Evidence for evolution
Palaeontology, comparative anatomy, embryology, divergent and convergent evolution, and industrial melanism.
The essentials
Palaeontology: fossils are remains of hard parts of life forms found in rocks. Rocks form sediments, and a cross-section of Earth's crust shows layers one over the other; different-aged rock sediments contain different life forms, which probably died during the formation of that sediment. Some fossils represent extinct organisms such as dinosaurs. The age of fossils can be estimated by radioactive dating.
Comparative anatomy and morphology: homologous organs have the same basic structure and origin but perform different functions, for example the forelimbs of whales, bats, cheetahs and humans, which have similar bones (humerus, radius, ulna, carpals, metacarpals and phalanges). The vertebrate hearts and brains are also homologous. In plants, the thorn of Bougainvillea and the tendril of Cucurbita are homologous. Homology indicates common ancestry and divergent evolution.
Analogous organs have different structures and origins but perform similar functions, for example the wings of butterflies and birds, the eyes of octopus and mammals, the flippers of penguins and dolphins, and sweet potato (root modification) and potato (stem modification). Analogy results from convergent evolution: similar habitats select similar adaptive features.
Embryology: Ernst Haeckel noted features common to all vertebrate embryos, such as a row of vestigial gill slits behind the head, which are functional only in fish. This proposal was disapproved by Karl Ernst von Baer, who noted that embryos never pass through the adult stages of other animals.
Industrial melanism: in England, before industrialisation (about 1850), white-winged moths outnumbered dark-winged (melanised) moths on trees. After industrialisation, the trunks became dark due to soot, and dark moths increased. Lichens, which do not grow in polluted areas, could no longer cover the trunks, so white moths were easily seen and eaten by predators. Similar natural selection is seen in the use of herbicides, pesticides and antibiotics, which leads to resistant varieties within months or years.
Common mistakes
- Similar function does not mean homology; check for common structural plan.