A model of domestication of plants in Mesoamerica based on selective management of plant populations and communities by silvicultural practices is analyzed. Archaeological and ethnobotanical information suggests that intentional manipulation of vegetation by Mesoamerican peoples has occurred in past and present times in order to control availability of useful plants. Forms of management of plant communities or populations have included tolerance, protection and enhancement of individual plants of particular species during clearance of vegetation and other ways of perturbation. Processes of artificial selection (selection in situ) may be carried out through these forms of plant management. These processes may cause significant morphological differences between wild and managed populations as illustrated by the cases discussed here of Anoda cristata, Crotalaria pumila, Leucaena esculenta and Stenocereus stellatus. Processes of artificial selection in situ are mechanisms of incipient domestication of plants which appear to have been carried out in Mesoamerica, perhaps since pre-agricultural times, and that could contribute to explain the processes that led to the origins of agriculture in this region.
Leucaena Benth. (Fabaceae, Mimosoideae) is an American genus of tropical and subtrop ical woody shrubs and trees with its center of diversity spanning Mexico. Some taxa of Leucaena are important plant resources contributing to the traditional diet in certain parts of Mexico, where selected taxa have been under cultivation since prehispanic time (Zarate 1982, 1984, in press, Casas et al. 1987). Opinions vary as to the number of species and subspecies in Leucaena. Sorensson and Brewbaker (1994) consider 16 species and 2 subspecies; Huges (1993) proposes 16 species; Zarate (in press) regognizes 14 species and 18 subspecies for Leucaena in Mexico. Table 1 shows species and subspecies of Leucaena studied in this paper, following Zarate's (in press) classification system and their equivalences in Sorensson and Brewbaker's (1994) system. Leucaena species have been reported as having n=26 and n=28 chromosomes. No species in the genus has been found with n=13 or n=14, and consequently species with n=26 (which show 26 bivalents) or n=28 chromosomes (showing 28 bivalents) were treated as diploids (Pan and Brewbaker 1988, Cardoso de Freitas et al. 1988). Because species with n=26 or 28 could have derived from an ancestor with n=13 or 14 chromosomes, the basic chromosome numbers of both x=13 and x=14 have been considered
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