  Very low levels of genetic diversity have been reported in vertebrates with
large genomes, notably salamanders and lungfish [1-3]. Interpreting differences
in heterozygosity, which reflects genetic diversity in a population, is
complicated because levels of heterozygosity vary widely between conspecific
populations, and correlate with many different physiological and demographic
variables such as body size and effective population size. Here we return to
the question of genetic variability in salamanders and report on the
relationship between evolutionary rates and genome sizes in five different
salamander families. We found that rates of evolution are exceptionally low in
salamanders as a group. Evolutionary rates are as low as those reported for
cartilaginous fish, which have the slowest rates recorded so far in vertebrates
[4]. We also found that, independent of life history, salamanders with the
smallest genomes (14 pg) are evolving at rates two to three times faster than
salamanders with the largest genomes (>50 pg). After accounting for
evolutionary duration, we conclude that more recently evolved species have
correspondingly smaller genomes compared to older taxa and concomitantly higher
rates of mutation and evolution.
