7.6 Evidence of Evolution

Morphological evidence = similarities and differences in body form (bones, organs, embryos) from living and fossil organisms that show descent with modification. Key types you should know for the AP exam (EK 7.6.B.1): homologous structures (same bones, different functions—e.g., mammal forelimbs), vestigial structures (reduced, like human tailbone), analogous structures (similar function but different origin), and transitional fossils (forms with intermediate traits). Comparative embryology also shows shared early development. Why it proves change over time: homologous and vestigial traits imply common ancestry and modification of structures across generations; transitional fossils document intermediate stages and timing (paired with radiometric/stratigraphic dating). On the AP, be ready to describe these morphological examples and explain how they support evolution and common ancestry (LO 7.6.B, EK 7.6.B.1). For more review, see the Topic 7.6 study guide (https://library.fiveable.me/ap-biology/unit-7/evidence-evolution/study-guide/Vy9P6fJvRt1ZTEWg31KI), Unit 7 overview (https://library.fiveable.me/ap-biology/unit-7), and practice questions (https://library.fiveable.me/practice/ap-biology).

Humans have a tailbone (the coccyx) because it’s a vestigial structure—a leftover from ancestors that had tails. Evolutionary evidence (morphological homology and vestigial structures, EK 7.6.B.1) shows that our skeleton still includes that series of fused vertebrae even though the external tail disappeared. Embryos even transiently form a tail-like structure, which is strong comparative/developmental evidence for common ancestry with tailed primates. Over time mutations and selection reduced the external tail because it wasn’t needed; parts of the vertebrae remained and now serve minor roles (attachment for pelvic muscles and support for pelvic organs). This is exactly the kind of morphological evidence the AP asks you to recognize for LO 7.6.B (vestigial structures as evidence of change over time). For a quick review of these ideas, see the Topic 7.6 study guide (https://library.fiveable.me/ap-biology/unit-7/evidence-evolution/study-guide/Vy9P6fJvRt1ZTEWg31KI) and hit the practice problems (https://library.fiveable.me/practice/ap-biology) to prep for exam-style questions.

Scientists date fossils using relative and absolute methods. Relative dating places fossils by rock layers (stratigraphy) and index fossils; absolute (radiometric) dating measures radioactive decay. For recent organic remains we use radiocarbon (carbon-14): C-14 has a half-life ≈ 5,730 years, so it works well up to ~40,000–50,000 years. Beyond that the remaining C-14 is too tiny to measure reliably. Accuracy depends on contamination, sample preservation, and proper calibration (calibration curves from tree rings and other records correct raw C-14 ages). For much older fossils we use other isotopes (U-Pb, K-Ar) on the surrounding volcanic rock to get absolute ages. On the AP exam, be ready to name methods (stratigraphy, radiometric dating, radiocarbon limits) and explain sources of error (contamination, half-life limits) as part of EK 7.6.B.1 (fossils dated by rock age and isotope decay). For a concise review, check the Topic 7.6 study guide (https://library.fiveable.me/ap-biology/unit-7/evidence-evolution/study-guide/Vy9P6fJvRt1ZTEWg31KI) or the full Unit 7 overview (https://library.fiveable.me/ap-biology/unit-7) and practice questions (https://library.fiveable.me/practice/ap-biology).

Homologous structures are body parts that share a common ancestral origin but may have different functions—like the mammal forelimb bones (human arm, whale flipper, bat wing). They’re evidence of common ancestry and are part of “morphological homologies” in the CED (EK 7.6.B.1). Vestigial structures (like a human appendix) are a related concept. Analogous structures look similar or do the same job but evolved independently because of similar environmental pressures—for example, insect wings vs. bird wings. That’s convergent evolution, not shared ancestry, so analogous traits don’t show close relatedness. Why it matters for AP: on the exam you’ll use these ideas to support claims about common ancestry (homology) versus similar selective pressures (analogy). For a quick Topic 7.6 review, check the Evidence of Evolution study guide (https://library.fiveable.me/ap-biology/unit-7/evidence-evolution/study-guide/Vy9P6fJvRt1ZTEWg31KI). For more practice, see the unit overview (https://library.fiveable.me/ap-biology/unit-7) and thousands of practice questions (https://library.fiveable.me/practice/ap-biology).

