Evidence for evolution
Evolution is supported by many independent lines of evidence that all point to the same conclusion.
The fossil record
Fossils show that life has changed over time: simple life appears first, then more complex forms, in a consistent order in rock layers. Transitional fossils link groups, such as Tiktaalik, a fish with limb-like fins, and Archaeopteryx, with features of both dinosaurs and birds.
Dating fossils
Radiometric dating uses the steady decay of radioactive isotopes to date rocks. Uranium-lead and potassium-argon dating give ages in millions of years, showing Earth is about 4.5 billion years old. Relative dating uses the order of rock layers: deeper layers are generally older.
Homologous structures
Homologous structures share an underlying design because of common ancestry, even when used differently. A human arm, a bat wing, a whale flipper, and a cat leg all have the same bone pattern: one upper bone, two lower bones, wrist bones, and digits.
Analogous and vestigial structures
Analogous structures do similar jobs but evolved separately, such as insect and bird wings. Vestigial structures are reduced remnants with little function, such as whale hip bones or the human tailbone, explained by descent from ancestors in which they were useful.
Embryology
Embryos of vertebrates look similar early in development: fish, chicks, and humans all have tail and throat structures resembling pharyngeal pouches. These shared features reflect shared developmental genes inherited from common ancestors.
Molecular evidence
All living things use DNA and the same genetic code. Comparing DNA sequences shows that closely related species have more similar sequences: humans and chimpanzees share about 98-99% of their DNA. Molecular trees built from DNA closely match those built from anatomy and fossils.
Biogeography
The distribution of species fits evolution. Islands have unique species related to those on the nearest mainland. Australia's marsupials evolved in isolation after the continent separated. Continental drift explains why related fossils appear on now-distant continents.
Observed evolution
Evolution has been directly observed: in Galapagos finches, beak size shifted measurably after droughts; in laboratory experiments, E. coli populations have evolved new abilities over tens of thousands of generations; and pests have evolved resistance to pesticides.
Key takeaways
Fossils show change through time, dated by radiometric methods. Homologous and vestigial structures reveal common ancestry, while analogous structures show convergence. Embryos and DNA sequences reflect shared descent. Biogeography and directly observed evolution add further evidence.
Flashcard
Shows life changing over time
Check your understanding
A bat's wing and a human arm have the same arrangement of bones. These are: