Crop Genetic Diversity

Genetic diversity in crops provides the raw material that allows breeders to improve yield, increase protein and vitamin content, and augment drought and pest resistance in food plants. The increasing food needs of a growing human population, combined with global climate change, regional water shortages, and altered environments, make it imperative to preserve crop genetic diversity. Loss of diversity has been severe in recent decades, as the number of crops grown has declined, and large-scale planting of genetically uniform, high-yield varieties has replaced traditional, locally grown crops.

Crops originated from wild plants. Through selective breeding, farmers have produced higher-yielding, more-nutritious forms of these plants. As people migrated around the globe, they brought their crop plants with them and selected forms best adapted to the new environmental conditions. Regionally adapted variants of crop plants, known as landraces, are the result of hundreds of years of local breeding and selection. The size, shape, color, and taste of different landraces result from genetic differences that have been selected in response to local tastes, customs, and markets. Selection in response to local soil characteristics, water availability, and cultivation practices has resulted in different varieties in different areas. Often, traditional farmers maintain many varieties that can be planted under different conditions and for different tastes.

The National Plant Germplasm System from the U.S. Department of Agriculture's Agricultural Research Service stores unusual varieties of Latin American maize like these, which may be used to enhance the genetic diversity of the U.S. corn crop.

Source: U.S. Department of Agriculture, Agricultural Research Service/Keith Weller

The green revolution in the mid-1900s dramatically increased crop yields for a rapidly growing human population. Scientific principles were applied to crop breeding: Better irrigation systems, mechanized cultivation, and the expanded use of pesticides, herbicides, and fertilizers quickly changed the nature of agriculture. Although landraces persisted in subsistence agriculture, modern agriculture no longer focused on local markets but provided large quantities of fewer crops to international markets. Now, of 7,000 crop plants that are grown globally, only 150 are widely cultivated, and only three—rice, wheat, and corn—provide the bulk of the world's calories. Genetically homogeneous, high-yield crops, cultivated in standardized ways, are planted over large expanses of land. However, such homogeneous crops are genetically vulnerable in that a disease, pest epidemic, or environmental change can decimate a crop. If high-yield varieties succumb to pest resistance, yields are significantly reduced within about 5 years. It takes 8 to 11 years to breed new varieties, using genes from wild relatives, landraces, and stock exposed to mutagens to induce variation. However, the loss of wild relatives through habitat destruction and [Page 112]the decline in cultivation of landraces have resulted in gene erosion, limiting options for change.

A few examples from the United Nations Food and Agriculture Organization (FAO) illustrate the extent of gene erosion:

• 96 percent of the 7,098 U.S. apple varieties cultivated before 1904 are extinct
• 95 percent of U.S. cabbage and 81 percent of tomato varieties are gone
• Only 10 percent of the 10,000 wheat varieties grown in China before 1940 remain
• 80 percent of Mexican maize varieties grown in the 1930s are lost

In addition to these types of gene erosion, researchers from Bioversity predict that global climate change will result in the extinction of 16–22 percent of the wild relatives of food plants.

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