Think of DNA like a text that changes slowly over time. Comparing DNA nucleotide sequences (or protein amino acid sequences) from different species lets you count differences: fewer differences = more recent common ancestor; more differences = they split longer ago. Those shared, similar sequences are molecular homologies—evidence of common ancestry (EK 7.6.B.2). If a gene is almost identical in two species, it was probably inherited from their common ancestor; if it’s different, mutations have accumulated since they diverged. Scientists use those differences to build phylogenetic trees and estimate split times with a molecular clock. On the AP exam, you should be able to explain how DNA/protein comparisons support evolution and common ancestry (LO 7.6.B; EK 7.6.A.1/EK 7.6.B.2). For a quick review, check the Topic 7.6 study guide (https://library.fiveable.me/ap-biology/unit-7/evidence-evolution/study-guide/Vy9P6fJvRt1ZTEWg31KI) and more practice problems at (https://library.fiveable.me/practice/ap-biology).

Vestigial structures are body parts that lost most or all of their original function but show common ancestry (morphological homology). Classic examples: human appendix and tailbone (coccyx), whale pelvic bones, reduced hind limbs in some snakes, and flightless wings in ostriches. They’re evidence organisms changed over time because their form matches ancestral structures that had a function. We still have them because evolution modifies existing structures slowly—if a reduced trait isn’t strongly harmful, natural selection won’t remove it quickly. Genetic drift, relaxed selection, or developmental constraints can also let vestigial traits persist. On the AP exam, you should link vestigial structures to morphological homologies and common ancestry (EK 7.6.B.1) and be ready to use them as evidence of evolution in short-answer or FRQ prompts. For a focused review, see the Topic 7.6 study guide (https://library.fiveable.me/ap-biology/unit-7/evidence-evolution/study-guide/Vy9P6fJvRt1ZTEWg31KI). For extra practice, check Fiveable’s AP Bio practice problems (https://library.fiveable.me/practice/ap-biology).

Good question—“missing links” don’t disprove evolution because the fossil record is inherently incomplete, but the patterns we do see strongly support descent with modification (CED LO 7.6.A & LO 7.6.B). Why gaps exist: fossilization is rare, rocks get destroyed or buried, and we’ve only sampled a tiny fraction of ancient environments. Those expected gaps don’t erase the evidence: fossils are dated (stratigraphy, radiometric/radiocarbon methods) and show ordered change over time. Transitional fossils (like Tiktaalik between fish and tetrapods) document intermediate morphologies. Comparative anatomy (homologous and vestigial structures) and molecular data (DNA/protein sequence comparisons, molecular clocks, phylogenies) independently match the same evolutionary trees, so multiple lines of EK 7.6.B.1–2 converge. For AP prep, focus on how dating methods + transitional fossils + homologies build a consistent argument for common ancestry (you’ll see this in exam questions). For a quick review, check the Topic 7.6 study guide (https://library.fiveable.me/ap-biology/unit-7/evidence-evolution/study-guide/Vy9P6fJvRt1ZTEWg31KI) and more unit resources (https://library.fiveable.me/ap-biology/unit-7). Practice questions: https://library.fiveable.me/practice/ap-biology.

