Natural Selection
Unit 7 of AP Biology, worth 13–20% of the exam. 14 questions below, each with the working. Every answer was checked by a second pass before it was published.
Natural and artificial selection, population genetics and Hardy-Weinberg, evidence for evolution, phylogeny, speciation, extinction, origin of life.
How this unit is tested
Most Unit 7 questions fall into five recognizable buckets: identify a mechanism of evolutionary change (selection, drift, gene flow, mutation), calculate allele or genotype frequencies with Hardy-Weinberg, read or build a phylogenetic tree, classify a piece of evidence for evolution, or explain a stage in speciation, extinction, or life's origin. Spend your first few seconds sorting the question into one of these buckets before you try to answer it, because each has its own toolkit.
For Hardy-Weinberg problems, always start from the number you can actually count directly — this is almost always the homozygous recessive phenotype frequency, q². Take its square root to get q, subtract from 1 to get p, then plug back into p² + 2pq + q² = 1 to get whatever genotype frequency the question actually asked for. Convert frequencies to counts (multiply by population size) only at the very last step.
For phylogeny questions, remember that a tree encodes nested sets of shared derived characters (synapomorphies), not a left-to-right ranking of 'primitiveness.' Two taxa are more closely related if they share a more recent common ancestor (a more recent node), regardless of how the tips are drawn on the page or how different the organisms look today.
For evidence-of-evolution and homology/analogy questions, ask whether the similarity comes from shared ancestry (homology, e.g., forelimb bones) or from independent adaptation to a similar environment (analogy/convergence, e.g., wings in insects vs. birds). For speciation, extinction, and origin-of-life items, focus on causal mechanism — what specifically stopped gene flow, what specifically drove the extinction, what specifically had to happen chemically before life could self-replicate — rather than just naming a vocabulary term.
What you have to know
Hardy-Weinberg equations
For a non-evolving population with two alleles at a locus: p + q = 1 (allele frequencies) and p² + 2pq + q² = 1 (genotype frequencies), where p is the frequency of the dominant allele, q of the recessive allele, p² homozygous dominant, 2pq heterozygous, and q² homozygous recessive.
Conditions for Hardy-Weinberg equilibrium
Allele frequencies stay constant across generations only if there is no mutation, no gene flow, random mating, no natural selection, an infinitely large population (no genetic drift), and non-overlapping generations. Real populations rarely meet all conditions, which is why allele frequencies actually change.
Natural selection
Differential survival and reproduction of individuals based on heritable phenotypic variation, resulting in a change in allele frequencies in a population over generations; selection acts on existing variation, it does not create new traits on demand.
Biological species concept
A species is a group of populations whose members can interbreed and produce viable, fertile offspring in nature, and are reproductively isolated from other such groups.
Principle of homology
Structures that are similar because they are inherited from a common ancestor (homologous) provide evidence of shared ancestry, whereas structures that are similar because of independent adaptation to similar environments (analogous) reflect convergent evolution, not common ancestry.
14 practice questions
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In a population of 500 wildflowers, 45 plants show the recessive white-flower phenotype (aa). Assuming Hardy-Weinberg equilibrium, what is the frequency of the dominant allele A?
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Answer. p = 0.7
q² = 45/500 = 0.09, so q = √0.09 = 0.3. Then p = 1 − q = 1 − 0.3 = 0.7. Only the homozygous recessive frequency can be square-rooted directly to find q. -
Human birth weight data show that mortality is lowest for babies at intermediate weights and higher for both very low and very high birth weights. What type of natural selection does this illustrate?
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Answer. Stabilizing selection
Stabilizing selection favors intermediate phenotypes and reduces variation at both extremes, unlike directional selection (favors one extreme) or disruptive selection (favors both extremes over the middle). -
A population of insects has two color morphs, dark and light, each camouflaged against a different substrate; intermediate-colored individuals are more visible to predators and rarely survive. What type of selection is acting, and what happens to the population's phenotypic distribution?
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Answer. Disruptive selection; the distribution becomes bimodal, with both extremes favored over the intermediate phenotype.
Disruptive (diversifying) selection favors both extreme phenotypes at the expense of intermediates, which can eventually contribute to speciation if the morphs stop interbreeding. -
A small group of birds is blown by a storm to a remote island and founds a new population. The allele frequencies in this new population differ substantially from the mainland population, even though no selective pressure differs between the two locations. What evolutionary mechanism best explains this?
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Answer. The founder effect (a form of genetic drift)
Because the founding group is small and represents only a random sample of the mainland gene pool, chance alone can produce large differences in allele frequencies — this is the founder effect, not selection, since there's no fitness advantage described. -
Explain the key difference between genetic drift and natural selection as mechanisms of allele frequency change.
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Answer. Genetic drift is random and unrelated to fitness, with the largest effect in small populations; natural selection is a non-random process driven by consistent differences in survival and reproduction tied to a heritable trait.
Drift can increase or decrease any allele's frequency by chance, even a harmful one, while selection systematically increases the frequency of alleles that improve fitness in a given environment. -
The forelimb bones of a human, a bat, and a whale have the same basic skeletal arrangement despite very different functions (grasping, flying, swimming). What is this an example of, and what does it suggest about these species?
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Answer. Homologous structures, indicating that humans, bats, and whales share a common ancestor.
