Effects on Diversity
Many of these trait changes reduce the ability of the plant to compete in the wild, and also decrease the genetic variability remaining in the crop. Crop domestication by its nature decreases genetic variation, since it deliberately selects only a small number of plants out of the many present in the wild population (those whose phenotype is considered desirable by the selectors, ie. humans). Figure 4 graphically demonstrates this by using different colored squares to represent different gene alleles. Through selection over time this has led to a decrease of genetic diversity in a crop’s gene pool.
Figure 4: Loss of genetic diversity with successive domestication. Artificial (e.g. human) selection creates a genetic bottleneck, decreasing diversity. (Image from Steven Tanksley)
One result of less diversity is that consumers and farmers are now accustomed to, and demand, uniformity – such as: round red apples, plants all the same height in the field (Figure 5).
Figure 5: Typical red apple found in grocery stores, all uniform in phenotype.
But the loss of genetic diversity can have devastating consequences, such as the Irish potato blight of 1850, the Southern corn leaf blight of 1970, and the current crisis in banana, Black Sigatoka disease, shown in Figure 6. In each of these cases what has happened is through the loss of genetic diversity, the vast majority of cultivated varieties grown by farmers were all susceptible to the same disease. Efforts have been made since then to globally increase diversity within commercialized varieties, while at the same time maintaining high yields and agronomic values.
Figure 6: Loss of genetic diversity in banana plants has led to increased vulnerability to Black Sigatoka disease. Banana image Copyright 2001 by The American Phytopathological Society, http://www.apsnet.org/education/feature/banana/; apple photo ourtesy of New York Apple Association