CONICET | Buscador de Institutos y Recursos Humanos

Estimating the ages of species or their component lineages based on genetic data is crucial for many studies in evolutionary biology. However, calibrations of the molecular clock in vertebrates have been done almost exclusively using cytochrome b. The existence of large, standardized COI libraries generated as a result of barcoding provides the opportunity to obtain a calibration for this mitochondrial gene. In this study we compare the evolutionary rate of COI relative to that of cytochrome b in birds using almost 6,000 publicly-available sequences from BOLD (COI) and GenBank (cytochrome b). We obtained divergence estimates for both genes for as many phylogenetically independent pairs of species as possible from eight orders of birds, including several families of passerines. For each pair of species we estimated the difference in divergence between the two genes, totaling around 300 independent comparisons of their evolutionary rates. An initial, general comparison between rates indicated that cytochrome b evolves, on average, between 14% and 22% faster than COI, depending on the way in which the values of the different orders are weighted. However, considerable variation among avian orders and families precludes the use of this result as a general molecular clock in birds. Moreover, the rate of COI relative to that of cytochrome b clearly depends on the level of divergence between species, a trend that in fact partially explains the differences among orders; cytochrome b evolves much faster than the COI barcode region in recently diverged species, but this difference diminishes as the species that are compared become more diverged and the distance in COI actually tends to be higher than that in cytochrome b in species pairs with COI divergences higher than 10%. Finally, we also analyzed the difference in evolutionary rate between regions for each of these genes.