Causes of Diversity Loss
The recent loss of genetic diversity is in large part due to the advent of new high yield varieties of staple food crops and the use of the farming methods necessary to cultivate them. The development of GMO crops has furthered the loss of diversity.
The Green Revolution and High Yield Varieties​
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Beginning around the 1940's, high yield varieties of wheat and rice were developed as a way to feed a rapidly growing global population that had already exploited much of its available farmland. The use of these few high yield varieties spread quickly; as they have spread, they have replaced native varieties of these crops that have been bred over centuries to be particularly suited to the area, sometimes causing them to die out (Harvesting). Furthering this problem is that the high yield varieties tend to be bred to be fairly broadly adapted so that they can be grown in as many climates as possible, and thus have lost the traits that made them hardy in a particular climate or region: resistance to a certain pest or blight, the ability to deal with drought or high winds, etc. For most efficient production of these genetically weaker high yield varieties, farmers grow monocultures, huge plantings of a crop in which the plants are genetically identical, so that they can be cared for with uniform methods (Siebert).
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Genetically modified organisms (GMOs) are organisms that contain specific genes from other organisms, generally that enhance them in some way; for example, the added genes may increase shelf life, increase cold-tolerance, provide resistance to certain pesticides or herbicides, or even, in the case of some GMO plants, allow crops to produce a pesticide-like toxin. Like high yield varieties, GMO crops, particularly those engineered to survive the applications of particular herbicides, are generally planted in large monocultures for efficiency of tending them. Companies such as Monsanto encourage this practice by producing herbicides that can only be used on fields planted with their herbicide-resistant GMO seeds (Paarlberg 163).
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To put the extent of the use of monocultures into perspective, in 2010 it was estimated that at least 70% of all food sold commercially in the US contains at least some GMO content (Paarlberg 166).
Crops in China being sprayed in an example of industrial farming.
Image from: <http://news.sciencemag.org/sciencenow/2009/10/05-02.html>
The Diminishing Seed Pool