The Origins and Taxonomy of Cassava

The origins of cassava (Manihot esculenta Crantz subspecies esculenta) have long been obscure. The three important questions to answer concern the botanical origin, (i.e. the wild species from which cassava descends), the geographical origin, (i.e. the area where the progenitor evolved in the geological past) and the agricultural origin (i.e. the area of initial cultivation of the wild ancestor by Amerindians). The core of the above argument rests on the assumption that if there is a living wild ancestor, its discovery would be likely to indicate the taxonomy of cassava. In turn, this would indicate the ancestry of the crop (the evolution of the ancestor and its phylogenetic relationships with related species) and the cradle of domestication. Current knowledge on the three topics shows that studies on the botanical origin of cassava have progressed far and stand on firm ground, those on the geographical origin have progressed and conjecture on the area where cultivation began has recently experienced a renewed surge following the appearance of novel ideas. Studies on the taxonomy of the genus Manihot in Brazil led unexpectedly to progress in matters with a bearing on the origins and phylogeny of cassava. Accumulated empirical knowledge derived from field experience culminated with the formulation of a classification in which the Brazilian species were arranged in groups. The model of classification proposed was thought to mirror some degree of phyletic kinship between the species (Fig. 1.1). A number of the prospective clades proposed in Fig. 1.1 (e.g. the Manihot nana group; the Manihot salicifolia group, etc.) match remarkably closely with former taxonomic classifications advanced for the same groups in earlier classic treatments including that of Rogers and Appan (1973). Group VI in particular, the M. esculenta group, has been tested extensively by the scientific community through genetic studies and phylogenetic investigations, which in turn influenced cladistic and taxonomic classifications. This group is particularly highlighted in the present review. No attempt has been made in Fig. 1.1 to suggest that the sequence of groups presented represents evolution towards greater complexity, although there is a tendency in the model to progress from herbaceous to tree species.