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      2.2. From narrative dispersal accounts to event-based methods (EBM)

As seen in Chapter 1, the introduction of the idea of evolution (Darwin 1859) provided early biogeographers with an explanation of why geographic regions sharing similar environments harbor different biotas. Since continents did not move their positions over geological time, it must be organisms that moved over Earth’s surface to achieve their present distribution. These early biogeographic reconstructions were mostly narrative dispersal accounts. A paradigm changed in the mid-twentieth century with the introduction of the concept of “plate tectonics” (the 1960s). If a global process is responsible for the current distribution of biodiversity on Earth, we should see its effects in the form of congruence in biogeographic relationships across species. This general process was termed vicariance. The first analytical school, cladistic biogeography, aimed to find general patterns of relationships among areas of endemism, indicating shared biogeographic history (“area cladograms”, Nelson and Platnick 1981). Dispersal and extinction were considered processes that depend on biological characteristics intrinsic to the species and, therefore, cannot generate shared distribution patterns (Humphries and Parenti 1999). Cladistic biogeographic methods are allegedly process-free: inference of the area cladogram is done with no consideration to the biogeographical events that may have generated the pattern. If any, these are inferred a posteriori through comparison of the area cladogram with the individual species patterns (Brooks 2005). Uncoupling the inference of biogeographic patterns from the underlying evolutionary processes made it difficult to compare alternative biogeographic scenarios (Sanmartín 2012).

      The next biogeographic school was “event-based biogeography” (EBM, Ronquist 1997, 2003). Biogeographic processes or events are tied to weights or “costs”, and the analysis consists of finding the pattern of area relationships with the minimum cost in terms of these processes. Four biogeographic events are considered in EBMs (Figure 2.1): vicariance, duplication, dispersal and extinction. The last two are tied to a speciation event and have also been termed “partial dispersal”, or “sorting, extirpation, and range contraction” for partial extinction. Within dispersal, we may distinguish “jump dispersal”, where a lineage migrates from one area to another (A to B) followed by speciation, and “range expansion”, where a lineage expands its range, leading to a temporally widespread distribution (A to AB); the latter is termed “geodispersal” when it affects multiple lineages (Lieberman 2003). Two biogeographic events are not considered in EBMs because they leave no observable traces in the phylogeny, that is, no descendants survive in the ancestral range (Sanmartín 2012): “full dispersal”, colonization of an area that is not followed by speciation, and “full extinction”, when the lineage entirely disappears from its ancestral range, that is, lineage extinction (Figure 2.1).