Whole-genome sequencing confirms multiple species of Galapagos giant tortoises

Galapagos giant tortoises are endemic to the Galapagos Archipelago, where
they are found in isolated populations. While these populations are widely
considered distinguishable in morphology, behavior, and genetics, the
recent divergence of these taxa has made their status as species
controversial. Here, we apply multispecies coalescent methods for species
delimitation to whole genome resequencing data from 38 tortoises across
all 13 extant taxa to assess support for delimiting these taxa as species.
In contrast to previous studies based solely on divergence time, we find
strong evidence to reject the hypothesis that all Galapagos giant
tortoises belong to a single species. Instead, a conservative
interpretation of model-based and divergence-based results indicates that
these taxa form a species complex consisting of a minimum of 9 species,
with most analyses supporting 13 species. There is mixed support for the
species status of taxa living on the same island, with some methods
delimiting them as separate species and others suggesting multiple
populations of a single species per island. These results make clear that
Galapagos giant tortoise taxa represent different stages in the process of
speciation, with some taxa further along in that evolutionary process than
others. Our study provides insight into the complex process of speciation
on islands, which is urgently needed given the threatened status of island
species around the world.