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The genus Phaseolus is New World in origin in contrast to Old World Vigna species, which includes related grain legumes such as cowpea, adzuki, and mung beans. Bean breeders are limited in their improvement efforts to working within the genus Phaseolus due to speciation barriers that prevent successful hybridization. Phaseolus includes five cultivated species separated into four gene pools (Figure 2.3) and over 70 wild species (Freytag and Debouck 2002). Within the five species of cultivated Phaseolus, common bean is the most widely adapted and economically important member compared to other species that include lima and tepary beans. Breeders have successfully transferred traits from the scarlet runner bean (P. coccineus) and P. costaricensis in the secondary gene pool (Schwartz and Singh 2013) and from members of tepary bean (P. acutifolius) in the tertiary gene pool (Singh and Miklas 2015; Kusolwa et al. 2016), but no successful introgression with lima bean (P. lunatus) in the quaternary gene pool has occurred.

Fig. 2.3. Genus Phaseolus includes five cultivated species that reside in four separate gene pools based on genetic distance. There is no restriction to intercrossing between members of the primary gene pool; crossing between primary and secondary gene pool requires that P. vulgaris is used as the pollen parent; crossing between primary and tertiary gene pool requires special techniques to rescue hybrid embryo; crossing is not possible between the primary and quaternary gene pools. In addition to the five cultivated species, there are over 50 wild Phaseolus species (Freytag and Debouck 2002).

Figure adapted from Gepts (2000).