glaciation was from several refugia in the peninsulas of Iberia, Italy, and the Balkans (Dumolin-Lapègue et al., 1997). In this case, each refugium was represented by a distinct haplotype lineage. Fine-scale phylogeographic analysis further indicated that chloroplast DNA polymorphisms are shared between several oak species and, in this case, are attributed to hybridization and introgression subsequent to the recolonization of Europe. Similarly, chloroplast DNA analysis has shown concordance between beech (Demesure et al., 1996) and black alder (King and Ferris, 1998) phylogeography. Both species are believed to have colonized Europe after glaciation from a refugium in the Carpathian Mountains. Moreover, the data indicate that an additional refugium for these species in Italy did not contribute to the recolonization of Europe. Similar concordance in phylogeographic patterns associated with postglacial spread is observed between plant species in the Pacific Northwest of North America. Soltis et al. (1997) have shown, via chloroplast DNA phylogenies, similar patterns in the structuring of variation among several different types of plants, including ferns, trees, and several members of the Saxifragaceae, suggesting that the present genetic structure of these species is strongly affected by their postglacial pattern of colonization.
We have used a genealogical approach in examining two questions involving the species M. esculenta (Euphorbiaceae): the origin of the staple root crop cassava (M. esculenta subsp. esculenta) and the phylogeography of cassava's closest wild relative (M. esculenta subsp. flabellifolia). Cassava (manioc) is the sixth most important crop in the world (Mann, 1997). It is the primary source of calories in sub-Saharan Africa and serves as the main carbohydrate source for over 500 million people in the tropics world-wide (Best and Henry, 1992; Cock, 1985). Cassava is mostly grown by subsistence farmers, and despite its global importance as a food crop, it has traditionally received less attention by researchers than have temperate cereal crops. One fundamental question that has remained unresolved concerns the crop's geographical and evolutionary origins. Cassava was traditionally proposed to be a “compilospecies” derived from multiple hybridizing progenitor species in the genus Manihot (Jennings, 1995; Sauer, 1993). Manihot includes ≈98 species occurring in both northern South America (≈80 spp.) and in Mexico/Central America (≈17 spp.); sites of domestication were proposed from much of this vast geographical area.
Traditional phylogenetic approaches were only partially successful in determining cassava's origin. Species of Manihot show low levels of diver-