Molecular Evidence for Evolution

Body shape can mislead you. Two species can look alike because they live the same way, not because they share recent ancestry. DNA and protein sequences cut through that. Every species inherited its genes from a common ancestor, and each lineage since then has picked up its own random mutations. Two species that split apart recently have had less time to accumulate differences, so their sequences look more alike. Two species that split long ago have had more time to diverge, so their sequences differ more. Counting the mismatches between two sequences gives a direct readout of shared ancestry that isn't confused by convergent appearances.

This works especially well with proteins like cytochrome c, used by nearly all aerobic organisms and slow to change because most mutations to it are harmful. To compare species, you line up the amino acid sequence of cytochrome c from each one, position by position, and count how many positions differ. Many proteins build up substitutions at a roughly steady average rate over long stretches of time, a pattern called a molecular clock. That steady rate means the difference count is roughly proportional to time since divergence: more differences, more time apart. A low count, like 3 differences, points to a recent split; a high count, like 18 or 19, points to a much older one.

This molecular evidence sits alongside fossils and anatomy as another line of support for common descent, and it often resolves cases where body structure alone is ambiguous. It also sets up speciation: once a molecular clock pins down roughly when lineages split, that timing connects directly to how populations isolate, diverge, and become separate species.