Evolution is central to the population-biology domain, roughly one-third of the CLEP exam. Questions here reward reasoning: you will classify examples, interpret evidence, and distinguish look-alike terms, not just recite Darwin.
Natural selection acts on populations, not individuals, and needs three things: 1. Heritable variation among individuals. 2. Differential reproductive success — some variants leave more offspring than others. 3. Time across generations.
The result: traits that improve reproduction become more common. Selection works on existing variation; organisms do not acquire useful traits during life and pass them on (that older idea is incorrect).
In evolutionary biology, fitness means reproductive success — the number of fertile offspring an individual leaves relative to others — not physical strength or size. An adaptation is an inherited trait that raises fitness in a given environment.
| Line of evidence | What it shows |
|---|---|
| Fossil record | Change in life forms over geological time; transitional forms |
| Comparative anatomy | Homologous structures (same origin, different function) point to common ancestry |
| Embryology | Similar early embryonic stages across related groups |
| Molecular/biochemical | Shared DNA and protein sequences; more similarity = closer relationship |
Contrast two anatomy terms: - Homologous structures — same underlying structure, different function (e.g., whale flipper, bat wing, human arm) → common ancestry (divergent evolution). - Analogous structures — same function, different origin (e.g., insect wing vs. bird wing) → convergent evolution.
[GRAPH: Three trait-distribution curves. Directional — the whole curve shifts toward one extreme. Stabilizing — the curve narrows around the mean, favoring intermediates. Disruptive — the middle is cut down, leaving two peaks at the extremes.]
A species (biological species concept) is a group that can interbreed and produce fertile offspring. Speciation requires reproductive isolation so gene pools stop mixing.
1. E — Heritable variation affects differences in reproductive success. Fix: Selection requires inherited differences that make some individuals reproduce more than others. - A: Identical individuals offer nothing to select among. - B: A never-changing environment is not a requirement; selection can act in stable or shifting conditions. - C: Acquiring and passing on traits gained during life is the discarded Lamarckian idea. - D: Sexual reproduction is common; asexuality is not required. - Fix rule: Selection needs heritable variation + differences in reproduction.
2. A — The number of fertile offspring it leaves relative to others. Fix: Fitness is reproductive success measured against the rest of the population. - B: Strength matters only if it raises reproduction; it is not fitness itself. - C: Body size is a trait, not a measure of fitness. - D: Long life without reproduction contributes nothing to fitness. - E: Carrying many mutations does not by itself mean more offspring. - Fix rule: Fitness = fertile offspring left, not brawn or size.
3. E — Homologous structures inherited from a common ancestor. Fix: Same bone plan with different jobs signals descent from a shared ancestor (divergent evolution). - A: Analogous structures share function but differ in origin — the reverse case. - B: These limbs are fully functional, not vestigial. - C: Coevolution involves two species shaping each other, not shared limb anatomy. - D: Drift is random change in allele frequency, unrelated to this anatomical pattern. - Fix rule: Same structure, different function = homologous = common ancestry.
4. B — Analogous structures. Fix: Same function but different origin marks analogous structures produced by convergent evolution. - A: Homologous structures share origin, not just function. - C: Vestigial structures are reduced remnants, not functional wings. - D: These are adult wings, not embryonic similarities. - E: Molecular homologies concern shared sequences, not wing anatomy. - Fix rule: Same job, different build = analogous = convergence.
5. C — Share a relatively recent common ancestor. Fix: The more alike two species' DNA, the more recently they diverged from a shared ancestor. - A: Sharing a habitat does not require shared sequences. - B: Body size is unrelated to DNA-sequence similarity. - D: Convergence produces similar function from different genes, not near-identical sequences. - E: Sequence similarity says nothing directly about interbreeding. - Fix rule: More shared DNA = closer (more recent) common ancestry.
