Overview

Genomes in organisms evolve primarily by microevolutionary changes in DNA sequences – substitutions, deletions, and insertions. Genomes can expand by gene duplication.

Genes evolve at different rates, depending on how the functions of RNAs and proteins are constrained by structure. The more a protein's structure is constrained, the more slowly does its gene evolve. Any given protein tends to evolve at a more or less characteristic rate, in extreme cases for hundreds of millions of years, making each protein useful in estimating when evolutionary lineages diverged.

All organisms arose from ancestral organisms, implying a single ancestor at the beginning. The organisms living today can be arranged as the twigs at the ends of a tree-like diagram whose trunk represents the original ancestor. The pattern of branching in the evolutionary tree is phylogeny. Much can be learned about phylogeny by comparing the nucleotide sequence of homologous genes – genes that descended from a common ancestral gene.

Evolution of Homologous Genes

Genes in different species that descended from a common ancestral gene are said to be homologous. There are two kinds of homology, orthology and paralogy. Orthologs are copies of a gene, present in two species and inferred to have existed in their most recent common ancestor – e.g., the β-hemoglobin genes of mice and humans. Paralogs are copies of a gene that arose by gene duplication – e.g., α-hemoglobin and β-hemoglobin.