Scientists use multiple independent lines of evidence to show evolution happened—the kind of stuff AP expects you to know for Topic 7.6. Key types: - Fossil record: sequence of transitional fossils and relative ages from rock layers plus absolute ages from radiometric/radiocarbon dating and geographical context. - Comparative morphology/embryology: homologous structures and vestigial organs show common ancestry; analogous structures show convergent evolution. - Biogeography: species distributions (islands vs continents) match historical divergence and dispersal. - Molecular and genetic data: DNA nucleotide and protein amino-acid sequence comparisons, molecular clocks, and phylogenetic trees quantify relatedness and timing. - Biochemical/physical patterns: shared metabolic pathways and conserved molecules (e.g., cytochrome) support common descent. On the AP exam you’ll need to describe and interpret these data and use them as evidence to support claims (LO 7.6.A/B). For a focused review check the Topic 7.6 study guide (https://library.fiveable.me/ap-biology/unit-7/evidence-evolution/study-guide/Vy9P6fJvRt1ZTEWg31KI), the Unit 7 overview (https://library.fiveable.me/ap-biology/unit-7), and practice questions (https://library.fiveable.me/practice/ap-biology).

Early-stage embryos of different species look similar because they share conserved developmental genes and inherited body-planning mechanisms from a common ancestor—this is comparative embryology, an important line of evidence for evolution (CED: EK 7.6.B.1, LO 7.6.B). Genes like Hox genes and other regulatory networks control basic body axes and organ primordia; those pathways are highly conserved, so initial stages produce similar shapes (pharyngeal arches, tailbud, limb buds). Later, species-specific genes and differential gene expression modify development, producing the adult differences. These shared embryonic patterns are morphological homologies that support common ancestry and are used with molecular data (DNA/protein sequence comparisons) on the AP exam (LO 7.6.B). Want a quick review and practice? Check the Topic 7.6 study guide (https://library.fiveable.me/ap-biology/unit-7/evidence-evolution/study-guide/Vy9P6fJvRt1ZTEWg31KI) and practice questions (https://library.fiveable.me/practice/ap-biology).

Molecular clocks use the steady accumulation of genetic changes (DNA nucleotide or protein amino acid substitutions) to estimate when species diverged. You compare homologous sequences between organisms, count differences, and—assuming a roughly constant mutation rate—convert that difference into time. Clocks are calibrated with independent dates (usually fossils or radiometric ages) so a number of substitutions = a number of years. They help build phylogenetic trees and give timing for common ancestry, complementing morphological and fossil evidence (EK 7.6.B.1–2). Limitations: mutation rates vary by gene and lineage, so multiple genes and fossil calibration give more reliable dates. For AP exam prep, link this to “molecular homology” and DNA/protein sequence comparison (LO 7.6.B). For a focused review, see the Topic 7.6 study guide (https://library.fiveable.me/ap-biology/unit-7/evidence-evolution/study-guide/Vy9P6fJvRt1ZTEWg31KI), the Unit 7 overview (https://library.fiveable.me/ap-biology/unit-7), and practice questions (https://library.fiveable.me/practice/ap-biology).

Morphological evidence = physical traits you can see or measure (bones, organs, embryos). AP examples: homologous structures (human arm vs bat wing), vestigial structures (pelvic bones in whales), and transitional fossils. Morphology shows how body plans changed over time and suggests common ancestry through shared structures and developmental patterns. Biochemical evidence = molecular-level data (DNA nucleotide sequences, protein amino-acid sequences, molecular homologies). AP examples: comparing cytochrome c sequences, using a molecular clock to estimate divergence times, and building phylogenetic trees from sequence data. Biochemical data give more precise, quantifiable measures of relatedness and can detect relationships that morphology misses. Why both matter: morphology shows form/function and fossil context (geological dating), while biochemical data let you test relatedness statistically and build trees. On the AP exam you may be asked to compare or use both kinds of evidence to support common ancestry or evolutionary timelines (see EK 7.6.B.1–2). For a focused review, check the Topic 7.6 study guide (https://library.fiveable.me/ap-biology/unit-7/evidence-evolution/study-guide/Vy9P6fJvRt1ZTEWg31KI) and practice questions (https://library.fiveable.me/practice/ap-biology).