6. C — Directional selection. Fix: A steady shift of the mean toward one extreme is directional selection. - A: Stabilizing selection favors the middle, not a shifting extreme. - B: Disruptive selection favors both extremes at once. - D: Sexual selection concerns mate choice, not feeding-driven size shifts. - E: Drift is random, not a consistent directional trend. - Fix rule: Mean marches toward one extreme = directional.
7. D — Stabilizing selection. Fix: Favoring intermediate values and removing both extremes is stabilizing selection. - A: Directional selection would shift the mean, not tighten it around the middle. - B: Disruptive selection favors the extremes, the opposite here. - C: Convergent evolution is about unrelated lineages, not a within-population pattern. - E: The founder effect is random change from a small starting group, not selection for intermediates. - Fix rule: Middle favored, extremes trimmed = stabilizing.
8. A — Disruptive selection. Fix: Favoring both extremes while selecting against intermediates is disruptive selection. - B: Stabilizing selection favors the middle, the reverse of this. - C: Directional selection favors only one extreme. - D: Gene flow is movement of alleles between populations, not this selection pattern. - E: Coevolution involves two interacting species. - Fix rule: Both extremes win, middle loses = disruptive.
9. B — Two species breed during different seasons of the year. Fix: Different breeding times prevent mating before any zygote forms, a prezygotic barrier. - A: Hybrid sterility acts after fertilization — postzygotic. - C: Hybrid death in development is postzygotic. - D: Weak, non-reproducing hybrids are a postzygotic outcome. - E: Chromosome pairing failure after fertilization is postzygotic. - Fix rule: Prezygotic = blocks mating/fertilization; postzygotic = problems with the hybrid.
10. A — Interbreed and produce fertile offspring. Fix: The biological species concept defines a species by successful, fertile interbreeding. - B: Sharing a range does not make organisms one species. - C: Similar appearance can be misleading; look-alikes may not interbreed. - D: Diet does not define a species. - E: Members of a species vary genetically; identical DNA is not required. - Fix rule: Biological species = interbreed to produce fertile young.
11. E — Convergent evolution under similar environmental pressures. Fix: Distant relatives evolving similar streamlined forms independently is convergence. - A: Homologous descent would require a recent common ancestor, which sharks and dolphins lack. - B: Coevolution is reciprocal shaping between two species, not this case. - C: Drift is random and would not reliably produce matching adaptations. - D: These are functional adaptations, not shared vestigial remnants. - Fix rule: Unrelated groups, similar solution, similar environment = convergence.
12. D — Coevolution. Fix: Two species that reciprocally drive each other's evolution over time are coevolving. - A: Convergent evolution involves unrelated species independently resembling each other, not mutual shaping. - B: Directional selection describes a trait shift within one population. - C: Adaptive radiation is one lineage diversifying into many niches. - E: Genetic drift is random and not a reciprocal partnership. - Fix rule: Two species evolving in lockstep with each other = coevolution.
1. E — Heritable variation affects differences in reproductive success. Fix: Selection requires inherited differences that make some individuals reproduce more than others. - A: Identical individuals offer nothing to select among. - B: A never-changing environment is not a requirement; selection can act in stable or shifting conditions. - C: Acquiring and passing on traits gained during life is the discarded Lamarckian idea. - D: Sexual reproduction is common; asexuality is not required. - Fix rule: Selection needs heritable variation + differences in reproduction.
2. A — The number of fertile offspring it leaves relative to others. Fix: Fitness is reproductive success measured against the rest of the population. - B: Strength matters only if it raises reproduction; it is not fitness itself. - C: Body size is a trait, not a measure of fitness. - D: Long life without reproduction contributes nothing to fitness. - E: Carrying many mutations does not by itself mean more offspring. - Fix rule: Fitness = fertile offspring left, not brawn or size.
3. E — Homologous structures inherited from a common ancestor. Fix: Same bone plan with different jobs signals descent from a shared ancestor (divergent evolution). - A: Analogous structures share function but differ in origin — the reverse case. - B: These limbs are fully functional, not vestigial. - C: Coevolution involves two species shaping each other, not shared limb anatomy. - D: Drift is random change in allele frequency, unrelated to this anatomical pattern. - Fix rule: Same structure, different function = homologous = common ancestry.
