History has proved the dangers inherent in genetic uniformity: it was the unrecognized cause of the Irish potato famine, which left millions dead in the mid-19th century. An informed few, mainly botanists, value traditional farmers for the rich variety of crops they produce. By cultivating numerous strains of corn, legumes, grains and other foods, they are ensuring that agricultural scientists have a vast genetic reservoir from which to breed future varieties. The genetic health of the world's potatoes, for example, now depends on Quechua Indians, who cultivate more than 50 diverse strains in the high plateau country around the Andes mountains in South America. If these natives switched to modern crops, the global potato industry would lose a crucial line of defence against the threat of insects and disease. Unfortunately, the same is not true for man other wild and domesticated varieties of crop plants - such as wheat, rice, millet, beans, yams, tomatoes, potatoes, bananas, limes and oranges - are already extinct and many more are in danger of following them.
The evolution of food crops over many centuries of domestication has increased the range of genetic diversity but development and promotion of high-yielding cultivars for modern intensive agriculture is now rapidly reversing this trend, leading to a dangerous reliance on genetically uniform crops, often ones that need high inputs of fertilizer and pesticides to perform effectively. As intensive agriculture has spread widely, many local varieties have been displaced and some have disappeared entirely. Wild relatives of cultivated species are also often threatened with extinction as a result of habitat change.
The International Board for Plant Genetic Resources of the FAO recently issued warnings that wheat, rice, sorghum, millet and barley were crops whose genetic diversity was imperilled. The USA National Academy of Sciences has warned that the USA's major crops were impressively uniform genetically and are therefore vulnerable to widespread attack by blight or pests.
Attempts to combat disease with resistant varieties have resulted in an increase in the incidence of disease. This occurs because the pathogen is at least as variable as the host crop. The newly released resistant variety can actually select a strain of the pathogen virulent on it, thus removing itself from usefulness and necessitating a replacement. For example, pure-line varieties of oats have an average useful life expectancy of only about 5 years. The entire acreage of the grass [Digitarta decumbens] in Central America and the West Indies originated from a single clone introduced in 1940, and already it is necessary to spray pastures of this grass in Central America as a protection against aphids and fungal diseases.
The situation has worsened considerably since hybridization involving single-gene resistance began, because now a single resistance gene can be quickly incorporated into crop varieties right across a continent. For example, various pure-line varieties of small grains were released in North America because they were highly resistant to prevalent races of rust fungi. Though these varieties resisted most rust races, they were susceptible to rare, virulent races which, though occurring in extremely small amounts in the rust population, could increase with sufficient rapidity in the absence of competition on a susceptible host to soon become devastating. Yet pure-line varieties of small grains continue to be released for commercial production.
An increasingly restricted genetic base appears to underlie periodic production failure in economically important crops, leading to increased yield variability and increased synchronicity of variation across large areas; for example, a 15 per cent reduction in maize harvest in 1970 in the United States was attributed to widespread cultivation of a blight-susceptible variety (WCMC 1992).