New evidence for the recent divergence of Devil's Hole pupfish and the plausibility of elevated mutation rates in endangered taxa

Sağlam etal. recently argued that the Devil's Hole pupfish (Cyprinodon diabolis), a conservation icon with the smallest known species range, was isolated 60 kya based on a new genomic data set. If true, this would be a radically long timescale for any species to persist at population sizes <500 individuals, in contrast to conservation genetics theory. However, here we argue that their analyses and interpretation are inappropriate. They placed highly restrictive prior distributions on divergence times, which do not appropriately model the large uncertainty and result in removing nearly all uncertainty from their analyses, and chose among models by assuming that pupfishes exhibit human mutation rates. We reanalysed their data with their same methods, only using an informative prior for the plausible range of mutation rates observed across vertebrates, including an estimate of the genomewide mutation rate from a pedigree analysis of cichlid fishes. In fact, Saglam etal.'s phylogenetic data support much younger median divergence times for C.diabolis, ranging from 6.2 to 19.9 kya, overlapping with our previous phylogenetic divergence time estimates of 2.5-6.5 kya. There are many reasons to suspect an even younger age and higher mutation rate in C.diabolis, as we previously estimated, due to their high metabolism, small adult size, small population size and severe environmental stressors. In conclusion, our results highlight the need for measuring mutation rate in this fascinating species and suggest that the ages of endangered taxa present in small, isolated populations may frequently be overestimated.