This infographic, titled "Our Dwindling Food Variety," from the August National Geographic looks pretty dire, no?:
But before you start stockpiling muskmelon seeds against inevitable genetic calamity, check out this study by Paul Heald and Susannah Chapman (highlighted by Jonathan Adler at the Volokh Conspiracy) which starts with the same data from 1903, but finds that crop diversity hasn't dwindled at all. The comparison between commercial seed catalogs in 1903 to the U.S. Department of Agriculture-run National Seed Storage Laboratory is not, as Adler punnily notes, an apples-to-apples comparison. In fact:
According to the conventional wisdom, the twentieth century was a disaster of monumental proportions for vegetable crop diversity. The conventional wisdom is wrong. Our study of 2004 commercial seed catalogs shows twice as many 1903 crop varieties surviving as previously reported in the iconic 1983 study on vegetable crop diversity. More important, we find that growers in 2004 had as many varieties to choose from (approximately 7100 varieties among 48 crops) as did their predecessors in 1903 (approximately 7262 varieties among the same 48 crops).
More on misunderstandings about genetic diversity here.