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Why are DNA transitions more common than transversions?

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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.

In the study of DNA and phylogeny, mutations are categorized based on the chemical nature of the base substitution. A transition mutation involves a substitution within the same chemical family—either a pyrimidine for another pyrimidine (cytosine ↔ thymine) or a purine for another purine (adenine ↔ guanine).12

Transitions are more common than transversions—which involve substituting a purine for a pyrimidine or vice versa—because they are chemically more probable. Because transitions involve replacing a base with one of a similar molecular structure, they occur more frequently during DNA replication and through specific chemical modifications, such as the deamination of cytosine to uracil, which leads to C-to-T transitions. Because these changes are not equally probable, methods used to reconstruct evolutionary trees, such as maximum parsimony, often account for these differing probabilities to more accurately model genetic change over time.23