Consequences of low genetic diversity

There are two well-known examples of the consequences of low genetic diversity, the Irish potato blight famine of 1850 and the Southern corn leaf blight of 1970.

The potato (Solanum tuberosum) originated in South America, but was carried back to Europe around 1530. By the early 1800s potatoes had become a staple food of Ireland, in particular of farmers and the rural population. In 1845 a disease caused by the protist Phytophthora infestans, commonly known as potato blight, began to spread. Because all the potatoes in Ireland descended from the small number that were introduced (producing a genetic bottleneck) and potatoes are usually grown from tubers from the mother plant (clones) rather than seed, there was very little genetic diversity among them. Since all the alleles in the population were the same, and susceptible to the disease, nearly all the potatoes in the country were killed. Since the Irish depending heavily on potatoes as a food source, this led to mass starvation during 1845-1852. The Great Famine decimated 25% of the Irish population and was one of the factors responsible for increased emigration to the US.  There were complicated political issues that exacerbated the famine, but the underlying cause was the inability of the potato population to respond to the pathogen, due to its low genetic diversity.

The second example is the Southern corn leaf blight. Nearly all maize varieties in the US are hybrids, varieties produced by crossing two inbred parents together. Pollen from one parent is used to fertilize the other parent. In the past, to ensure that all kernels were really from the cross and not from self-pollination, all the plants used as females in the cross were detasseled by hand (this removed the pollen source, so any kernels that developed must have been pollinated from the other parent). To avoid the labor-intensive work of detasseling, plants that are male-sterile were subsequently developed for use as the female parent. Thus, self-fertilization was impossible. However, all the plants initially developed this way had very little diversity, as they were derived from a single parental type. In the 1970s, a fungus, Bipolaris maydis, began to spread throughout the US, devastating corn production (losses in the U.S. were estimated at one billion dollars; in some regions fields were completely lost). Because all maize varieties at the time were so genetically similar, none of them had genes that were resistant to this fungus; whereas, if there had been many diverse types growing, some would have been likely to contain genes resistant to the fungus. Plant breeders realized that genetic diversity among their populations is crucial, and have since been careful to avoid growing single crop systems (monocultures).

These types of devastating epidemics can be avoided by maintaining good levels of genetic diversity in crop populations; that is, making sure that all the plants are not too genetically similar.

For more on the corn blight, see:
http://aboutbiodiversity.org/agbdx/cornblight.html

Ullstrup AJ (1972) The impacts of the Southern Corn Leaf Blight Epidemics of 1970-1971. Annual Review Phytopathology 10: 37-50.