Natural Selection and Evolution — AP Biology Bio Study Guide

For: AP Biology candidates sitting AP Biology.

Covers: Evidence for evolution, Hardy-Weinberg equilibrium, directional/stabilising/disruptive selection, speciation, and phylogenetic trees as required for AP Biology Unit 7.

You should already know: High-school biology, basic chemistry.

A note on the practice questions: All worked questions in the "Practice Questions" section below are original problems written by us in the AP Biology style for educational use. They are not reproductions of past College Board papers and may differ in wording, numerical values, or context. Use them to practise the technique; cross-check with official College Board mark schemes for grading conventions.

1. What Is Natural Selection and Evolution?

Evolution is defined as the change in heritable characteristics of biological populations over successive generations, while natural selection is the primary mechanism driving adaptive evolutionary change: individuals with traits better suited to their local environment survive and reproduce at higher rates, passing their advantageous alleles to offspring. Also referred to as "descent with modification" (Darwin’s original term), this topic makes up 13–20% of your AP Biology exam score, and forms the unifying framework for all biological concepts from cell biology to ecology.

2. Evidence for evolution

Four core lines of evidence for evolution are tested consistently on the AP exam, each directly supporting the theory of common ancestry for all living organisms:

1. Fossil records: Preserved remains of extinct organisms show transitional species that bridge gaps between modern taxonomic groups. For example, Archaeopteryx fossils have both dinosaurian skeletal features and bird-like feathers, confirming birds evolved from theropod dinosaurs. Radiometric dating of fossil layers allows you to sequence evolutionary changes chronologically.
2. Homologous structures: Anatomical traits that share a common evolutionary origin but serve different functions across species. For example, the forelimbs of humans (grasping), bats (flying), and whales (swimming) have identical bone arrangements, confirming they descended from a common tetrapod ancestor. Note that analogous structures (same function, different origin, e.g. bat wings and insect wings) are evidence of convergent evolution, not shared ancestry.
3. Molecular homology: Shared DNA or amino acid sequences between species indicate recent common ancestry. For example, humans and chimpanzees share 98.8% of their coding DNA sequences, while humans and gorillas share 98.4%, confirming humans are more closely related to chimps than gorillas.
4. Biogeography: The geographic distribution of species matches evolutionary history. For example, Galapagos finch species are closely related to finch species from the nearest South American mainland, not African finch species, confirming they colonized the Galapagos from South America and adapted to local island conditions.

Exam tip: Examiners frequently ask you to distinguish between homologous and analogous traits; only homologous traits count as evidence for shared ancestry.

3. Hardy-Weinberg equilibrium

The Hardy-Weinberg (H-W) equilibrium is a null model of evolution that describes allele and genotype frequencies in a non-evolving population. If any of the 5 required H-W conditions are violated, the population is undergoing evolutionary change.

Required conditions for H-W equilibrium

1. No new mutations introduce new alleles
2. Random mating (no sexual selection for specific traits)
3. No natural selection acting on any traits
4. Extremely large population size (eliminates random genetic drift)
5. No gene flow (no immigration or emigration that adds/removes alleles)

Core H-W formulas

First, for allele frequencies: $$p+q=1$$ Where $p$ = frequency of the dominant allele, $q$ = frequency of the recessive allele in the population. For genotype frequencies: $$p^2+2pq+q^2=1$$ Where $p^2$ = frequency of homozygous dominant individuals, $2pq$ = frequency of heterozygous individuals, $q^2$ = frequency of homozygous recessive individuals.

Worked example

In a population of 2000 sunflowers, 9% of plants have dwarf stems, a homozygous recessive trait. Calculate the number of heterozygous tall sunflower plants in the population, assuming H-W equilibrium.

