Sequencing of DNA recovered from dodo remains, including material held in Oxford, placed the species within Columbidae alongside the extinct Rodrigues solitaire, its sister taxon. The closest surviving relative identified is the Nicobar pigeon, Caloenas nicobarica, an iridescent ground-feeding bird of island Southeast Asia. The two lineages diverged long before humans reached Mauritius.
Island life relaxed the selective pressure that keeps flight muscles large. With no terrestrial mammal predators on Mauritius, the dodo's ancestors grew heavier, the keel of the breastbone shrank, and the wings reduced to short stubs. This is a repeated pattern across oceanic islands, seen also in the moa of New Zealand and several rails.
The familiar image of a grotesquely fat, waddling bird is largely an artefact. Several of the most-copied paintings appear to depict captive individuals overfed in European menageries. Skeletal reconstructions suggest a more upright, leaner animal, though estimates of live mass still vary widely because no reliable weight was ever recorded.