Mid‐Oligocene (30 Ma)‐gap narrowed but still a clear deep‐water passage formed by oceanic crust
There are two basic versions of the centre of origin hypothesis. The first is what Bellwood et al. (2012) have called the ‘spreading dye model’ in which species originating within the Coral Triangle expanded their geographic range by chance. The second version states that new species with superior competitive abilities displaced older, inferior competitors towards the periphery of the region’s boundaries. Thus, in theory, newer species will be at the centre of the boundary and that there would be a “peripheral halo” of predominantly older species that will also have greater geographic range.
The second hypothesis is the ‘Centre of Overlap’ (or Vicariance) model. This idea states that the centre of biodiversity consists of overlapping distribution ranges that extend into either the Pacific or the Indian Ocean resulting from either larval dispersal or ancient plate tectonics. These biogeographic boundaries are well recognised, but any subsequent expansion or movement of ranges would lead to an overlap of geographic ranges and a localised increase in species richness in the overlap zone (Santini and Winterbottom 2002). The inherent complexity of the physical environment within the Coral Triangle makes it highly likely that this area exhibited many such divisions that were susceptible to frequent changes through time. It may thus be considered a ‘dynamic mosaic’ (Bellwood et al. 2012) of constantly changing distributions driven by continual climatic, geologic, and oceanographic processes. This hypothesis has found some support from reef fish (Woodland 1986), corals (Wallace 2002), crustaceans (Fransen 2007), and gastropods (Reid et al. 2006).
The third hypothesis is the ‘Centre of Accumulation’ model that is the opposite of the centre of origin hypothesis. It proposes that species arose in peripheral locations, around, or at some distance from the margin of the centre and that they subsequently moved into the centre. New species can be from anywhere outside the centre. It does not require overlap with related taxa. There are two distinct versions of this hypothesis. First, the centre of accumulation by individuals places the emphasis on isolation on peripheral oceanic islands in underpinning the speciation process. Second, the centre of accumulation by faunas reflects the accumulation of entire faunas on moving land masses. In this version, species richness may be enhanced by the merging of entire faunas via the merging of island arcs on the north coast of New Guinea with the numerous land fragments from Asia, Gondwana, and Australia.
The fourth hypothesis is the ‘Centre of Survival’ idea that is a composite hypothesis emphasising persistence and survival in an area rather than on origin of the species in question. The key aspect of this hypothesis is regional variation in the relative rates of extinction, and it makes no assumptions about the rates or location of origin of species. Speciation may thus have occurred anywhere. The centre of biodiversity is just an area of survival with species extinctions outside the centre’s boundaries. The maintenance of habitat diversity and the availability of sufficient abundance for each species is an important condition for this hypothesis.
The fifth hypothesis is the ‘Centre of Mid‐Domain Overlaps’ idea that is a variation on the second hypothesis. In this version, a maximum in species richness in the middle of a geographical area has formed by overlying randomised distributions of the locations of individual geographic ranges within species groups. The maximum is predicted to occur where the probability of maximum of species range overlaps is highest, that is, if the ranges are randomly placed within the Indo‐Pacific, the resultant pattern of species richness forms a peak in the middle. The hotspot is thus the result of random placement of species’ geographic ranges (Bellwood et al. 2012).
The sixth hypothesis is ‘Reticulate Evolution’ that is arguably the main mechanism of evolutionary change in most marine taxa (Veron et al. 2009). It recognises “(a) that currents are both genetic barriers (as in vicariance) and paths of genetic connectivity, (b) that species fuse as well as divide over time and space, (c) that species are generally not isolated units and (d) that evolution is driven by the physical environment (especially ocean currents) rather than biological mechanisms (e.g., competition). Furthermore, reticulate evolution does not deny the existence of Darwinian evolution which could become uppermost were genetic mixing to weaken sufficiently to create isolated gene pools. This would allow evolution to occur through biological selection. Conditions which promote reticulate evolution are at a maximum in the Coral Triangle (sometimes referred to as an ‘evolutionary cauldron’) because of habitat diversity and the ever‐changing complexity of ocean surface currents.”
Another hypothesis based on fossil and molecular evidence is the idea of ‘hopping hotspots’ (Renema et al. 2008). There is good evidence for the fact that during the past 50 Ma, there have been at least three marine biodiversity hotspots that have moved across half the globe, with their timing and locations coinciding with major movements of the earth’s plates. Based on generic diversity of large benthic foraminifera, three successive movements of biodiversity hotspots have been identified: (a) in the late Middle Eocene (42–39 Ma), (b) in the early Miocene (23–16 Ma), and (c) in the present day (Renema et al. 2008). These are known as the West Tethys, Arabian, and Indo‐Australian Archipelago biodiversity hotspots. During the Eocene, diversity peaked in SW Europe, NW Africa, and along the eastern shore of the Arabian Peninsula, Pakistan, and West India. The fossil record of mangroves and reef corals suggests maximal global diversity in the West Tethyan hotspot. By the late Eocene, the highest diversity was recorded in the Arabian hotspot that has an overlapping taxonomic
