Partial cytochrome b and 12S rDNA mitochondrial DNA sequences of eight representatives of the Ramphastidae family were analyzed. We applied the linearized tree method to identify sequences evolving at similar rates and estimated the divergence times among some of the taxa analyzed. After excluding Ramphastos tucanus and Capito dayi from our data set, the remaining taxa presented a constant rate of DNA substitution, and branch lengths could be re-estimated with a clock constraint using the maximum likelihood method. Branch lengths were calibrated assuming that Galliformes and Piciformes split around 100 million years ago (mya). Our results indicate that Ramphastinae, and probably Capitoninae, diverged from other Piciformes in the Late Cretaceous (~82 mya), suggesting that Piciformes is another avian order that survived the mass extinction event occurred 65 mya at the Cretaceous/Tertiary (K/T) boundary. The divergence times estimated within the Ramphastinae genera cover the period from the Middle Eocene (around 47 mya) through the Late Miocene (9.5 mya). Our estimate of divergence time is coincidental with the split of the African and the South American continents and other intense geologic activities and modifications of the areas which correspond to the current Neotropics. These events might have influenced the diversification of Ramphastinae in South America.

Com o advento das técnicas de seqüenciamento de DNA, a organização do genoma mitocondrial de vertebrados mostrou variação entre níveis taxonômicos superiores. A organização "conservada" é encontrada em mamíferos placentários, tartarugas, peixes, alguns lagartos e Xenopus. As aves, outras espécies de lagartos, crocodilianos, mamíferos marsupiais, cobras, tuatara, lampréia, uma espécie de peixe e alguns outros anfíbios apresentam ordens que diferem do que foi inicialmente considerado a "ordem gênica conservada". O mecanismo mais provável de novos rearranjos gênicos parece ser duplicação in tandem e múltiplos eventos de deleção, sempre associados com seqüências de tRNAs. Alguns novos rearranjos parecem ser típicos de grupos monofiléticos e o uso destes dados pode ser útil em questões de filogenia envolvendo níveis taxonômicos superiores. Outros aspectos como a estrutura secundária de tRNA e códons de início e de parada de transcrição de genes codificantes para proteínas podem ser úteis para comparações entre os genomas mitocondriais de vertebrados.

With the advent of DNA sequencing techniques the organization of the vertebrate mitochondrial genome shows variation between higher taxonomic levels. The most conserved gene order is found in placental mammals, turtles, fishes, some lizards and Xenopus. Birds, other species of lizards, crocodilians, marsupial mammals, snakes, tuatara, lamprey, and some other amphibians and one species of fish have gene orders that are less conserved. The most probable mechanism for new gene rearrangements seems to be tandem duplication and multiple deletion events, always associated with tRNA sequences. Some new rearrangements seem to be typical of monophyletic groups and the use of data from these groups may be useful for answering phylogenetic questions involving vertebrate higher taxonomic levels. Other features such as the secondary structure of tRNA, and the start and stop codons of protein-coding genes may also be useful in comparisons of vertebrate mitochondrial genomes.