|Biogeography and Evolution of the Galapagos|
This account includes illustrations and information from Grehan (2001).
The 900 km of ocean separating the Galapagos from the nearest mainland represents a formidable biogeographic challenge. Most evolutionists and biogeographers explain the origin of the terrestrial animals and plants on the Galapagos as the result of dispersal over the surrounding ocean, mostly from America. Darwin also suggested there may have been some kind of former continuous land connection between the Galapagos and the American continent. This theory did not find favor because there was no geological evidence and the islands appeared to be entirely volcanic in origin.
Traditional Galapagos model
This geological reconstruction allowed for overwater dispersal as the initial origin of colonists, with a series of subsequent dispersal events onto newly emergent volcanoes while the original islands were transported east and eventually submerged. This conveyer-belt mechanism conforms to a general model proposed for many Pacific island biotas, and was used to 'explain' Galapagos organisms with molecular divergence estimates that exceeded the age of the modern islands.
There are at least two distinct biogeographic relationships between the Galapagos and the American mainland: a track between western North and South America via the Galapagos, and between the Galapagos and the Caribbean, sometimes including Central America.
Intersection between the two tracks at the Galapagos led to the biogeographic prediction that the Galapagos Islands were associated with a major tectonic center in the Eastern Pacific, and that there were former island clusters in the Eastern Pacific from which at least some of the Galapagos biota originated.
Geology and tectonics
Historical models for eastern Pacific geology are in a greater state of flux and uncertainty than one may perceive solely from the traditional Galapagos literature. A widely accepted geological model suggests the Caribbean plate formed from oceanic flood basalt produced at the Galapagos hotspot about 90 million years ago. Associated with the origin of the Caribbean plate in the Eastern Pacific are two island arc systems - an inner or eastern Greater Antillean arc positioned between Mexico and Ecuador at 90 Ma, and an outer or western Costa Rica-Panama island arc along the eastern boundary of the Caribbean plate between Mexico and Ecuador by 76 million years ago.
Where an island arc moved over the hotspot island arc biota could disperse onto the volcanic landscape while their relatives were transported eastwards until the arcs accreted with the American mainland. This model suggests the American relatives originated to the west of their current position rather than themselves representing eastern sources of the Galapagos biota as represented in traditional colonization models
With the earliest geological formation of the Galapagos hotspot at 90 Ma, the temporal geological window for island island-arc colonization of the Galapagos ranges from late Cretaceous through early and mid-Tertiary time. Falling well within this time frame is the 33-48 million year molecular divergence estimate for Tropidurus. Biogeographic correlation of distributions and tectonic patterns provide an alternative, and perhaps more accurate estimation of phylogenetic age than fossil-based methods (Craw et al., 1999). Five of the American Galapaganos species, for example, are confined to the Piñón terrane (Craw et al., 1999) - a geological system that originated in the Eastern Pacific with an estimated accretion period between late Cretaceous and Eocene time (Feininger, 1987; Kellogg & Vega, 1995).