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This paper discusses the centers of origin and diversity for bread wheat; sketches historical patterns in the sources and use of wheat genetic resources in modern plant breeding; identifies and compares indicators of genetic diversity used by social and biological scientists; reviews the relationship of wheat genetic diversity to yield stability and vulnerability to disease; develops a profile of the structure of genetic variation in wheat in the developing world today; and investigates how scientific plant breeding has influenced the structure of genetic variation among the major bread wheats grown in developing countries. Modem plant breeding appears to have contributed to genetic variation in several ways. The number of different landraces in pedigrees has steadily increased over the past 30 years, and the geographical origins of the landraces have broadened. The pedigree complexity of the more successful cultivars planted in the developing world has grown over time. Yield stability, both in terms of the performance of individual lines and in terms of regional yields, on farms, appears to have increased in the past decades. The diversity that confers resistance to the wheat rusts has also increased. Diversity over space and time varies among regions of the developing world. Spatial diversity, measured as the percentage area sown to leading modem cultivars, is lowest in West Asia and highest in Mexico/Guatemala. Temporal diversity is highest in Mexico/Guatemala, lowest in North Africa, and also fairly low in South Asia. However, both spatial and temporal diversity are fairly high for many developing countries compared to some of the major industrialized producers, such as Canada. Latent diversity, calculated from pedigree analysis, is fairly high across the developing world and does not appear lower than in the major industrialized wheat producers. Genetic distance measures calculated from genealogies demonstrate the comparative diversity among the top 10 bread wheats grown in West Asia and the marked similarity of wheats grown in Mexico/Guatemala and Canada. Numerous socioeconomic factors, which future research must examine more closely, have shaped the structure of genetic variation in farmers' fields. These factors are related to the adoption and diffusion of new varieties and include pricing policies for seed and associated inputs, and the structure of the seed multiplication and distribution system.