1. Step 1: Calculate $q$ from the recessive phenotype frequency: $q^2=0.09$, so $q=\sqrt{0.09}=0.3$
2. Step 2: Calculate $p$: $p=1-q=1-0.3=0.7$
3. Step 3: Calculate heterozygote frequency: $2pq=2\ast 0.7\ast 0.3=0.42$
4. Step 4: Number of heterozygous plants = $2000\ast 0.42=840$

4. Selection types — directional, stabilising, disruptive

Natural selection acts on phenotypic variation in populations, and can be categorized into three types based on which segment of the phenotypic distribution is favored:

1. Directional selection: Favors one extreme of the phenotypic range, shifting the entire distribution of the trait toward that extreme over generations. Example: During the 19th century Industrial Revolution in England, dark-colored peppered moths were favored over light-colored moths because soot-covered tree bark camouflaged dark moths from predators. The frequency of the dark pigment allele increased from <10% to 90% in 50 years.
2. Stabilizing selection: Favors the intermediate phenotype, and selects against both extreme phenotypes, reducing overall phenotypic variation in the population. Example: Human birth weight is under stabilizing selection: infants with very low or very high birth weight have significantly higher mortality rates, so average birth weight stays consistent at ~3–3.5 kg across most human populations.
3. Disruptive (diversifying) selection: Favors both extreme phenotypes, and selects against the intermediate phenotype, increasing phenotypic variation and often leading to speciation over time. Example: African seedcracker finches have either very small or very large beaks: small beaks are specialized for eating soft small seeds, large beaks for eating hard large seeds, while intermediate-sized beaks cannot efficiently eat either seed type, leading to lower survival rates for medium-beaked finches.

Exam tip: You will often be given a graph of phenotypic frequency before and after selection; directional selection shifts the peak left/right, stabilizing selection narrows the peak, disruptive selection creates two distinct peaks.

5. Speciation

Speciation is the process by which new, distinct species arise from a single ancestral population, occurring when populations are reproductively isolated long enough to accumulate genetic differences that prevent interbreeding even if they are reunited. The AP exam uses the biological species concept: a species is defined as a group of organisms that can interbreed to produce viable, fertile offspring.

Types of speciation

1. Allopatric speciation: Occurs when a geographic barrier (e.g. a river, mountain range, or ocean) separates a population into two isolated groups, preventing gene flow. Over time, the two groups accumulate different mutations and adapt to their separate environments, eventually becoming reproductively isolated. Example: The Grand Canyon split a population of antelope squirrels ~3 million years ago; today, squirrels on the north rim and south rim are separate species that cannot interbreed.
2. Sympatric speciation: Occurs without geographic isolation, most commonly in plants via polyploidy (an error during cell division creates individuals with extra sets of chromosomes). Polyploid individuals cannot interbreed with the parent diploid population, so they are immediately a new species. Example: Modern bread wheat is a hexaploid (6 sets of chromosomes) formed from the hybridization of three different diploid grass species, and cannot interbreed with any of its parent species.

Reproductive barriers

• Prezygotic barriers: Prevent fertilization from occurring: temporal isolation (different mating seasons), behavioral isolation (different mating rituals), mechanical isolation (anatomical incompatibility), gametic isolation (sperm cannot fertilize eggs of the other species)
• Postzygotic barriers: Act after fertilization: hybrid inviability (hybrid embryo dies before maturity), hybrid sterility (hybrid is alive but cannot reproduce, e.g. mules, the hybrid of a horse and donkey, are sterile)

6. Phylogenetic trees

Phylogenetic trees are branching diagrams that show evolutionary relationships between groups of organisms, constructed using shared homologous traits or molecular sequence data. Key terms you need to know:

• Nodes: Represent the most recent common ancestor of all groups branching from that point
• Sister taxa: Two groups that share an immediate common ancestor, so they are each other’s closest relatives
• Root: The common ancestor of all taxa included on the tree
• Branch length: In scaled phylogenetic trees, branch length indicates the amount of evolutionary change or time since divergence; in cladograms (unscaled trees), branch length has no meaning.

Critical reading rule

The left-to-right order of terminal taxa on a phylogenetic tree is arbitrary. Relatedness is determined only by the branching pattern: the more recent the common ancestor two groups share, the more closely related they are. For example, if a tree lists (left to right) gorilla, human, chimpanzee, humans and chimpanzees are still sister taxa, as they share a more recent common ancestor with each other than either does with gorillas, even though gorilla is listed next to human.

Exam tip: Examiners often test this rule by rearranging terminal nodes to trick you into thinking unrelated groups are close; always trace back to common nodes to determine relatedness.

