The loss of crop varieties over time is often called genetic erosion. This ongoing loss has been calculated in a number of different ways by different researchers, usually based on special plant species. Seed Savers Exchange of Decorah, Iowa has tabulated the number of varieties of many garden vegetables offered by commercial seed companies early in the twentieth century. They compared these totals to the number of varieties still being offered by seed companies at the end of the twentieth century. In all cases, the number of varieties available commercially declined, often dramatically. One of the broadest estimates of genetic erosion deals with apples.
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