Radioisotopes decay over time. Rocks can be dated by comparing the ratios of different isotope forms. Comparative anatomy - basic structure is modified by evolutionary pressures. Bird wing, dolphin flipper, human hand have similar bones, but no mechanical need for structural similarity. Homologous structures - features in different species that are similar in underlying structure because of common evolutionary origin.
Analogous structures don t have a common evolutionary origin.
They look the same because they have similar function, like wings of bugs and birds. Convergent evolution means that populations of different ancestry may adapt in similar ways to similar environments, like flippers on whales and fins on fish. Vestigial organs are small or nonfunctional structures in present organism leftover from functional organs in ancestors. Convey no adaptive advantage, so fade out gradually. River Dolphins live in very muddy water, and are mostly blind.