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Multiple domestications and their taxonomic consequences: Phaseolus vulgaris
Effects of dispersal and hybridisation
To qualify for taxonomic recognition, a taxon must persist over time. Both
Mesoamerican and Andean races have been dispersed widely by man subsequent to
their domestication. They may now be grown together in the same area, ecological
differences notwithstanding, and may then intercross. There has therefore been ample
opportunity for distinctions between gene pools or races to become blurred by
hybridisation. This has not in fact occurred on any large scale. For example, BEEBE et
al. (2000) studied RAPD banding patterns in 269 Mesoamerican landraces and found
that nine had several bands characteristic of Andean landraces so probably resulted
from introgression. This is a very small proportion of the total sample. KHAIRALLAH et
al. (1990) studied isozymes and mitochondrial DNA in beans from farmers’ mixtures
in Malawi, where beans from Mesoamerican and Andean gene pools have been cultivated
together for at least three centuries, and found that most lines could be placed
clearly in one or other gene pool. Lines showing evidence of inter-gene pool hybridisation
were very rare. ZEVEN et al. (1999) failed to distinguish gene pools or races in
a core collection of Dutch common bean when they used 14 characters of predominantly
horticultural significance, but could classify them into gene pool and race when
they used the appropriate diagnostic characters.
There is therefore sufficient justification for recognising taxa corresponding to wild
versus domesticated beans, different races within domesticated beans, and possibly
different gene pools. However, neither the Botanical nor the Cultivated Code provides
adequately for recognition of taxa that reflect or cut across the divide between wild
and cultivated plants.
Conclusions
Names for wild common beans are governed by the Botanical Code. This provides
two categories, subspecies and variety, to treat infraspecific differentiation. One or
other of these could be used to provide formal names for the wild beans in the
Mesoamerican and Andean gene pools, though the requirement that every plant be
classifiable into a named taxon at a particular rank causes problems when dealing
with intermediates, as STACE (1986) has already noted. The lineages of chloroplast
haplotypes do not correspond well with morphological differences among wild beans.
Integrating molecular information on evolutionary lineages with formal taxonomic
classifications is currently a problem at many levels of the taxonomic hierarchy.
In domesticated beans, as in other crops, human selection after domestication has
partitioned variation within the domesticate into different use groups, different agronomic
groups, etc. These are treated, under the Cultivated Code, as different cultivargroups.
How satisfactory this is depends on the complexity of the variation and the
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