Think of a protein sequence like a sentence made of amino-acid “letters.” Over time random mutations in DNA change those letters, so related species will have similar sentences because they inherited most letters from a common ancestor. The fewer amino-acid differences between two species’ proteins, the more recently they likely shared an ancestor. That’s molecular homology—comparing protein (or DNA) sequences to build phylogenetic trees and estimate divergence times (molecular clock). Conserved proteins used in essential processes (e.g., cytochrome c, hemoglobin) change slowly, so similarities there are strong evidence of common ancestry. AP idea to remember: EK 7.6.B.2 says protein amino-acid comparisons provide evidence for evolution—use sequence alignment to count differences, infer relationships, and place taxa on trees. For extra practice and examples (including sample phylogenies and practice questions), check the Topic 7.6 study guide (https://library.fiveable.me/ap-biology/unit-7/evidence-evolution/study-guide/Vy9P6fJvRt1ZTEWg31KI), the Unit 7 overview (https://library.fiveable.me/ap-biology/unit-7), and Fiveable’s practice problems (https://library.fiveable.me/practice/ap-biology).

Scientists date fossils two main ways: relative and absolute dating. For relative dating, geologists use stratigraphy—deeper rock layers are usually older—plus index fossils (species that lived for a short, known time) to order fossils in time. For absolute (numeric) ages, they use radiometric dating: measuring decay of radioactive isotopes in the rock or fossil (e.g., carbon-14 for recent samples up to ~50,000 years; other isotopes like uranium-lead for much older rocks). Geochemical and geographical data (like volcanic ash layers) help tie relative and absolute dates together. Together these methods give strong, cross-checked evidence about which fossils are older, which is exactly what the CED lists under EK 7.6.B.1 (age of rocks, isotope decay, geographical data). Want practice applying these ideas to AP-style questions? Check the Topic 7.6 study guide (https://library.fiveable.me/ap-biology/unit-7/evidence-evolution/study-guide/Vy9P6fJvRt1ZTEWg31KI) and more unit review (https://library.fiveable.me/ap-biology/unit-7).

Because whales, bats, and humans inherited the same basic limb plan from a common tetrapod ancestor, their forelimbs share homologous bones (humerus, radius/ulna, carpals, metacarpals, phalanges) even though each limb is modified for different functions (swimming, flying, manipulating). That’s a classic morphological homology—same structure, different function—and it’s exactly the kind of evidence for common ancestry listed in the CED (EK 7.6.B.1). This differs from analogous structures (like insect wings), which look similar because of convergent evolution, not common descent. Molecular data (DNA/protein sequence comparisons) and fossils that show transitional forms reinforce the homology story and help place these species on phylogenetic trees (LO 7.6.A / LO 7.6.B). For a quick AP-aligned review, check the Topic 7.6 study guide (https://library.fiveable.me/ap-biology/unit-7/evidence-evolution/study-guide/Vy9P6fJvRt1ZTEWg31KI) and more unit resources at (https://library.fiveable.me/ap-biology/unit-7). You can practice related questions at (https://library.fiveable.me/practice/ap-biology).

Geological evidence for evolution comes from rock layers (stratigraphy), radiometric dating of those rocks, and large-scale Earth changes (like continental drift) that explain where fossils are found. Sedimentary strata give a relative timeline—older layers contain older fossils—while radiometric methods (e.g., decay rates of isotopes; note C-14 is only useful for ~50k years) provide absolute ages. Index fossils and dated layers let us place transitional fossils in time, showing gradual change in form. Biogeography (continental positions and habitat shifts) explains why related fossils show up in certain regions. Together with morphological and molecular data, geological dating ties the fossil record into a time-calibrated tree of life—exactly what EK 7.6.B.1 and EK 7.6.A.1 describe. For a focused review, see the Topic 7.6 study guide (https://library.fiveable.me/ap-biology/unit-7/evidence-evolution/study-guide/Vy9P6fJvRt1ZTEWg31KI) and more unit resources (https://library.fiveable.me/ap-biology/unit-7). For practice, try the AP problems at (https://library.fiveable.me/practice/ap-biology).