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How do DNA sequences help scientists compare the relatedness of species?

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A diagram showing the tentative evolutionary relationships among hominid species over the past four million years, with skull images and labeled species names connected by solid and dotted lines to indicate proposed evolutionary pathways. — phylogeny Tentative phylogenetic scheme for the evolution of the human lineage. Solid bars indicate the time ranges during which species are thought to have existed, based on fossil evidence. Dotted lines signify evolutionary relationships between hominin species that have been proposed on the basis of the fossil evidence.

DNA sequences are fundamental to phylogenetics, the study of ancestral relatedness among organisms. Scientists compare homologous DNA sequences—sequences inherited from a common ancestor—to estimate evolutionary distance by measuring the number of nucleotide substitutions that have accumulated over time.123

This process functions similarly to a "molecular clock." Because mutations occur over time, the number of differences between the DNA sequences of two species is generally proportional to the time elapsed since they diverged from a common ancestor. By analyzing these differences, researchers can reconstruct the branching patterns of the "tree of life," identifying which species are more closely related based on how recently they shared a common ancestor.124

Different types of DNA are selected based on the evolutionary timescale being studied. For instance, mitochondrial DNA evolves rapidly and is useful for examining recently diverged groups, while other sequences, such as those for ribosomal RNA, evolve more slowly and are better suited for deeper evolutionary history.3