/reproduction of individuals with favorable heritable traits leads to adaptive evolution |
| $p + q = 1$ | Sum of allele frequencies for 2 alleles at a locus equals 1 (Hardy-Weinberg) |
| $p^2 + 2pq + q^2 = 1$ | 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.

- [AP Biology Introduction to Natural Selection](https://www.owlsprep.com/study/ap-biology-u7-introduction-to-natural-selection/)
- [Natural Selection](https://www.owlsprep.com/study/ap-biology-u7-natural-selection/)
- [Artificial Selection](https://www.owlsprep.com/study/ap-biology-u7-artificial-selection/)

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From [OwlsPrep](https://www.owlsprep.com) — free study guides for A-Level, IB, AP and IGCSE, written against the official syllabus. Canonical page: https://www.owlsprep.com/study/ap-biology-u7-overview/