4. B — Analogous structures. Fix: Same function but different origin marks analogous structures produced by convergent evolution. - A: Homologous structures share origin, not just function. - C: Vestigial structures are reduced remnants, not functional wings. - D: These are adult wings, not embryonic similarities. - E: Molecular homologies concern shared sequences, not wing anatomy. - Fix rule: Same job, different build = analogous = convergence.
5. C — Share a relatively recent common ancestor. Fix: The more alike two species' DNA, the more recently they diverged from a shared ancestor. - A: Sharing a habitat does not require shared sequences. - B: Body size is unrelated to DNA-sequence similarity. - D: Convergence produces similar function from different genes, not near-identical sequences. - E: Sequence similarity says nothing directly about interbreeding. - Fix rule: More shared DNA = closer (more recent) common ancestry.
6. C — Directional selection. Fix: A steady shift of the mean toward one extreme is directional selection. - A: Stabilizing selection favors the middle, not a shifting extreme. - B: Disruptive selection favors both extremes at once. - D: Sexual selection concerns mate choice, not feeding-driven size shifts. - E: Drift is random, not a consistent directional trend. - Fix rule: Mean marches toward one extreme = directional.
7. D — Stabilizing selection. Fix: Favoring intermediate values and removing both extremes is stabilizing selection. - A: Directional selection would shift the mean, not tighten it around the middle. - B: Disruptive selection favors the extremes, the opposite here. - C: Convergent evolution is about unrelated lineages, not a within-population pattern. - E: The founder effect is random change from a small starting group, not selection for intermediates. - Fix rule: Middle favored, extremes trimmed = stabilizing.
8. A — Disruptive selection. Fix: Favoring both extremes while selecting against intermediates is disruptive selection. - B: Stabilizing selection favors the middle, the reverse of this. - C: Directional selection favors only one extreme. - D: Gene flow is movement of alleles between populations, not this selection pattern. - E: Coevolution involves two interacting species. - Fix rule: Both extremes win, middle loses = disruptive.
9. B — Two species breed during different seasons of the year. Fix: Different breeding times prevent mating before any zygote forms, a prezygotic barrier. - A: Hybrid sterility acts after fertilization — postzygotic. - C: Hybrid death in development is postzygotic. - D: Weak, non-reproducing hybrids are a postzygotic outcome. - E: Chromosome pairing failure after fertilization is postzygotic. - Fix rule: Prezygotic = blocks mating/fertilization; postzygotic = problems with the hybrid.
10. A — Interbreed and produce fertile offspring. Fix: The biological species concept defines a species by successful, fertile interbreeding. - B: Sharing a range does not make organisms one species. - C: Similar appearance can be misleading; look-alikes may not interbreed. - D: Diet does not define a species. - E: Members of a species vary genetically; identical DNA is not required. - Fix rule: Biological species = interbreed to produce fertile young.
11. E — Convergent evolution under similar environmental pressures. Fix: Distant relatives evolving similar streamlined forms independently is convergence. - A: Homologous descent would require a recent common ancestor, which sharks and dolphins lack. - B: Coevolution is reciprocal shaping between two species, not this case. - C: Drift is random and would not reliably produce matching adaptations. - D: These are functional adaptations, not shared vestigial remnants. - Fix rule: Unrelated groups, similar solution, similar environment = convergence.
12. D — Coevolution. Fix: Two species that reciprocally drive each other's evolution over time are coevolving. - A: Convergent evolution involves unrelated species independently resembling each other, not mutual shaping. - B: Directional selection describes a trait shift within one population. - C: Adaptive radiation is one lineage diversifying into many niches. - E: Genetic drift is random and not a reciprocal partnership. - Fix rule: Two species evolving in lockstep with each other = coevolution.