Genetic Diversity and the Green Revolution

One of the common criticisms environmentalists make of the Green Revolution is that it supposedly reduced the genetic diversity of crops. Usually environmentalists complain that by doing so, the world’s crops are open to being wiped out by a single catastrophic crop disease. This scenario is certainly scary, but is it true?

Melinda Smale of the International Maize and Wheat Improvement Center set out to examine the status of genetic diversity in wheat. Her paper on the subject, “The Green Revolution and Wheat Diversity: Some Unfounded Assumptions,” maintains there is insufficient evidence for claims that the Green Revolution caused either genetic erosion of wheat crops or an increase in genetic vulnerability to disease.

First, Smale notes that much of the environmentalist concern is highly exaggerated even if their worst fears were accurate. As she notes, there doesn’t at the moment seem to be any shortage of potential genetic wheat combinations,

Given the size of the wheat genome and possibilities of incorporating genes from wild relatives or unrelated species through biotechnology and other breeding techniques, genetic combinations do not seem determinate in number.

There also doesn’t seem to be much risk of genetic vulnerability to disease in and of itself leading to catastrophic crop failure. The classic case of such a scenario is the destruction of 15% of the U.S. corn crop in 1970 due to disease, but as Smale notes that was hardly catastrophic. The real weakness to catastrophic crop failure is not so much genetic vulnerability but a lack of infrastructure in some countries to deal quickly and efficiently with outbreaks.

That said, when it comes to bread wheat the act of domestication itself appears to have significantly decreased genetic diversity. Smale writes,

According to some scientists, the utilization of bread wheat landraces maintained in collections offers only limited possibilities for diversification within the gene pool constituted after domestication … Continued farmer selection and modern plant breeding have extended wheat cultivation into new and different area, creating diversity in one sense and narrowing it in another. Statements about the relative breadth of the genetic base of wheat over time seem more a matter of scientific intuition than of scientifically proven fact.

In other words, what those who claim a lack of genetic diversity fail to realize is that agriculture by its very nature involves a selection process which reduces genetic diversity.

Which explains when it comes to the supposed increased in vulnerability to disease, Green Revolution varieties seem no more prone to disease and may in fact be more resistant thanks to the process whereby hybrids such as dwarf wheat were chosen than traditional varieties. Smale writes,

Data from screening nurseries for advanced breeding lines used in the developing world also show a gradual increase in the level of resistance to rust since the late 1960s.

Smale’s paper certainly does not rule out the possibility of declining genetic diversity in wheat and other crops, but it does mean that those who make the claim must step up to the plate with more concrete numbers and analysis beyond the vague claims and predictions that have so far been presented.

The full text of Smale’s article appears in World Development, Vol. 25, No. 8, pp. 1257-1269, 1997.

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