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Biogeography of the Leguminosae

World map showing four generalised legume distribution patterns at the biome level: Red= Succulent (S) biome; Brown =Grass (G) biome; Green = Rainforest (R) biome and Blue = Temperate (T) biome.

This project aims to address the following questions:  1) What does the evidence of legume distributions and the phylogeny of the family say about a wet versus dry megathermal origin of Leguminosae?  2) Can the crown clade ages of a range of trans-oceanic disjunctions between sister legume taxa inform on whether vicariance (continental history) or dispersal, is largely responsible for the existing pattern of legume distributions?

Four global legume biomes (SUCCULENT, GRASS, RAINFOREST and TEMPERATE) were delimited by Schrire et al. (2005a, b), which at the broadest level have greatly increased understanding of biogeographical patterns in the Leguminosae. The long-standing West Gondwana ‘equatorial megathermal’ hypothesis for the origin of legumes was rejected for the following reasons. The vicariance analyses of Schrire et al. (2005a) firmly place the semi-arid SUCCULENT biome as the only biome diagnosing the critical nodes of diversification in the legume phylogeny. A dry origin for legumes is in accordance with key morphological synapomorphies and a high nitrogen metabolism in the family. Lineages confined to the SUCCULENT biome gave rise to sublineages occupying all other biomes and the RAINFOREST biome taxa are primarily derived from dry SUCCULENT and GRASS biome taxa. The essentially Tethyan Seaway, SUCCULENT biome (i.e., excluding the extensions into the Southern Hemisphere) largely overlaps with the belt of seasonally dry to arid tropical climate shown by Scotese (2001), which links the Caribbean and C America with N Africa (including the Horn) and extends to Asia. This palaeoclimatic belt also coincides with the known spatial and temporal distribution of legume fossils (Herendeen et al., 1992).

An argument can be supported for vicariance as an explanation for the present disjunct distributions of a number of Palaeocene to Oligocene-aged clade diversifications in legumes, e.g. most tribal and many super-generic-level clades. At the biome (metacommunity)-level, however, the processes of ecological drift, immigration, extinction, and resident speciation have played out over such enormous spatial and temporal scales that any historical signal of the Palaeogene is likely to have been largely swamped if not obliterated altogether (Hubbell, 2001). The majority of trans-oceanic crown clades in legumes are Miocene to Holocene in origin (Lavin et al., 2004), thus much of the present diversity in the family is Neogene in age. This is too recent for disjunctions to be explained by continental history, e.g., large-scale continental movements and land-bridges. The historical legacy of these metacommunities is thus one of dispersal assembly within similar ecological settings world-wide.

During the next phase of the project, a college-based sandwich course student position has been allocated from 09/2006 to 08/2007 to provide a scoping study of how to develop Legumes of the World Online (see separate legume project), with a major aim being to enable the updating of biogeographical and other taxonomic legume data electronically in conjunction with Legumes of the World (Lewis et al., 2005) and ILDIS (International Legume Database and Information Service).

This project started in 2001 and is ongoing.  So far the products/outputs include publication of a ground-breaking scientific paper, one conference proceedings paper and one book chapter.

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