Unravelling the peculiarities of island life: vicariance, dispersal and the diversification of the extinct and extant giant Galápagos tortoises

Abstract

In isolated oceanic islands, colonization patterns are often interpreted as resulting from dispersal rather than vicariant events. Such inferences may not be appropriate when island associations change over time and new islands do not form in a simple linear trend. Further complexity in the phylogeography of ocean islands arises when dealing with endangered taxa as extinctions, uncertainty on the number of evolutionary 'units', and human activities can obscure the progression of colonization events. Here, we address these issues through a reconstruction of the evolutionary history of giant Galápagos tortoises, integrating DNA data from extinct and extant species with information on recent human activities and newly available geological data. Our results show that only three of the five extinct or nearly extinct species should be considered independent evolutionary units. Dispersal from mainland South America started at approximately 3.2 Ma after the emergence of the two oldest islands of San Cristobal and Española. Dispersal from older to younger islands began approximately 1.74 Ma and was followed by multiple colonizations from different sources within the archipelago. Vicariant events, spurred by island formation, coalescence, and separation, contributed to lineage diversifications on Pinzón and Floreana dating from 1.26 and 0.85 Ma. This work provides an example of how to reconstruct the history of endangered taxa in spite of extinctions and human-mediated dispersal events and highlights the need to take into account both vicariance and dispersal when dealing with organisms from islands whose associations are not simply explained by a linear emergence model.

Amplitudes of orbitally induced climatic cycles and patterns of hominin speciation

“…Climate change, particularly at the rapid, half-precessional pace detected by Maslin et al (2005), could have acted as a 'vicariance knife' (Rosenzweig, 1995), dividing areas of existing habitat into smaller, isolated refugia tied to limiting resources. Vicariance is often associated with speciation in allopatry, but it may also lead to extinction when it prompts the multiple fission of already relatively small populations such as those considered likely for PlioPleistocene hominins (Keppel et al, 2009;Poulakakis et al, 2012). The extent to which environmental remodelling affects a particular species will depend upon that species' ability to tolerate habitat zones between those that comprise its ideal habitat; thus a specialist might experience severe vicariance effects during a period of climatic change that leads to only moderate effects on a closely related generalist.…”

Invaders of pollination networks in the Galápagos Islands: emergence of novel communities

“…This young volcanic archipelago (0.035 -4 Ma [32]) comprises 18 islands larger than 1 km 2 and numerous islets. Arid zones dominate the lowland on all islands (around 60% of total land area); they show the highest plant diversity and endemicity, and a relatively low fraction of alien plant species compared with the transition and humid zones [29].…”

“…We built a lowland pollination network for each of five islands (figure 1), differing in age and degree of disturbance, from youngest/pristine to oldest/disturbed: Fernandina (0.035-0.07 Ma), Pinta (earlier than 0.7 Ma, undetermined maximum), Santiago (0.8-1.4 Ma), Santa Cruz (1.1-2.3 Ma) and San Cristóbal (2.4-4 Ma) [32].…”

Twenty Years of Molecular Biogeography in the West Mediterranean Islands of Corsica and Sardinia: Lessons Learnt and Future Prospects

“…Remote oceanic islands can only be colonised through dispersal while both vicariance and dispersal play a role in determining the biological diversity of continental islands [49,50]. The contribution of either factor is related to a large extent to the dispersal ability of organisms.…”