How do genes help explain evolutionary relationships among living organisms?

Genes provide compelling evidence for evolutionary relationships by acting as a record of an organism's ancestry. Because all living organisms share a common genetic code and use the same fundamental building blocks—nucleotides in DNA and amino acids in proteins—scientists can quantify the degree of relatedness between species.1

This process, central to phylogenetics, involves comparing homologous DNA or protein sequences. The number of differences, or substitutions, between these sequences serves as a molecular clock:23
* Quantifying Divergence: By measuring nucleotide or amino acid substitutions, researchers can estimate how long ago two species diverged from a common ancestor.23
* Testing Hypotheses: These molecular data allow scientists to test and refine evolutionary trees, or cladograms, which represent the inferred relationships between species.21
* Consistency: Because thousands of genes provide independent tests of evolutionary history, the consistency of these molecular findings offers some of the most robust evidence for the common descent of all life.1

Essentially, genes function like a shared language; the more similar the "text" of the genetic code between two organisms, the more closely related they are.1