**6. Local genetic adaptation**

Landraces are generally adapted to local environment. With the continued cycles of local planting, harvesting and farmers' selection, over time landraces will be selected for local environmental and agroecosystem conditions and practices, just as ecotypes of wild species are adapted to the local environmental conditions. Landraces 'are adapted to their growing conditions' [11]; 'possess adaptive complexes associated with the special conditions of cultivation, pure-stand associations, harvesting and others factors' [44]; 'are not only adapted to their environment, both natural and man-made, but they are also adapted to each other' [7]; 'are adapted to the areas in which they grow' [12]; 'are specifically adapted to local conditions' [13]; and 'are adapted to local conditions' [26].

Bennett [44] made the assumption that landraces are more suited to cultivation in particular locations than highly bred cultivars that are bred for cultivation in the most common environmental conditions. Inevitably, cultivars will be less suited to grow in suboptimal conditions and therefore have less of a competitive advantage in marginal environments where the local landraces are likely to have an adaptive advantage. These local conditions may be defined as abiotic (e.g. salinity, drought, etc.), biotic (e.g. pests, diseases, weeds) and human (e.g. cultivation, management and use). Landraces are perceived to have the ability 'to sensitively respond to even minor environmental influences' [44]; 'to have some built-in insurance against hazards' possibly due to their inherent population structure [7]; 'to accumulate resistance genes to limiting factors in the physical and biological environment—drought, cold, diseases, pests' [24]; and 'to be capable of producing in any but disaster seasons at a level which safeguards the survival of the cultivator' and so provide yield stability [24].

Several studies have demonstrated the relationship between landraces and local adaptation, for example; Frankel [8] and Brown [39] discuss landrace adaptation to marginal conditions associated with climatic, soil and disease stress. The evolution of local adaptation over millennia in these stressed environments ensures yield stability even in extremely adverse years. In this sense, Zeven [27] considers yield stability to be a principal characteristic of landraces.

However, even though there are numerous references to a specific relation between a landrace and local environmental conditions, there are exceptions. Zeven [3] indicated that 'some landraces are able to adapt themselves to a wide range of environments, whereas others are able to adapt themselves only to a few environments'. Wood and Lenne [19] disagree with the assumption 'that all traditional varieties are locally adapted' and state that 'evidence against specific local adaptation in crop varieties is provided by the extensive interchange of traditional varieties of all crops'. Farmers employing an 'open' cultivation system where there is regular local or more exotic landrace introduction are less likely to have locally adapted landraces. Zeven [20] provided evidence of farmers' traditional practice of periodic seed replacement to combat socalled degradation, which indicates that in certain situations a 'closed' cultivation system that results in local adaptation of landraces may be deleterious. The farmer's criteria for seed selection also do not necessarily lead to selection for local adaptation; the varying environmental conditions under which traditional agriculture is carried out may in certain conditions not actually favor specific local adaptation. In this sense, some authors consider that local adaptation can comprise both wide adaptation in certain landrace characters and narrow adaptation in others.
