  Growing evidence suggests that many vertebrate lineages are evolving at
significantly different rates. As a first approximation of evolutionary rates,
we assessed the amount of neutral (dS) and non-neutral (dN) substitutions that
have accumulated within and across sister clades since the time of their
divergence. We found that in fish, tetraodontiformes (pufferfish) are evolving
at faster rates than cypriniformes (fresh water teleosts), while cypriniformes
are evolving faster than elasmobranchs (sharks, skates and rays). A similar
rate variation was observed in salamanders: plethodontidae were found to evolve
at a rate nearly two fold faster than the hydromantes lineage. We discuss
possible explanations for this striking variation in substitution rates among
different vertebrate lineages that occupy widely diverse habitats and niches.
