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Multiple domestications and their taxonomic consequences: Phaseolus vulgaris
Classification involves assignment of the objects being classified into classes, followed
by arrangement of those classes in an order that reflects a principle of some
kind, whether the arrangement is purely artificial (e.g., alphabetical), is based on
overall similarity (natural), or represents putative evolutionary relationships (phylogenetic).
The International Code of Nomenclature for Cultivated Plants (TREHANE et al.
1995) provides only two categories for classifying variation within any given cultigen:
cultivar and cultivar-group. This suffices for constructing an organised catalogue of
names for commercial purposes, but is insufficient to reflect the hierarchy of variation
developed through human selection in many crops, as discussed previously for Capsicum
pepper, faba bean, peanut and banana (PICKERSGILL 1986, PICKERSGILL and
KARAMURA 1999).
Previous discussions of the taxonomy of cultivated plants have not considered explicitly
the consequences of multiple domestications of the same species in different
parts of the range of its wild ancestor. Different parts of the genetic diversity of the
wild progenitor are thereby included in different lineages within the crop: a fact of
potential significance to plant breeders and curators of gene banks, who are important
users of crop classifications. On the other hand, human selection may produce
parallel changes within each lineage and hybridisation may result in gene exchange
between lineages, so that it may be neither feasible nor useful to distinguish these
lineages in practice.
Common bean (Phaseolus vulgaris L.) is an example of a species that has clearly
been domesticated more than once. It may therefore be used as a case study of the
extent to which taxonomy of a crop can or should reflect its evolution under domestication.
Multiple domestications in Phaseolus vulgaris
Wild common beans range from northern Mexico to north-western Argentina (GEPTS
and DEBOUCK 1991). Mesoamerican wild beans differ from those of the Andean region
in various morphological characters (VANDERBORGHT 1983), some of which may
relate to ecological differences. Small-seeded Mesoamerican beans occur in disturbed
shrubby vegetation; large-seeded Andean beans in less disturbed moister and
cooler forests (KOENIG et al. 1990). Wild beans from the two continents are reproductively
isolated, at least partially, by complementary lethal genes (KOINANGE and GEPTS
1992), which may have developed as a by-product of independent evolution in each
continent. Similar genetic divergence has occurred in isozymes (GEPTS 1990), phaseolin
seed proteins (GEPTS et al. 1986, GEPTS 1990), nuclear DNA, both RFLPs
(BECERRA and GEPTS 1994) and AFLPs (TOHME et al. 1996), and mitochondrial DNA
(KHAIRALLAH et al. 1992). Domesticated common beans show differences in morphology,
isozymes, phaseolins and nuclear and mitochondrial DNA polymorphisms which
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