It would seem that with the advent of fertilizers, pesticides and biotechnology, our ability to produce crops should be limitless. What chance do insects and plant diseases stand against mighty modern agricultural technology?
But biological systems evolve. Insects and diseases evolve as new technologies are introduced. Variables change, because evolution is change over time through descent with modification. So in the fields of agriculture and economics, just as in medical science and conservation, history matters.
courtesy of UC Berkeley
Refugees of Genetic Variation: Controlling Crop Pest Evolution
Insects persistently nibble away at crops in the fields and at the narrow profit margin they offer. What’s a farmer to do? Spray, of course. Or, plant crops that have been genetically engineered to produce their own pesticides.
But evolutionary theory tells us that these solutions will not work indefinitely. Pest insects have short generation times and large population sizes — which means that they evolve quickly. If pesticides are widely applied, or if fields are widely planted with pesticide-producing plants, insects resistant to the pesticide will evolve. Some degree of resistance has been documented for every major class of insecticide used in agriculture.
Refugia
Is there any way that we can slow the spread of resistant genes? Evolutionary theory points to an answer: we can provide havens for non-resistant insects (and their non-resistant genes!). These havens are called refugia — they are fields without pesticides (sprayed or plant-produced) located near fields planted with pesticide-producing crops.
The diagram below illustrates how refugia slow down the evolution of pesticide-resistant pests by allowing non-resistant pest strains to survive.
Refugia may be particularly important in slowing the spread of insects resistant to the pesticide Bt (produced by a gene in the bacterium Bacillus thuringiensis). As one of the few pesticides used by organic farmers, Bt kills a small subset of insects and does not harm many beneficial organisms. However, resistance to this pesticide has become an imminent threat as corn and cotton genetically engineered to produce their own Bt fill more and more fields. These genetically engineered crops increase the selective pressure for Bt resistance on insect populations. If Bt resistant insects become common, organic farmers will have lost one of the few pesticides they are able to use.
Refugia slow the evolution of widespread Bt resistance by providing havens in which the non-resistant insects survive. The allele for Bt resistance happens to be recessive — that means that the resistant allele can be masked by the dominant non-resistant allele. So if a resistant insect (rr) surviving in the Bt-producing field mates with a non-resistant insect (RR) surviving in the refuge, all of their offspring will be non-resistant (Rr).
When two heterozygous pests mate, only one in four offspring (on average) will be homozygous recessive (rr) and therefore resistant to the pesticide.
By keeping refuges for the non-resistant alleles, we can prevent many of the resistant alleles from being expressed. More insects will be vulnerable to Bt and the spread of the resistant allele will slow.
Only by understanding evolutionary theory and by recognizing how our short-term solutions are likely to affect evolving insect populations can we figure out ways to control the evolution of resistance.
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Corn and It’s Untamed Cousins: Wild Genes in Domestic Crops
Corn viruses can seriously damage crops and the profitability of farms. The Maize Chlorotic Dwarf Virus, for example, stunts plants and decreases yields. Plants resistant to this virus and others have been a boon to corn growers.
But if these resistant varieties don’t already exist, how can they be developed? Resistant corn strains could be genetically engineered. But genetic engineers can’t actually create new genes — they can only move around genes that already exist. In order to develop virus-resistant corn varieties, an engineer or breeder would need a source for these resistant genes.
In this case, an evolutionary perspective — one that considers the history of corn — pointed scientists towards teosinte for these genes. Although they don’t look it, evolutionary studies indicate that wild teosinte and domesticated corn are close relatives.
In 1977, Rafael Guzman, a Mexican biologist, discovered a previously unknown teosinte species, Zea diploperennis, in South-central Mexico. This species happens to carry particularly useful genes—including genes for resistance to seven viral diseases that affect domestic corn. Using these genes, scientists developed virus-resistant domestic corn varieties.
Understanding the evolutionary history of domestic crops and other organisms helps scientists identify valuable stores of genetic variation.
courtesy of UC Berkeley
Monoculture and the Irish Potato Famine: Cases of Missing Genetic Variation
Lumpers
In the 1800s, the Irish solved their problem of feeding a growing population by planting potatoes. Specifically, they planted the “lumper” potato variety. And since potatoes can be propagated vegetatively, all of these lumpers were clones, genetically identical to one another.
The lumper fed Ireland for a time, but it also set the stage for human and economic ruin. Evolutionary theory suggests that populations with low genetic variation are more vulnerable to changing environmental conditions than are diverse populations. The Irish potato clones were certainly low on genetic variation, so when the environment changed and a potato disease swept through the country in the 1840s, the potatoes (and the people who depended upon them) were devastated.
The importance of diversity
The genetically identical lumpers were all susceptible to a rot caused by Phytophthora infestans, which turns non-resistant potatoes to inedible slime. Because Ireland was so dependent on the potato, one in eight Irish people died of starvation in three years during the Irish potato famine of the 1840s.
Although the famine ultimately had many causes, the disaster would likely not have been so terrible had more genetically variable potatoes been planted. Some potatoes would have carried the right genes to make it through the epidemic, and more of the resistant varieties could have been planted in the years following the first epidemic. Later, scientists identified resistance genes in a potato from South America, where farmers have preserved the genetic variation of potatoes by growing many cultivated varieties alongside the potato’s wild cousins.
The image below compares the effect of a blight on diverse and cloned crops.
Ignoring history
Despite the warnings of evolution and history, much agriculture continues to depend on genetically uniform crops. The widespread planting of a single corn variety contributed to the loss of over a billion dollars worth of corn in 1970, when the U.S. crop was overwhelmed by a fungus. And in the 1980s, dependence upon a single type of grapevine root forced California grape growers to replant approximately two million acres of vines when a new race of the pest insect, grape phylloxera (Daktulosphaira vitifoliae) attacked in the 1980s.
Although planting a single, genetically uniform crop might increase short term yields, evolutionary theory and the lessons of history highlight an undesirable side effect. Planting genetically uniform crops increases the risk of “losing it all” when environmental variables change: for example, if a new pest is introduced or rainfall levels drop.
courtesy of UC Berkeley
Understanding Evolution is Important to Agriculture
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Evolution and Agriculture: Helpful Links
Evolution in Agriculture: The Domestication of Wheat