Lesson 16: Evidence for EvolutionAP

Evolution is not supported by one argument but by the convergence of many independent lines of evidence — fossils, anatomy, embryos, geography, and above all molecules — all pointing to the same family tree. Unit 7 of the CED (Topics 7.6–7.8) asks you to deploy that evidence precisely: to distinguish homologous from analogous structures, to read sequence-similarity data as a measure of relatedness, and to recognise evolution happening in real time. The organising claim is common ancestry: all known life descends, with modification, from a shared ancestor (LUCA) — which is why the genetic code is nearly universal (Lesson 12) and why every organism runs its metabolism on ATP (Lesson 4).

Evidence from the fossil record

Fossils document both the sequence of life's history and the transitions between groups. Radiometric dating (for example, potassium-argon) anchors the record in absolute time; the order of strata matches the order predicted by trees built from living species. Transitional forms link groups in exactly the places the theory says they should: Archaeopteryx combines dinosaur teeth, claws and a bony tail with bird feathers and wings; Tiktaalik is a fish with wrist-like fin bones, found in Devonian rocks of the predicted age and environment for the fish–tetrapod transition; the whale series (Pakicetus → Ambulocetus → Basilosaurus → modern whales) tracks hind limbs shrinking and nostrils migrating as a land mammal lineage returned to the sea. A predicted transitional fossil found after the prediction (Tiktaalik was located by targeting rocks of the right age) is among the strongest kinds of evidence science can offer.

Evidence from comparative anatomy

Homologous structures — same underlying architecture, different functions — record descent from a common ancestor: the forelimbs of humans, bats, whales and cats contain the same bones in the same arrangement (one upper bone, two forearm bones, wrist and digit bones), reshaped for grasping, flying, swimming and running. Analogous structures — similar functions, different architecture — record convergent evolution, not kinship: insect wings (extensions of the exoskeleton) and bird wings (modified forelimbs) both fly, but their structures betray separate origins. Vestigial structures are reduced remnants of ancestral features: the human appendix and tailbone, hip bones buried inside whale bodies, and the tiny limb buds of pythons. Comparative embryology adds a developmental line: vertebrate embryos pass through similar early stages (including pharyngeal arches) before diverging — evidence of shared developmental programmes inherited from common ancestors, whose genes (such as the Hox family, present across bilaterian animals) can still be compared directly.

Evidence from biogeography

The geographic distribution of species matches evolution plus continental drift. Islands hold species found nowhere else, closely related to mainland forms — Darwin's finches and tortoises in the Galápagos, the lemurs of Madagascar. Australia's marsupials diversified in isolation into forms that converge remarkably on placental mammals elsewhere (marsupial "wolves," "moles" and "mice") — the same selection pressures sculpting unrelated lineages into analogous shapes. Oceanic islands characteristically lack whole groups (no native land mammals or amphibians on Hawaii) because only organisms that could cross oceans arrived — a pattern inexplicable by any "placed there" account but inevitable under dispersal and evolution.

Evidence from molecules — the decisive test

Molecular data let us compare organisms that share almost no anatomy. The logic: the more recently two species shared an ancestor, the fewer sequence differences have accumulated between them. Cytochrome c, a respiratory protein (Lesson 6), differs between humans and chimpanzees by zero amino acids, between humans and monkeys by one or two, humans and ducks by about eleven, and humans and yeast by over forty — matching, rank for rank, the tree built from anatomy a century earlier. Universal features speak to the deepest ancestry: the near-universal genetic code, ATP as universal energy currency, and shared core metabolic pathways. Genomes also carry historical scars: pseudogenes (broken gene copies, like the human GULO pseudogene — we cannot make vitamin C because an ancestral primate's gene was disabled and survives as a fossil in our DNA, in the same broken form in other primates) and endogenous retroviruses — viral DNA inserted into ancestral genomes at identical chromosomal positions in related species, an inheritance pattern only common descent explains.

Evolution observed directly

Selection in action has been recorded repeatedly: antibiotic resistance evolving in hospitals (Lesson 13), pesticide resistance in insects, the Grants' finches changing with drought, industrial melanism rising and falling, and Richard Lenski's decades-long E. coli experiment, in which twelve initially identical populations have diverged over tens of thousands of generations — one even evolved the entirely new ability to consume citrate. These cases matter because they are the mechanisms of Lessons 13 and 15 caught in the act, repeatedly and measurably.

Worked example 1: reading a cytochrome c table

Amino acid differences in cytochrome c, relative to the human sequence: chimpanzee 0, rhesus monkey 1, dog 11, chicken 12, frog 19, yeast 42. Rank the organisms by relatedness to humans and explain the reasoning, including the placement of chicken and dog.

Reasoning: fewer differences = less time since the common ancestor = closer relationship. Ranking: chimpanzee > monkey > dog ≈ chicken > frog > yeast. The interesting pair is dog (11) versus chicken (12): despite chickens being "more different" from us anatomically in obvious ways, the molecular count puts the dog marginally closer — consistent with both being amniotes whose lineages split deep in time, with the mammal line (dog and human) sharing a more recent ancestor than either does with birds. The exam's point is methodological: trust the count, not the impression, and note that molecular and anatomical trees cross-check each other.

Worked example 2: classifying structures

Classify each pair as homologous or analogous, and state what each is evidence for. (a) The wing of a bat and the flipper of a whale. (b) The wing of a bat and the wing of a butterfly. (c) The spines of a cactus and the thorns of a rose.

