AP Biology Unit 7: Natural Selection — Free Review Games.
This unit covers evolution, natural selection, speciation and Hardy-Weinberg — essential concepts for AP Biology. Use our interactive study games to test your understanding, or review questions in traditional format below.
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This unit covers evolution, natural selection, speciation and Hardy-Weinberg — essential concepts for AP Biology. Use our interactive study games to test your understanding, or review questions in traditional format below.
Key Concepts Breakdown
1 Evolution
Evolution is a change in allele frequencies in a population over time, not in individuals. Students must understand that evolution is driven by heritable variation and differential reproductive success. The AP exam tests whether students can distinguish between evolution acting on populations versus individuals, and connect evidence types (fossil record, molecular homology, comparative anatomy) to evolutionary conclusions.
Key Points
- Evolution acts on populations, not individuals — individuals do not evolve during their lifetime
- Heritable variation is required; non-heritable traits cannot be selected
- Homologous structures indicate common ancestry; analogous structures indicate convergent evolution
- Molecular evidence (DNA/protein sequence similarity) is the most direct evidence of common ancestry
Two species of fish have similar streamlined body shapes but are not closely related. Their DNA sequences are very different. Explain what type of evolution this represents.
The similar body shapes despite distant ancestry indicate convergent evolution, producing analogous structures — similar function, different origin. Because their DNA sequences are very different, common ancestry is ruled out. This pattern arises when unrelated species face similar selective pressures (e.g., aquatic locomotion) and independently evolve similar solutions.
2 Natural Selection
Natural selection is the mechanism of evolution: individuals with heritable traits better suited to their environment survive and reproduce at higher rates, increasing those allele frequencies over generations. The AP exam requires students to identify the four conditions for natural selection (variation, heritability, differential survival/reproduction, time) and predict directional, stabilizing, or disruptive outcomes from given scenarios.
Key Points
- Four conditions: variation exists, variation is heritable, variation affects fitness, sufficient time passes
- Directional selection shifts the mean phenotype toward one extreme (e.g., antibiotic resistance)
- Stabilizing selection reduces variation by favoring the mean (e.g., human birth weight)
- Disruptive selection favors both extremes over the mean, can lead to speciation
In a population of beetles, green beetles are eaten by birds more often than brown beetles. Over 20 generations, the proportion of brown beetles increases from 20% to 85%. Identify the type of selection and the condition that made this possible.
This is directional selection — the population mean shifts toward brown coloration over time. The condition that made evolution possible is heritable variation: color differences must be genetically encoded so offspring inherit the trait. If color were purely environmental (non-heritable), selection could not change allele frequencies across generations.
3 Speciation
Speciation is the formation of new species, defined as the evolution of reproductive isolation between populations. Students must distinguish allopatric speciation (geographic isolation precedes reproductive isolation) from sympatric speciation (reproductive isolation without geographic separation). The AP exam frequently asks students to construct or interpret scenarios involving isolation mechanisms and predict whether speciation will occur.
Key Points
- Biological species concept: a species is a group of organisms that can interbreed and produce fertile offspring
- Allopatric speciation: physical barrier → genetic divergence → reproductive isolation; most common mechanism
- Sympatric speciation: occurs via polyploidy (especially in plants), habitat differentiation, or sexual selection without geographic separation
- Prezygotic barriers prevent mating or fertilization; postzygotic barriers reduce hybrid fitness after fertilization
A river changes course and splits a squirrel population into two isolated groups. After 10,000 years, the two groups are brought back together. They attempt to mate but produce infertile offspring. Has speciation occurred? Which type of barrier is this?
Yes, speciation has occurred — the two groups can no longer produce fertile offspring, satisfying the biological species concept for reproductive isolation. The barrier is postzygotic because mating and fertilization do occur, but the resulting hybrids are sterile (reduced hybrid fitness). This scenario exemplifies allopatric speciation: geographic isolation drove genetic divergence until reproductive isolation was complete.
4 Hardy-Weinberg Equilibrium
Hardy-Weinberg equilibrium (HWE) describes a non-evolving population where allele and genotype frequencies remain constant across generations. Students must know the five conditions required for HWE, use the equations p + q = 1 and p² + 2pq + q² = 1 to calculate allele/genotype frequencies, and interpret deviations from HWE as evidence of evolution. The AP exam commonly provides a phenotype frequency and requires students to work backward to allele frequencies.
Key Points
- Five conditions: large population, random mating, no mutation, no gene flow, no natural selection
- p = frequency of dominant allele, q = frequency of recessive allele; p + q = 1
- p² = homozygous dominant frequency, 2pq = heterozygous frequency, q² = homozygous recessive frequency
- If a population deviates from HWE predictions, at least one evolutionary force is acting on it
In a population of 1,000 flowers, 160 are white (homozygous recessive, aa). Assuming HWE, calculate the frequency of heterozygous carriers (Aa).
First, find q²: 160/1000 = 0.16, so q = √0.16 = 0.4. Then p = 1 − q = 1 − 0.4 = 0.6. The heterozygous frequency is 2pq = 2(0.6)(0.4) = 0.48. Therefore 48% of the population (480 individuals) are carriers. This is a standard AP problem type: always start from the homozygous recessive phenotype because it is the only genotype directly readable from phenotype data.
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What is Natural Selection?
Natural Selection is Unit 7 of AP Biology, covering evolution, natural selection, speciation and Hardy-Weinberg.
How to study for AP Biology Unit 7?
Start with the Quick Summary above, review the Key Concepts, then test yourself with our interactive study games. Aim for 80%+ accuracy before moving on.
How many questions are in this unit?
This unit has 30+ review questions across 5 different game modes.