[Taxacom] Galapagos tortoise biogeography

John Grehan calabar.john at gmail.com
Mon Jul 8 09:57:18 CDT 2019

I incorporated some of the recent information into my notes (not formatted
here) regarding the Galapagos tortoises and have appended below my current
draft notes (emphasis on draft). Happy to have any corrections of factual
errors, misrepresentations of other work or other such blunders. As this is
part of a larger paper there are elements of justification and context
(methodology, geology etc.) that are not included within this specific

John Grehan

(17) Eastern Pacific affinities of Galapagos Chelonoidis (Reptilia:
A systematic analysis by Caccone et al. (1999) identified the Galapagos
tortoise Chelonoidis nigra (species complex) as the sister species of
Chelonoidis chilensis followed by the species pair C. carbonaria and C.
denticulata (Caccone et al. 1999). In this systematic arrangement the
closest relative of the Galapagos species is disjunct to Argentina and
Bolivia east of the Andes (Fig. 25a). The species C. carbonaria and C.
denticulata are distributed mostly in or around the Amazon basin and only
C. denticulata has a range extending to the Pacific shore in Panama. This
distribution is consistent with vicariance of a widespread ancestor with
the first phylogenetic break between the Galapagos-Argentina ancestor and a
mostly northeastern ancestor of the mostly Amazonian species.

Because the Galapagos archipelago has never been connected to the mainland,
Caccone et al. (1999) concluded that the ancestral tortoises “probably”
reached the islands by rafting up the coast of Chile and Peru on the
Humboldt Current. They did not offer an explanation of how these tortoises
found their way to the Humboldt Current from Argentina. A divergence
estimate mostly between 6-12 Ma was calculated by Caccone et al. (1999)
using published mtDNA divergence rates for other turtles and vertebrate
ectotherms while Galapagos islands age calibrations led to a divergence
estimate of 3.2 Ma (in Poulakakis et al 2012), but this precludes the
possibility that the tortoises first colonized older islands that have now
submerged. Both the fossil and island calibrated estimates can only
represent minimum ages for the origin of the Galapagos tortoises. Ernst
(1998) noted that the fossil Geochelone gringorum (Simpson 1942) from
Patagonian Argentina is closely related to G. chilensis with only minor
morphological differences and this early to Mid Miocene species has been
considered to be basal to the Galapagos- C. chilensis group (Vlachos &
Sterli 2017).

A modified phylogeny was recently proposed by Kehimaier et al. (2017) which
included the Caribbean fossil  Chelonoidis alburyorum as the sister taxon
to the Galapagos-C. chilensis clade (Fig. 25b). This pattern is also
consistent with vicariance where the Galapagos is involved with a standard
relationship with the Caribbean as well as South America. The
Caribbean-Galapagos-southern South America clade is allopatric to the C.
carbonaria-C. denticulata clade. Divergence of C. alburyorum could have
resulted from the tectonic displacement of the Caribbean from the eastern
Pacific resulting in an initial phylogenetic break from a Galapagos-South
America ancestor. Kehimaier et al. (2017) estimated that C. alburyorum
diverged from the last common ancestor of C. chilensis and the Galapagos
tortoises about 15.5 mya, whereas C. chilensis and the Galapagos C. vicina
diverged approximately 12 mya. But since these estimates were calibrated by
fossils they can only represent minimum dates. Kehimaier et al. (2017) note
that overseas dispersal from Africa has been postulated to explain
Chelinoidis in South America but the proposal by Le et al (2006) does not
present any empirical evidence while their DIVA analysis has center of
origin reasoning built into the algorithm - i.e. certain patterns of
relationship will be coded as chance dispersal rather than vicariance
(Heads 2017).

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