Homology reflects shared ancestry with subsequent modification for different functions, distinguishing it from analogy, which reflects independent adaptation without common descent. -
Insect wings and bird wings both allow flight but have completely different underlying structures and developmental origins. What term describes this kind of similarity, and what process produces it?
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Answer. Analogous structures, produced by convergent evolution.
Because insects and birds do not share a winged common ancestor, their wings evolved independently in response to similar selective pressures (the need to fly) — this is convergence, not homology. -
On a phylogenetic tree, species X and species Y branch off from the same node, and that node is more recent than the node connecting them to species Z. What can you conclude about their relatedness?
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Answer. Species X and Y are more closely related to each other than either is to species Z.
Relatedness on a tree is determined by recency of common ancestry (how close the shared node is), not by how the tips are visually arranged or by overall similarity in appearance. -
Two species of fruit fly are morphologically almost identical, but molecular sequencing reveals many differences in their mitochondrial DNA. Why is DNA sequence comparison considered strong evidence for evolutionary relationships even when morphology is misleading?
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Answer. DNA sequences accumulate mutations over time at a roughly predictable rate, so the number of differences between two species' sequences reflects how long ago they diverged from a common ancestor, independent of how their outward appearance has changed.
Molecular data can reveal evolutionary divergence that morphology alone might mask, especially between morphologically conserved or convergent species. -
A river changes course and splits a population of rodents into two groups that can no longer interbreed. Over thousands of years, the two groups accumulate enough genetic differences that they can no longer produce viable offspring even if reunited. What type of speciation is this, and what initiated it?
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Answer. Allopatric speciation, initiated by a physical geographic barrier (the river) that stopped gene flow.
Allopatric speciation requires a geographic barrier separating populations; genetic divergence and reproductive isolation then accumulate independently in each isolated group. -
Two plant species living in the same field cannot successfully hybridize because their flowers bloom at different times of year, so pollinators never transfer pollen between them. Is this a prezygotic or postzygotic reproductive isolating mechanism, and what specific type is it?
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Answer. Prezygotic isolation; specifically, temporal (timing) isolation.
Prezygotic mechanisms prevent fertilization from occurring at all (e.g., temporal, behavioral, mechanical, or gametic isolation); postzygotic mechanisms act after fertilization, such as hybrid sterility or reduced hybrid viability. -
After a mass extinction event eliminates most large predators and competitors, a surviving lineage rapidly diversifies to fill many newly available ecological niches. What is this pattern called, and what classic example illustrates it?
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Answer. Adaptive radiation; Darwin's finches on the Galápagos Islands are a classic example.
Adaptive radiation occurs when a single ancestral lineage diversifies into many species with different adaptations, often after a mass extinction or colonization of a new habitat removes competition. -
The Miller-Urey experiment simulated early Earth's atmosphere (with gases such as methane, ammonia, hydrogen, and water vapor) and applied electrical sparks to mimic lightning. What did this experiment demonstrate about the origin of life?
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Answer. That simple organic molecules, including amino acids, can form spontaneously from inorganic precursors under conditions thought to resemble early Earth, supporting abiotic synthesis as a step toward life's origin.
The experiment did not create life itself, but it showed that the building blocks of life (monomers such as amino acids) could plausibly arise through purely chemical processes before life existed. -
Why do many scientists favor the RNA world hypothesis, proposing that RNA rather than DNA or protein was the first self-replicating genetic material?
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Answer. RNA can both store genetic information and catalyze chemical reactions (as a ribozyme), so a single RNA molecule could potentially replicate itself and carry information, unlike DNA (which needs proteins to catalyze replication) or proteins alone (which cannot store heritable information).
This dual capability makes RNA a plausible candidate for bridging the gap between simple chemistry and self-replicating, information-carrying life, before the more specialized division of labor between DNA and proteins evolved.
What people get wrong
- Taking the square root of an observed phenotype frequency that is NOT homozygous recessive — only q² (the fully recessive phenotype) can be square-rooted directly; the dominant phenotype frequency is p² + 2pq and cannot be square-rooted to get p.
- Assuming a population is automatically in Hardy-Weinberg equilibrium without checking whether the listed conditions (random mating, no selection, no drift, no gene flow, no mutation, large population) are actually met in the scenario.
- Treating genetic drift and natural selection as the same process — drift is random and has the largest effect in small populations (founder effect, bottleneck), while selection is non-random and driven by fitness differences; a question describing a small isolated population after a storm or colonization event is testing drift, not selection.
- Reading a phylogenetic tree left-to-right as if the order of the tip labels indicates evolutionary relatedness — relatedness is determined only by the position of branch points (nodes), so two tips can look far apart on the page yet share a very recent common ancestor.
- Calling similar structures homologous just because they look alike or serve the same function — check whether the underlying anatomy suggests common descent (homology, e.g., pentadactyl limb) versus convergent adaptation to a similar niche (analogy, e.g., wings of bats and insects).
- Describing evolution as goal-directed ('the giraffe stretched its neck to reach food, so its offspring got long necks') — natural selection acts on variation that already exists in the population; it does not cause individuals to develop new traits in response to need.
Drill this unit until it sticks
These questions come back on a schedule built from what you get wrong, alongside the rest of AP Biology. Free, and no account needed to start.