This means their DNA is so similar that they are very susceptible to virus and disease. Maintaining this diversity is a key element to success of a single species. The key to genetic diversity is to have a big enough differential percentage in a species so that mutations can lead to evolution.
Hybridization is the ability for different species to mate, and it has lead to the vast variety of plants we see today. Hybridization is not restricted, and it does happen in the wild and in domestic areas as long as they are reproductively compatible. Natural hybridization typically has a positive impact on genetic diversity, but many are concerned that if a GMO x Natural cross occurs somewhere other than in a controlled area the fitness advantage of the GM plant will evolve the plants. Thus eventually creating a lack of genetic diversity. In addition to crossbreeding, GMOs can also affect genetic diversity through uncontrolled growth of a genetically engineered population. If an advantageous gene is introduced into the GM species, it might allow them to become more fit than their wild relatives. This situation would be catastrophic to the natural wildlife, because it would allow the GM plants to grow larger faster and suck all the nutrients from the ground away from non GM