Calibrating the molecular clock beyond cytochrome b: assessing the evolutionary rate of COI in birds

Estimating the age of species or their component lineages based on sequence data is crucial for many studies in avian evolutionary biology. Although calibrations of the molecular clock in birds have been performed almost exclusively using cytochrome b (cyt b), they are commonly extrapolated to other mitochondrial genes. The existence of a large, standardized cytochrome c oxidase subunit I (COI) library generated as a result of the DNA barcoding initiative provides the opportunity to obtain a calibration for this mitochondrial gene in birds. In this study we compare the evolutionary rate of COI relative to cyt b across ten different avian orders. We obtained divergence estimates for both genes from nearly 300 phylogenetically independent pairs of species through the analysis of almost 5000 public sequences. For each pair of species we calculated the difference in divergence between COI and cyt b. Our results indicate that COI evolves on average 14% slower than cyt b, but also reveal considerable variation both among and within avian orders, precluding the use of this value as a standard adjustment for the COI molecular clock for birds. Our findings suggest that this variation is partially explained by a clear negative relationship between the difference in divergence in these genes and the age of species. Distances for cyt b are higher than those for COI for closely related species, but the values become similar as the divergence between the species increases. This appears to be the result of a stronger pattern of negative time‐dependency in the rate of cyt b than in that of COI, a difference that could be related to lower functional constraints on a small number of sites in cyt b that allow it to initially accumulate mutations more rapidly than COI.