You may have noticed that while your grocery store or supermarket has several types of apple for sale, there is usually only one type of banana. In fact, it’s possible that every banana you have ever eaten has been the same variety. Now the popular yellow fruit may be in trouble.

Years ago, the Western world favored a banana variety known as the Gros Michel. In the first part of the twentieth century, a devastating fungal infection called Panama disease wiped out Gros Michel plantations around the world. In the 1950s, banana growers turned instead to the Cavendish, a variety that displayed some natural resistance to Panama disease. Half a century later, the Cavendish remains the top-selling banana in supermarkets in North America and beyond.

Agriculture has transformed our planet and our species; without it, modern civilization would not be possible.

Yet the reign of the Cavendish may be coming to an end. For years, banana growers have battled black sigatoka, a fungal infection that can cause losses of 50% or more of the banana yield in infected regions. Making matters worse, Panama disease is once again posing a threat. A strain of the disease has emerged against which the Cavendish plants have no resistance. The disease has spread across Asia and Australia, and experts fear it’s only a matter of time before it reaches prime banana-growing regions in Latin America.

For those who enjoy bananas sliced into their breakfast cereal, the loss of bananas would be disappointing. For the millions of people in tropical countries who depend on bananas and related plantains for daily sustenance, the destruction of those crops would be much more serious. Agriculture has transformed our planet and our species; without it, modern civilization would not be possible. But as the case of the banana shows, there are challenges to overcome if we’re to continue feeding a growing population.

Cultivated bananas are vulnerable to infection in a way that wild bananas are not. The bananas that we eat are sterile; they do not produce seeds. This means they must be propagated vegetatively by replanting cuttings taken from parent plants. As a result, banana plantations contain almost no genetic diversity. As Panama disease and black sigatoka are proving, that lack of diversity can have disastrous effects.

As sterile clones, bananas are particularly vulnerable to disease outbreaks. Other agricultural crops face similar threats. For example, a new, virulent strain of yellow wheat rust fungus emerged in the Middle East in 2010. In its first year, the disease wiped out as much as half of Syria’s wheat crop and has since spread to several other countries in the region. Meanwhile, crop scientists have warned that Ug99, a new, virulent strain of an even more damaging fungal pathogen that first surfaced in eastern Africa, could devastate global wheat crops as it spreads.

Wheat was one of the first crops that our ancestors cultivated when agriculture emerged 10,000 years ago. The early farmers learned to choose wheat plants that were easiest to harvest and whose seeds germinated without delay. Over time, wheat evolved by this process of artificial selection. Those earliest farmers set in motion a long chain of genetic modifications that ultimately led to the high-yield wheat varieties grown today.

In the wild, natural selection favored wheat whose seeds were protected by tough barbs and separated easily from the plant, allowing efficient dispersal across the landscape. Farmers, on the other hand, preferred wheat plants whose seeds remained attached to the plant and so were easier to harvest. But the tendency for seeds to remain attached would interfere with the plant’s ability to self-seed in the wild. By selecting for traits that would be disadvantageous in nature, humans created a plant dependent on humans for its survival. The codependency between domestic plants and people is a hallmark of agriculture.

In addition to selecting desirable traits in their crop plants, early agriculturalists altered the environment to be favorable for those plants. They provided water, chose planting sites to ensure optimal sunlight, and removed nearby plants that would compete with the crops for vital resources such as water and nutrients. By changing the community structure and the availability of resources to support growth and reproduction, humans allowed domesticated crops to thrive.

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By the 1940s, scientists had developed industrially fixed nitrogenous fertilizers that significantly boosted plant growth. They had also learned enough about genetics and DNA to begin applying that knowledge to plant breeding. These advances led to an amazingly productive period during the 1960s and 1970s now known as the Green Revolution. During that time, plant yields ballooned. Food production actually out-stripped population growth through the second half of the twentieth century.

Today, we are entering a new phase of agriculture, with genetic engineering being added to the traditional approaches of crossbreeding and artificial selection. Most of the corn and soy products now consumed in the United States come from plants that have been genetically engineered to resist certain pests and herbicides. Some of the genes employed come from different species altogether. For example, corn and cotton plants are often engineered to contain a gene from the bacterium Bacillus thuringiensis (Bt) that confers pest resistance.

At a glance, it would seem that we’ve successfully taken domesticated species under our control, and indeed, we’ve made great strides in increasing yields to feed an ever-growing population. But evolution never takes a break. Crop pests and pathogens continue to evolve ways to evade our savviest plant breeders.

The nature of modern agriculture only compounds the problem. Most modern farms consist largely of monocultures, single types of crop each grown over a large area. Growing only a single crop at a time makes it much easier to mechanize planting and harvesting. In a given field of corn or wheat, therefore, the individual plants are often genetically quite similar to one another. With so little genetic variation, each plant is vulnerable to pathogens in exactly the same way. This means that a pathogen that can overcome one plant’s natural defenses has the potential to wipe out the entire field.

Lately, many consumers are taking a hard look at these problems and at the source of the food on their plates. Critics of modern agriculture argue that monocultures, with their heavy reliance on fungicides and pesticides, are unsustainable and unhealthy. Proponents of genetic modification and high-tech agriculture argue that we will need every tool to meet food-production demands for the expanding human population. Sometime in 2011, the human population surpassed 7 billion people. The population continues to grow, expanding at its fastest rate at any time in human history. By 2100, experts predict, our planet will be home to more than 9 billion people. One point that both sides agree on is that unless agricultural productivity continues to increase, more and more land will be needed to produce food.

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In many parts of the world, agricultural productivity is limited by the availability of water or nutrients or both. Global climate change is predicted to cause regional droughts and flooding that may further weaken the global food supply. There will be no easy solutions as we move forward into civilization’s next phase of agriculture. One thing that is certain, however, is that understanding how plants grow, reproduce, and protect themselves from pests will help us survive in an increasingly crowded world.

CASE 6 QUESTIONS

Special sections in Chapters 29–34 discuss the following questions related to Case 6.