摘要：This study makes use of data on wheat production in the Punjab of Pakistan from 1979 to 1985 to 1) examine patterns of varietal diversity in farmers' fields both at the regional and district levels and 2) identify how and in what ways genetic resources have contributed to wheat productivity and yield stability-important considerations to farmers and national authorities where wheat is a staple food crop. Five indicators are used to describe the system of wheat genetic resource use and diversity in farmers' fields. The contribution of farmers' previous selections is expressed as the number of different landraces appearing in the pedigree of a cultivar . The contribution of scientific breeding efforts is expressed as the number of parental combination appearing in a cultivar's pedigree. The diversity of wheat varieties in a geographical area, as related to productivity, is captured by measures of area concentration (diversity in space) and age of varieties (diversity in time). Finally, the relative dissimilarity of cultivars grown in a geographical area is measured using a distance indicator constructed from genealogical information. Disaggregated analysis at the district level demonstrates how diversity patterns are influenced by the production environment and by possible differences in the availability of suitable varieties. The study finds no indication that modern plant breeding technologies have reduced diversity among the wheats grown in the districts of the Punjab of Pakistan during the study period, although brief. Analysis of the genealogical background of the varieties grown by farmers reveals patterns of greater use of genetic resources and dissimilarity of parentage. For some factors related to genetic resource use and diversity, there are large differences between production environments (specifically, irrigated and rainfed areas) and individual districts, which suggest that efforts to increase genetic diversity in farmers' fields will require policy instruments tailored to the individual circumstances of each production environment. Econometric results suggest that greater genealogical dissimilarity and higher rates of varietal replacement are likely to have positive payoffs relative to aggregate yield stability, while in areas where production constraints inhibit farmers' ability to exploit the yield potential of their varieties, better production management is likely to have greater yield enhancing effects than the varietal attributes related to diversity.
