7.6Natural Selection

Evidence of Evolution — AP BIOLOGY Revision Notes & Topical Questions

5 min read 9 parts Lesson 7 of 13 2 quizzes

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7.6 Evidence of Evolution — Notes — AP Biology notes: Describe the types of data that provide evidence for evolution.Evolution — the change in allele frequencies of a population over generations; the term also covers descent of all species from common ancestors with modification …

Homologous structure
Analogous structure
Vestigial structure
Transitional fossil
Relative dating
Radiometric dating

Section 1 of 9Types of evidence

Evolution — the change in allele frequencies of a population over generations; the term also covers descent of all species from common ancestors with modification over geological time.

Independent fields — the evidence comes from extant organisms, fossils and comparisons of molecular, morphological and genetic data,

and the more varied the sources that agree, the stronger the conclusion.

Geological — fossil distribution and rock strata trace changes over time.

Geographical — distribution of organisms links to shared ancestry (Galápagos finches and Hawaiian honeycreepers show adaptive radiation).

Physical — fossils and homologous structures.

Biochemical — DNA and protein sequence comparisons (cytochrome c similarity across species).

Mathematical models — calculate divergence times and genetic similarity; more nucleotide differences mean an older common ancestor.

Robustness — independent, consistent evidence makes evolution a robust scientific theory (a well-tested explanation, not a guess).

Section 2 of 9Fossils and dating

Fossils — preserved remains or traces of past organisms, usually in sedimentary rock, in strata laid down in order so lower (older) layers hold simpler or more ancestral forms.

Transitional fossils — show intermediate features, e.g. Tiktaalik, Archaeopteryx (a bridge between dinosaurs and birds) and early whales with legs.

Incompleteness — few organisms fossilise, but the record shows extinct groups, the order in which groups arose and mass extinctions.

Relative dating — lower layers were deposited earlier, so their fossils are older; it gives order, not an age in years.

Radiometric dating — uses the fixed decay rate of isotopes; each half-life halves the parent isotope and age = number of half-lives × half-life.

Carbon-14 (half-life about 5730 years) dates organic material up to about 50 000 years.
Potassium-40 (about 1.3 billion years) and uranium-238 (4.5 billion years) date very old rocks.

Timelines — fossil dating plus geographical data build evolutionary timelines.

Section 3 of 9Anatomy and embryology

Homologous structures — same underlying anatomy inherited from a common ancestor even when functions differ (pentadactyl limb of humans, bats, whales, cats, birds, frogs and alligators; humerus, radius, ulna, carpals and digits); evidence of divergent evolution.

Analogous structures — similar function but different origin (insect and bird wings),

from convergent evolution, and not evidence of close relatedness.

Vestigial structures — reduced remnants of ancestral structures (human appendix and tailbone, whale pelvic bones, wings of flightless birds, eyes of blind cave fish) showing ancestors in which they worked.

Embryology — vertebrate embryos share pharyngeal pouches and a post-anal tail through shared developmental genes.

Reasoning — the same limb bones in a bat wing and a whale flipper are best explained by inheritance from a common ancestor.

Section 4 of 9Molecular evidence

Sequences — more similar DNA or amino acid sequences (e.g. Hox genes, haemoglobin, cytochrome c) mean closer relationship and a more recent common ancestor.

Sequence divergence — differences accumulate over time and are greater between distantly related species; cytochrome c is nearly identical across mammals but differs more between mammals and yeast.

Genome similarity — humans and chimpanzees share about 98 to 99 per cent in comparable regions.

Shared errors — pseudogenes (the broken vitamin C gene in primates) and retrovirus insertions at the same position are explained only by common ancestry.

Molecular clock — uses the fairly steady rate of neutral mutations to estimate when lineages diverged.

Evolutionary trees — built from DNA and protein sequencing.

Conserved processes and structures — DNA replication, respiration, protein synthesis, ribosomes, the genetic code and phospholipid bilayers show shared ancestry.

Molecular versus fossil — molecular evidence is quantitative and testable statistically,

while fossils give dates and extinct forms but are incomplete.

Section 5 of 9Biogeography and observed evolution

Biogeography — distribution reflects evolutionary history and continental drift (marsupials concentrated in Australia, island species resembling nearby mainland species).

Antibiotic resistance — MRSA shows natural selection in action,

as do pesticide (DDT) and herbicide resistance and cancer cells resisting chemotherapy.

Emerging pathogens — seasonal flu changes antigens through mutation, making vaccines ineffective, and COVID-19 shows rapid pathogen evolution.

Other direct observations — beak changes in Galápagos finches after drought, long-term bacterial evolution experiments, genomic divergence and the fossil record.

Agreement — confidence rises when independent lines give the same family tree.

Section 6 of 9Lines of evidence compared

Type	Shows	Example	Limitation
Fossils	Change over time	Whale ancestors with legs	Gaps in the record
Homologous structures	Common ancestry	Pentadactyl limb	Divergent evolution
Analogous structures	Similar pressures	Bird and insect wings	Convergent evolution
Vestigial structures	Ancestral function lost	Whale pelvis	Reduced remnants
Molecular data	Relatedness	Cytochrome c	Needs sequence comparison

Section 7 of 9More from the specification

Section 8 of 9Key terms

Term	Meaning
Homologous structure	similar anatomy because of shared ancestry
Analogous structure	similar function but different origin
Vestigial structure	reduced structure inherited from an ancestor
Transitional fossil	fossil with features of two groups
Relative dating	ordering fossils by rock layer
Radiometric dating	dating by the decay of radioactive isotopes
Half-life	time for half of an isotope to decay
Molecular clock	use of mutation rate to estimate divergence time

Section 9 of 9Exam technique

Confusing homologous and analogous structures — homologous share ancestry; analogous share function.

Treating analogous structures as evidence of close relationship — they come from convergent evolution.

Saying fossils show a straight-line sequence — evolution is branching.

Calling evolution "just a theory" in the everyday sense — a scientific theory is a well-supported explanation.

Saying relative dating gives an age in years — it gives only the order.

Last updated: 14 October 2026