(a) Homologous — same bone-by-bone forelimb plan inherited from a common tetrapod ancestor; evidence for shared ancestry of the two mammals. (b) Analogous — a modified vertebrate limb versus an outgrowth of insect exoskeleton; evidence for convergent evolution under the shared pressure of flight. (c) Analogous — cactus spines are modified leaves, rose thorns are modified stem tissue (technically prickles); similar defensive function, different developmental origin; evidence of convergence in dry/defended niches. The classification procedure is always: compare the underlying structure and development, not the function — function is what misleads.

Where this shows up in the real world

Evolutionary evidence is working infrastructure. Phylogenetic comparison identified the bat origins of coronaviruses and tracks influenza's drift each year to choose vaccine strains. Comparative genomics underlies the model-organism strategy of all biomedical research: we test drugs in mice because molecular evidence shows we share most genes and pathways with them — a prediction of common ancestry that pays out daily in laboratories — and human pseudogene patterns are used in ancestry testing and forensics.

Common mistakes

• Citing analogous structures as evidence of common ancestry. They are evidence of similar selection pressures. Homologous structures and shared DNA are the kinship evidence.
• "Humans evolved from chimpanzees." Both species evolved from a common ancestor neither resembles exactly; chimps are our cousins, not our grandparents.
• Calling vestigial structures "useless." Vestigial means reduced from the ancestral condition — the whale pelvis still anchors muscles; the point is the mismatch between structure and current need, not total functionlessness.
• Treating the fossil record's gaps as fatal. The record is incomplete (fossilisation is rare), yet every discovered transitional form has landed in the predicted position; absence of a fossil is not evidence of absence of the organism.
• Confusing homology with similarity of function. Homology is similarity due to ancestry — judge by structure, development and position, never by job description.
• Claiming acquired characteristics as evidence. A bodybuilder's children are not born muscular; only heritable variation feeds evolution (Lesson 13).

AP Exam Experience

Question 1. The forelimbs of a human, a bat and a whale contain the same set of bones in the same arrangement, although the limbs are used for grasping, flying and swimming respectively. These limbs are best described as

• (A) analogous structures resulting from convergent evolution
• (B) homologous structures resulting from descent from a common ancestor
• (C) vestigial structures with no current function
• (D) evidence that the three species evolved from each other

Question 2. Two species of frog look nearly identical and live on different continents, but their DNA sequences differ by 18%, while two dissimilar-looking frog species on the same continent differ by only 2%. The most reasonable conclusion is that

• (A) appearance is a more reliable guide to relatedness than DNA
• (B) the similar-looking species are closely related despite the DNA evidence
• (C) the similar-looking species evolved similar features independently (convergence) and are not close relatives, while the dissimilar pair shares a recent common ancestor
• (D) DNA evidence cannot be used for frogs

Question 3. Endogenous retroviruses are found at identical positions in the genomes of humans and chimpanzees far more often than chance would predict. This is best explained by

• (A) the viruses preferentially infect the same locations in both species today
• (B) the viral insertions occurred in the genome of a common ancestor and were inherited by both species
• (C) humans and chimpanzees exchange DNA in the wild
• (D) convergent evolution of genome structure

Answer key

• 1. (B) — Shared structural plan despite different function is the definition of homology, and it is explained by inheritance from a common ancestor with modification for different uses.
• 2. (C) — Molecular divergence measures time since common ancestry directly; similar appearance across continents is the classic signature of convergence under similar pressures.
• 3. (B) — Independent insertions at identical sites in two species would be an astronomically improbable coincidence; shared insertions are inherited scars from an ancestral genome.

Free-response question. The Hawaiian Islands are home to more than 50 species of honeycreeper birds found nowhere else on Earth, with beaks ranging from thin probing bills to massive seed-cracking bills. DNA evidence shows all descend from a single finch-like ancestor that reached the islands a few million years ago. The islands have no native woodpeckers. (a) Explain how one ancestral species gave rise to more than 50 species on the islands, naming the process involved. (b) One honeycreeper, the akiapolaau, uses its bill to pry insects from bark — the ecological role woodpeckers fill on continents. Explain this resemblance in evolutionary terms. (c) Predict how the cytochrome c sequences of two honeycreeper species would compare with the difference between a honeycreeper and a mainland finch, and justify the prediction. (d) Explain why the absence of native land mammals in Hawaii before human arrival supports the dispersal-and-evolution account of the islands' biology.

Scoring guideline

• (a) 1 point: adaptive radiation — the colonising ancestor's descendants spread into many unoccupied ecological niches across islands, and divergent selection on beak shape for different foods, combined with isolation between islands, produced speciation repeatedly (point requires the mechanism elements: colonisation, empty niches, divergent selection/speciation).
• (b) 1 point: convergent evolution / analogy — in the absence of woodpeckers, selection favoured bark-prying bills in the honeycreeper lineage independently; the resemblance is functional, not evidence of kinship with woodpeckers.
• (c) 1 point: predicts the two honeycreepers differ less from each other than a honeycreeper differs from the mainland finch. 1 point: justification — the honeycreepers share a more recent common ancestor (a few million years, within the radiation), so fewer neutral mutations have accumulated between them than since their lineage split from mainland finches.
• (d) 1 point: under dispersal-and-evolution, only groups capable of crossing the ocean (flying birds, wind- or sea-borne organisms) could found populations; land mammals could not cross, so their absence — and the explosive diversification of the groups that did arrive — is the predicted pattern.