New insights into the origin of the Galápagos tortoises with a tip-dated analysis of Testudinidae

ABSTRACT

The giant tortoises of the Galápagos Archipelago influenced Darwin’s early thinking on transmutation and now constitute a famous example of island-based speciation and evolutionary dynamics. Questions regarding their evolutionary origin still exist and include whether their giant size evolved before or after the lineage arrived in the Galápagos from the South American continent. Here we present new evidence supporting the ‘before’ hypothesis based on previously reported fossil specimens from the Late Pleistocene of Ecuador. Despite the isolated and fragmentary nature of these tortoise fossils, phylogenetically informative features are preserved. A tip-dated phylogenetic analysis recovers the Ecuador form as the sister taxon to the Galápagos lineage, thus supporting the existence of a closely related, giant tortoise population at approximately the same time and place that the Humboldt Current is thought to have carried the ancestral Galápagos tortoise to the archipelago. The phylogeny also raises the possibility that the same evolution of continental gigantism also facilitated the dispersal of South American tortoises into the Caribbean Basin where they diversified before succumbing to Holocene extinction.

DISCLOSURE STATEMENT

No potential conflict of interest was reported by the author(s).

ACKNOWLEDGMENTS

We thank J. Abella and the Museo Paleontológico Megaterio for specimen access. O. Torres-Carvajal, D. Cerio, W. Foster, and J. Wilson provided many helpful discussions and comments. We also thank two anonymous reviewers whose critical observations and suggestions led to a much-improved paper. We thank T. Lyson and all JVP editors for their efforts.

AUTHOR CONTRIBUTIONS

All authors contributed to project design, data interpretation, and approved the final manuscript. FT and GB wrote the manuscript. FT and EJH ran the phylogenetic analyses.

DATA ACCESS STATEMENT

The datasets supporting this article have been uploaded as part of the supplementary material.

SUPPLEMENTARY FILES

Testud_FBD_R3.xml: file for analysis ready to be run in BEAST v2.7.4, which includes all the settings and data for the tip-dated analysis.

Testud_FBD_R3.log: file with sampled parameter estimates that are recorded during the run of Testud_FBD_R3.xml in BEAST v2.7.4.

Testud_FBD_R3.trees: file with a posterior sample of phylogenetic time-trees that result from using Testud_FBD_R3.xml in BEAST v2.7.4.

Testud_FBD_R3.tre: Summary tree that results from using Treeanotator on the Testud_FBD_R3.trees file.

NEXUS_file_parsimony_analysis_R3.nex: NEXUS file with data for parsimony analysis.

TNT_file_parsimony_analysis_R3.tnt: TNT file with the same data as the “NEXUS_file_parsimony_analysis_R3.nex” file that can be used for analysis in TNT.

MPTs_R3.tre: File with most parsimonious trees after running the parsimony analysis.

TNT_file_parsimony_analysis_constrained_R3.tnt: TNT file with the same data as the “NEXUS_file_parsimony_analysis_R3.nex” file that contains the enforced constraints and can be used for analysis in TNT.

MPTs_contrained_R3.tre: File with most parsimonious trees after running the parsimony analysis with constraints.

start_tree_data_R3.phy: Data used for maximum likelihood analysis in RAxML.

RAxML_bestTree.start_tree_rooted_R3.tre: Tree file resulting from the maximum likelihood analysis.

RAxML_info.start_tree_R3.tre: File with information of the RAxML run.