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Biogeography and Evolution of the Galapagos This account includes illustrations and information from the following article: INTRODUCTION The 900 km of ocean separating the Galapagos from the nearest mainland represents a formidable biogeographic challenge. Most evolutionists and biogeographers account for the terrestrial animals and plants of the islands as the result of dispersal over the surrounding ocean, mostly from America. Darwin also suggested there may have been some kind of formly continuous land connecting the Galapagos with America. With the islands being entirely volcanic the land connection theory was generally not favored. Traditional Galapagos model Alternative Galapagos model East Pacific tracks Panbiogeographic analysis confirms a precise geographic separation of different distribution patterns regardless of differences in the dispersal ability of individual taxa. As the geographic intersection between these vicariant tracks, the Galapagos is biogeographically nodal with respect to the biogeographic history of the region. Even taxa with trans-Pacific distributions may locally conform to distinct tracks with respect to the eastern Pacific (e.g. Nicotiana) and Caribbean (e.g. Cymatopus). These alternative track relationships are indicated below as a general "Pacific track" merging with American tracks with respect to the Galapagos.
Geology and tectonics Discovery of the 'Galapagos Gore,' a geological and topographical region with its apex at the triple junction of the Pacific, Cocos, and Pacific plates corroborated Croizat's (1958) prediction for an important tectonic structure being associated with the Galapagos. The east Pacific rise and Galapagos rift system, extending between the triple junction and the PanaMyrfracture zone, confirm a tectonic connection between Galapagos, Central America and western North and South America paralleling the biogeographic relationships between these areas. Given the nodal position of the Galapagos, Croizat predicted a former Pacific 'shoreline' that included Galapagos and also extended out into the Pacific south to Chile and northwest to Hawaii. Croizat referred to this former land-limit as a mobile coastline extending along the rims of geosynclinal forelands and insular island clusters. Historical models for eastern Pacific geology are in a greater state of flux and uncertainty than one may perceive solely from the Galapagos literature. Much of geological debate for the eastern Pacific is centered on the origin and evolution of the Caribbean plate. A widely circulated model suggests the Caribbean plate formed from oceanic flood basalt produced at the Galapagos hotspot about 90 Myr with two major 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 Myr Island arc model for an eastern Pacific origin of the Caribbean plate (Pindell, 1993, Fig. 6H) at 76 Myr with the Panama-Costa Rica arc along the western boundary at an undetermined longitude, and the Antillean arc along the eastern boundary.(Illustration to be added)
Spatial correlation of Galapagos tracks with eastern Pacific tectonics represents the best available evidence of direct contact between the Galapagos hotspot and proposed former Pacific-Caribbean island arcs. 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. Sequential dispersal onto newer volcanoes by direct geographic contact or geographic proximity adjacent volcanic islands would be necessary for the island arc biota to persist at the Galapagos into the present. Croizat referred to the Galapagos as a "fragment of geological America" reduced to a series of volcanic islands in the early Tertiary with animal and plant life corresponding to the 'American' biota of that time. The geosynclinal mechanism proposed by Croizat is compatible with the plate tectonic depiction of submarine trenches associated with volcanic island arcs and it is also compatible with Caribbean plate tectonic models that allow for indirect contact between the Galapagos hotspot and American mainland mediated by island arcs transport. Even though volcanic activity is responsible for the geological formation and persistence of the Galapagos, organisms inherited from an island arc or other mobile Pacific landscape represent an older 'geological' layer re-deposited onto a younger geological stratigraphy. Pacific history Historical geological models for the Pacific represent theoretical and methodological developments in historical geology that attempt to address circum-Pacific terranes as a general, rather than local, geological problem. The panbiogeographic approach similarly links the origin of Galapagos endemics to the origin of Galapagos relatives rather than treating each in isolation. The presence of Pitnus in Galapagos, for example, is not simply a Galapagos matter, but a more general problem of why Pitnus also happens to be in the Caribbean and Australia. This global context is exemplified by the distribution of iurid scorpions where the western American and Galapagos range is disjunct with respect to the eastern Mediterranean and Caucasus region. This disjunction may be due to the opening of the Atlantic basin with subsequent extinction in eastern North and South America. However, the localization of the family to western North and South America conforms to a common pattern for Pacific groups, suggesting the Galapagos Iuridae originated as elements of a Pacific group possibly connected with the Mediterranean-Caucasus outlier via the Tethys geosyncline. Evolutionary age Molecular calibrations that rely on the fossil record as an accurate or realistic estimator of lineage duration and divergence generally fail to consider or acknowledge the oldest fossil records as the minimal phylogenetic age of a lineage. Fossils may even lack the full repertoire of phylogenetic characters necessary for their taxonomic placement with respect to modern groups. Divergence calibrations based on calibrations for other lineages removes the procedure from immediate verification other than accepting the authority of the sources as being accurate and precise. These problems suggest molecular techniques, while emphasizing the antiquity of Galapagos taxa relative to the age of the modern islands are methodologically suspect, or at the very least problematic, and do not constrain with any reliability the upper phylogenetic age of the Galapagos lineages. 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 Myr molecular divergence estimate for Tropidurus based on "geological" as well as fossil information. 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 with an estimated accretion period between late Cretaceous and Eocene time (Feininger, 1987; Kellogg & Vega, 1995). It may be possible in the future to develop a statistical test of association between Galapagos endemics and their relatives by comparing endemic and non-endemic Galapagos taxa and their track relationships with respect to allochthonous terranes. CONCLUSIONS (1) A comprehensive Galapagos biogeography is needed to assess the relative merits of geologically or |