7. Common Pitfalls (and how to avoid them)

• Wrong move: Confusing analogous and homologous structures when answering evidence for evolution questions. Why: Students mix up "same function" and "same origin". Correct move: First ask if the trait comes from a shared common ancestor; if yes it is homologous (evidence for shared ancestry), if it evolved independently it is analogous (evidence for convergent evolution).
• Wrong move: Calculating $p$ directly from dominant phenotype frequency in Hardy-Weinberg problems. Why: Students forget that dominant phenotypes include both homozygous dominant and heterozygous individuals. Correct move: Always start calculations with the recessive phenotype frequency to find $q$ first, then calculate $p=1-q$.
• Wrong move: Stating that natural selection acts on genotypes, not phenotypes. Why: Students confuse selection on traits with selection on underlying alleles. Correct move: Natural selection acts on expressed phenotypes; a recessive harmful allele will not be selected against if it is masked in a heterozygous individual.
• Wrong move: Assuming all speciation requires geographic separation. Why: Students learn allopatric speciation first and overlook sympatric cases. Correct move: Remember sympatric speciation occurs frequently in plants via polyploidy, and in animals via resource partitioning, with no geographic barrier required.
• Wrong move: Reading phylogenetic trees by terminal node order instead of branching points. Why: Students assume leftmost taxa are more primitive, or adjacent terminal taxa are closely related. Correct move: Trace branches back to the most recent common ancestor to determine relatedness, ignore the order of labels at the ends of branches.

8. Practice Questions (AP Biology Style)

Question 1

A population of 1500 mice has a recessive allele for black fur that occurs at a frequency of 0.2. Assume the population is in Hardy-Weinberg equilibrium. a) Calculate the number of heterozygous brown-furred mice in the population. b) If a wildfire kills all black-furred mice, what is the new frequency of the black fur allele in the surviving population?

Solution

a) Given $q=0.2$, so $p=1-0.2=0.8$. Heterozygote frequency = $2pq=2\ast 0.8\ast 0.2=0.32$. Number of heterozygous mice = $1500\ast 0.32=480$. b) All surviving mice are either homozygous dominant or heterozygous. The total number of surviving alleles = $(1500-(1500\ast 0.2^2))\ast 2=(1500-60)\ast 2=2880$. Number of remaining black fur alleles = number of heterozygotes = 480. New $q=480/2880\approx 0.17$.

Question 2

A researcher observes that in a population of songbirds, individuals with very bright plumage are more likely to be eaten by predators, while individuals with very dull plumage are less likely to find a mate. a) Identify the type of selection acting on plumage brightness. b) Explain how this selection will affect phenotypic variation in the population over time.

Solution

a) Stabilizing selection, as it selects against both the very bright and very dull plumage extremes, favoring intermediate plumage brightness. b) Stabilizing selection reduces phenotypic variation over time: the frequency of intermediate plumage will increase, while the frequency of both bright and dull plumage will decrease, leading to a narrower distribution of plumage brightness in the population.

Question 3

The table below shows the number of amino acid differences in the hemoglobin protein between four species:

Solution

Fewer amino acid differences indicate a more recent common ancestor. The smallest difference is between W and X (3 differences), so W and X are sister taxa. Next, Z is the closest relative of the W-X clade (6 differences from X, 7 from W), and Y is the outgroup (10+ differences from all other species). The cladogram branching order is: root splits first into Y, then the remaining branch splits into Z, then the final branch splits into W and X.

9. Quick Reference Cheatsheet

10. What's Next

This topic forms the unifying framework for all remaining AP Biology content, particularly Unit 8 (Ecology), where you will apply evolutionary principles to explain species interactions, community dynamics, and ecosystem responses to environmental change. It also connects back to earlier units: mutations and gene expression changes (Unit 6) are the raw material for natural selection, while Mendelian inheritance (Unit 5) explains how advantageous traits are passed between generations to drive evolutionary change.

If you struggle with any of the concepts covered in this guide, from Hardy-Weinberg calculations to interpreting phylogenetic trees, you can ask Ollie, our AI tutor, for personalized explanations, additional practice problems, or step-by-step walkthroughs of exam questions anytime on the homepage. Make sure to test your understanding with official College Board AP Biology past papers to get comfortable with the exam question format and grading rubrics before your test date.