The basics
What it covers: Natural and artificial selection; population genetics and Hardy-Weinberg equilibrium; evidence of evolution and common ancestry; continuing/observable evolution; phylogeny; speciation; population variation (drift, bottleneck, founder effect, gene flow); and origins of life.
Exam weight: 13–20% of the AP Biology exam — the single largest unit.
The big question: How do populations change genetically over time, and what evidence do we have that this has happened across the history of life on Earth?
Big Ideas covered: Evolution (BI 1), Information Storage & Transmission (BI 3), Systems Interactions (BI 4).
Key topics at a glance
Requirements for Natural Selection
Heritable variation + differential reproductive success based on that variation. Without all three (variation, heritability, fitness differences), selection can't occur.
Natural vs. Artificial Selection
Same mechanism — differential reproduction — but in artificial selection humans (not the environment) choose which traits get passed on.
Hardy-Weinberg Equilibrium
p² + 2pq + q² = 1 and p + q = 1. Five conditions for "no evolution": no mutation, random mating, no selection, infinite population, no gene flow.
Genetic Drift
Random allele frequency changes, biggest in small populations. Bottleneck: disaster shrinks population. Founder effect: small group starts a new population.
Evidence of Evolution
Fossils, biogeography, homologous structures (shared ancestry) vs. analogous structures (convergent evolution), and molecular/DNA evidence.
Common Ancestry
All life shares DNA, the genetic code, and core metabolic pathways — strong molecular evidence that all organisms descend from a common ancestor.
Continuing Evolution
Evolution is observable today: antibiotic resistance and pesticide resistance are real-time natural selection acting on existing genetic variation.
Reading Phylogenetic Trees
Relatedness is determined by the most recent shared node/common ancestor — NOT how physically close branches are drawn on the page.
Speciation
Allopatric: geographic separation. Sympatric: no geographic separation (e.g., polyploidy). Both require reproductive isolation.
Origins of Life
The RNA world hypothesis: early self-replicating life may have used RNA, which can both store information and catalyze reactions.
The key terms you must know
- Natural selection — differential survival/reproduction of individuals based on heritable variation, changing allele frequencies over generations.
- Artificial selection — humans, rather than nature, select which individuals reproduce.
- Allele frequency — the proportion of a specific allele among all alleles for a gene in a population.
- Hardy-Weinberg equilibrium — the state in which allele/genotype frequencies are NOT changing; the mathematical baseline for "no evolution."
- Genetic drift / bottleneck effect / founder effect — random, chance-driven changes in allele frequency, strongest in small populations.
- Gene flow — movement of alleles between populations via migration.
- Homologous structure — shared structure due to common ancestry (may serve different functions).
- Analogous structure — similar-function structure that evolved independently (convergent evolution), not from shared ancestry.
- Phylogenetic tree / cladogram — a branching diagram of evolutionary relationships based on shared derived characteristics.
- Speciation — formation of new, reproductively isolated species from a single ancestral population.
- Allopatric / sympatric speciation — speciation with vs. without geographic separation.
- Reproductive isolation — any mechanism preventing successful interbreeding between populations.
- RNA world hypothesis — the idea that early life used self-replicating RNA before DNA and proteins took over.
Key themes to remember
- Selection needs three ingredients. Variation, heritability, and differential reproductive success — missing any one means selection can't occur.
- Hardy-Weinberg is a null hypothesis, not a real-world expectation. Its purpose is to give you a baseline so you can recognize when and how a population IS evolving.
- Not all evolution is selection. Drift, gene flow, and mutation also change allele frequencies — selection is just the only one that's consistently directional based on fitness.
- Shared traits aren't always shared ancestry. Homologous structures indicate common ancestry; analogous structures (convergent evolution) do not.
- Tree relatedness = shared node, not visual proximity. Always trace back to the most recent common ancestor when reading a cladogram.
- Evolution is happening right now. Antibiotic and pesticide resistance are natural selection in action, not just historical/fossil evidence.
Common exam traps
- "Survival of the fittest" doesn't mean strongest — it means best reproductive success in a given environment. Fitness is about reproduction, not strength.
- p and q are allele frequencies; p² and q² are genotype frequencies. Mixing these up is the most common Hardy-Weinberg math error.
- Genetic drift is random — it is NOT natural selection. Drift has nothing to do with fitness; it's pure chance, and it matters most in SMALL populations.
- Homologous ≠ same function; analogous ≠ same ancestry. Don't assume structural similarity always means shared evolutionary history.
- A trait being common doesn't make it "more evolved." Evolution has no inherent direction toward complexity or "improvement" — only toward better fit with the current environment.
- Speciation requires reproductive isolation, not just physical distance. Geographic separation (allopatric) is one path to it, but isolation itself is the key requirement.