Community assembly and in-situ ecological and phenotypic diversification in the Socotra Archipelago
A lot of attention has been invested to study the mode and tempo of diversification in Oceanic Islands. We have learn from ionic examples of explosive radiations how trans-oceanic dispersals rapidly fill empty ecological niches, initially by colonization from a source pool, and eventually by in-situ diversification. However, considerably less attention has been devoted to study diversification in Continental Islands, which are essentially fragments of the continental shelf that have become isolated from the mainland through geological processes or rising sea levels. On such island types, since they have been in contact to continental landmass, the ecological niche space will already be largely filled at the point of the isolation (Gillespie & Roderick, 2002). And, opposing to the Oceanic Islands, within Continental Islands is largely believed that the number of species will, if anything, decline through the process of “relaxation” (Whittaker and Fernandez - Palacios, 2007).
Still, high levels of endemism and diversity can be found in some Continental Island systems. Thus, the question remains: does diversification occur within the island, or somewhere else (in the continent and drift with the island)? The Archipelago of Socotra offers a great opportunity to study the evolution of endemism formation, since although its origin is continental; its long isolation and complex topography, together with a very dynamic climate regime have created elevated levels of endemism (Cheung et al. 2006). With a very long geological history of about 700 million years old, and a very long isolation history of about 20 million year old, endemic biota has had ample opportunity to evolved in either the continent or within the island (Fig. 1). A special interesting group to address this the roles of isolation, dispersal, and niche in patterns of island diversification is the diurnal semaphore geckos Pristurus (Pristurus Rüppell, 1835). At least seven recognised endemic species of Pristurus occur in the Archipelago of Socotra (Arnold, 2009). Morphologically primitive forms occur mostly in rocky surfaces while, similar to the mainland radiation, at least two species (P. guichardi and P. obsti) moved independently to the trunks and branches of trees (Arnold, 2009; Razetti et al. 2011).
Still, high levels of endemism and diversity can be found in some Continental Island systems. Thus, the question remains: does diversification occur within the island, or somewhere else (in the continent and drift with the island)? The Archipelago of Socotra offers a great opportunity to study the evolution of endemism formation, since although its origin is continental; its long isolation and complex topography, together with a very dynamic climate regime have created elevated levels of endemism (Cheung et al. 2006). With a very long geological history of about 700 million years old, and a very long isolation history of about 20 million year old, endemic biota has had ample opportunity to evolved in either the continent or within the island (Fig. 1). A special interesting group to address this the roles of isolation, dispersal, and niche in patterns of island diversification is the diurnal semaphore geckos Pristurus (Pristurus Rüppell, 1835). At least seven recognised endemic species of Pristurus occur in the Archipelago of Socotra (Arnold, 2009). Morphologically primitive forms occur mostly in rocky surfaces while, similar to the mainland radiation, at least two species (P. guichardi and P. obsti) moved independently to the trunks and branches of trees (Arnold, 2009; Razetti et al. 2011).
Here we attempted to explain the mode and tempo of the diversification process of Pristurus in Socotra. Twenty Pristurus species were placed in the phylogenetic context of the main Gekkota tree and the tree was time-calibrated to date the Socotra node and further speciation events. In addition, a phylogeographic framework was employed to study the speciation process of the endemic Socotra species.
We confirm that the semaphore geckos Pristurus arrive to the archipelago of Socotra in at least two different occasions. All but one of the species of the archipielago falls in a monophyletic group in the island of Socotra together with the Two Brothers (Samha and Darsa) with at least six endemic species. And, as already recognised by Papenfuss et al. (2009), P. abdelkuri represents an independent colonisation in the Abd al Kuri Island; 105km southwest of Socotra. The time-dated phylogeny seems to demonstrate that present-day Pristurus arrive to the islands after it detached from the continent. However there is a great uncertainty associated with this time-measure, and with the geological estimates of exactly when the archipelago became isolated. Further speciation events can definitely be placed to occur in-situ within the islands. In terms of within island diversity, great genetic intraspecific divergence was found in the two more widespread species. P. socotranus subdivides into six highly supported clades, while P. insignis seemed to contain three highly differentiated clades (Fig. 2).
We confirm that the semaphore geckos Pristurus arrive to the archipelago of Socotra in at least two different occasions. All but one of the species of the archipielago falls in a monophyletic group in the island of Socotra together with the Two Brothers (Samha and Darsa) with at least six endemic species. And, as already recognised by Papenfuss et al. (2009), P. abdelkuri represents an independent colonisation in the Abd al Kuri Island; 105km southwest of Socotra. The time-dated phylogeny seems to demonstrate that present-day Pristurus arrive to the islands after it detached from the continent. However there is a great uncertainty associated with this time-measure, and with the geological estimates of exactly when the archipelago became isolated. Further speciation events can definitely be placed to occur in-situ within the islands. In terms of within island diversity, great genetic intraspecific divergence was found in the two more widespread species. P. socotranus subdivides into six highly supported clades, while P. insignis seemed to contain three highly differentiated clades (Fig. 2).
Figure 2. Intra-island genetic diversity of Pristurus. Thirteen well differentiated mitochondrial lineages were found within the endemic species of Pristurus in the Island of Socotra and Samha.. All the clades are geographically distinguishable. a) P. socotranus, b) P. samhaensis, c) P. guichardi/ P. obsti, and d) P. insignis/P. insignoides. The colours correspond to the mtDNA lineages |
At least 14 endemic genetic lineages were identified in the entire archipelago of Socotra, 13 of which seemed to evolve in-situ after the islands became isolated. While this is a remarkable level of diversity for a continental island of the size of Socotra (132 x 50 km.), the evolutionary forces that drove speciation in the island are not clear. On one hand, the isolation history of Socotra is very long, and the tempo of the speciation process seems to be quite slow, which is not consistent with explosive islands radiations. On the other hand, ecological drivers are likely to play a role since the topographic complexity and climatic variation in the island is outstanding. Socotra posses an altitudinal range between 9 meters below sea level, to 1,440 meters above sea level, and its climate is highly dynamic (i.e. reversing monsoon, El Niño, the Indian Ocean Dipole and episodic occurrence of tropical cyclones). In addition, the species composition of Pristurus geckos in Socotra seem to respond to a differential adaptation to particular ecological conditions; differences in body size and in habitat preference are persistent between clades (Razetti et al. 2011).
The relative effects of vicariance, dispersal, niche evolution and island size and shape that account for the high levels of diversity of Pristurus observed in Socotra were assessed (Garcia-Porta J, Morales H, et al. in preparation). Stay tuned for the publication!
Joan Garcia-Porta, Hernan Morales, Elena Gómez-Díaz, Roberto Sindaco, Fabio Pupin, Mauro Fasola & Salvador Carranza.. (in preparation). Community assembly and in-situ ecological and phenotypic diversification in continental islands. What do we learn from the geckos of the Socotra Archipelago?
The relative effects of vicariance, dispersal, niche evolution and island size and shape that account for the high levels of diversity of Pristurus observed in Socotra were assessed (Garcia-Porta J, Morales H, et al. in preparation). Stay tuned for the publication!
Joan Garcia-Porta, Hernan Morales, Elena Gómez-Díaz, Roberto Sindaco, Fabio Pupin, Mauro Fasola & Salvador Carranza.. (in preparation). Community assembly and in-situ ecological and phenotypic diversification in continental islands. What do we learn from the geckos of the Socotra Archipelago?