Darwin and the Evidence for Evolution

During a five-year voyage on HMS Beagle, Charles Darwin observed, among many things, that finches across the Galapagos Islands had beak shapes closely matched to the specific food source available on each different island. Influenced by Charles Lyell's geology, which showed Earth was far older than previously assumed, and by Thomas Malthus's writing on population growth outstripping resources, Darwin proposed in his 1859 book On the Origin of Species that living things show "descent with modification," driven by natural selection. Biologists define evolution precisely: a change in the heritable characteristics of a population across generations -- not a change within a single organism's own lifetime.

Four independent lines of evidence converge on the same conclusion. The fossil record documents transitional forms across deep time, such as a well-documented series of whale-ancestor fossils showing gradual change from a four-legged land mammal to a fully aquatic body plan. Comparative anatomy reveals homologous structures -- the same underlying bone arrangement in a human arm, a whale flipper, a bat wing, and a cat leg, despite very different functions, pointing to a shared ancestor -- as well as vestigial structures, reduced structures with little or no current function, such as the small, unused hip and leg bones found in some whale skeletons. Embryology shows strikingly similar early embryonic structures across very different vertebrate species. And molecular biology shows that DNA and protein sequence similarity between species tracks closely with how anatomically similar those species are, with the near-universal genetic code from Unit 7 itself a data point pointing toward common ancestry.