Lines of evidence
Fossils, anatomy, biogeography.
Definition of evolution. A change in the heritable characteristics of a population over successive generations.
Fossil evidence. Fossils show:
- Older rocks contain simpler / more ancient life forms.
- Younger rocks contain more recent forms.
- Transitional fossils link groups: Archaeopteryx (feathered dinosaur, ~150 Ma) links reptiles and birds; Tiktaalik (~375 Ma) links fish and tetrapods.
The fossil record is incomplete (most organisms don't fossilise) but its patterns consistently support a history of descent with modification.
Homologous structures. Same underlying anatomy, different functions, due to common ancestry. The pentadactyl limb (5-fingered limb) is present in all tetrapods — human arm, whale flipper, bat wing, horse leg — all built from the same bones (humerus, radius, ulna, carpals, metacarpals, phalanges) repurposed for different tasks.
Analogous structures. Similar function but different anatomy — independent (convergent) evolution. Bird wings (modified forelimbs) and insect wings (cuticle outgrowths) both fly but share no common ancestor.
Vestigial structures. Reduced versions of features that had function in ancestors but are now non-functional or much less developed.
- Human appendix (was a digestive sac for plant material).
- Whale pelvis (whales' ancestors walked on land).
- Snake hind-limb buds (snakes evolved from limbed lizards).
Biogeography. Species distribution reflects evolutionary history. Marsupials are largely confined to Australia because the continent broke from Gondwana while marsupials still dominated there; placental mammals later out-competed marsupials elsewhere. Darwin's finches on the Galápagos diversified from one ancestral colonist.
Molecular evidence.
- The genetic code is nearly universal, suggesting all life shares a common ancestor.
- DNA / protein sequence similarity is greater between closely related species. Humans and chimpanzees share ~98.8% of DNA; humans and yeast still share ~30%.
- Sequence comparison allows construction of phylogenetic trees showing evolutionary relationships.
- Fossils: transitional forms link groups.
- Homologous = same plan; analogous = same function.
- Vestigial: evolutionary leftovers.
- Biogeography reflects history.
- DNA similarity quantifies relatedness.