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Whole-genome sequencing confirms multiple species of Galapagos giant tortoises

Gaughran, Stephen1; Gray, Rachel2; Ochoa, Alexander3; Jones, Menna4; Fusco, Nicole3; Miller, Joshua M5; Poulakakis, Nikos6; de Queiroz, Kevin7; Caccone, Adalgisa3; Jensen, Evelyn L2

Published Nov 24, 2024 on Dryad. https://doi.org/10.5061/dryad.b2rbnzsqs

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Abstract

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.