If you are interested in learning more about the dwindling variety of seeds available on the market, read the 2009 article by Philip H. Howard of Michigan State Unversity. Following is the article abstract as published in the Sustainability Open Access Journal.
"Abstract: The commercial seed industry has undergone tremendous consolidation in the last 40 years as transnational corporations entered this agricultural sector, and acquired or merged with competing firms. This trend is associated with impacts that constrain the opportunities for renewable agriculture, such as reductions in seed lines and a declining prevalence of seed saving. To better characterize the current structure of the industry, ownership changes from 1996 to 2008 are represented visually with information graphics. Since the commercialization of transgenic crops in the mid-1990s, the sale of seeds has become dominated globally by Monsanto, DuPont and Syngenta. In addition, the largest firms are increasingly networked through agreements to cross-license transgenic seed traits."