1.3 Artificial Selection and Early Hybridization
1_Plant-Domestication
Plant Domestication
Overview
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Introduction
Lesson Objectives
Explain the domestication of plants for agriculture.
Explain the transition to the agrarian lifestyle.
Key Terms
artificial selection - occurs when humans select an organism for desired characteristics (phenotype), which can lead to changes on a genetic level (genotype)
cultivation - the growing or tending of crops
domestication - the process of modifying wild plants and animals by selective breeding into forms more suited to cultivation by humans
hybridization - the creation of offspring from two unlike parents, often the product of two different species or two different varieties
The Transition to the Agrarian Lifestyle
Archeologists believe that for much of our history, humans lived a nomadic lifestyle: hunting animals and gathering plants for food. These early humans would follow wild game, collecting fruit, nuts, grains, and other plant-based foods along the way. Their movement was largely driven by seasonal changes in food availability.
Some of the earliest plants that were harvested for long-term storage were grains, such as wheat and barley. These cereals grew wild throughout much of an area in the Middle East known as the Fertile Crescent. Using simple tools, such as flint-bladed sickles, early humans could harvest as much as 2 lbs. of wheat an hour. With just a few weeks of work, a small family could store enough grain to sustain itself through an entire year (Standage, 2005). Dried grains were able to be stored for long periods of time, which provided a consistent source of food through lean seasons. However, stored food needed to be monitored and guarded to prevent loss due to pests and environmental damage.
About 10,000 years ago, some groups of humans began to deliberately cultivate their food by planting seeds with the intent to harvest. This development led to the formation of agricultural villages centered around cultivating and harvesting crops (Figure 9.1.1). Agricultural concepts and practices seem to have independently arisen in several areas across the globe over a span of only about six thousand years, rather than originating in just one area and spreading from there (Diamond, 2002; Diamond, 2005). Agriculture spread from those centers as populations and farmable land use increased. Knowledge of agricultural techniques and use of tools and seeds also spread by trade.
Artificial Selection and Early Hybridization
The process of artificial selection happens when humans select a plant or animal, based on qualities of appearance (phenotype), for extensive cultivation or further breeding, which can lead to changes on a genetic level (genotype). To help us better understand the process of artificial selection, let us consider what researchers believe were likely the steps early humans made in domesticating three common crops: wheat, maize, and the cole crops.
A Brief History of Wheat
Primitive einkhorn wheat (Trititicum monococcum) is similar to modern wheat in that it is edible and relatively easy to harvest when compared to other crops. However, there are some differences between the wild ancestors of our modern wheat and the wheat that we grow today.
For example, wild wheat tends to produce much smaller grains held on smaller heads. These heads of grain are delicate and prone to scatter their seeds in response to the slightest touch. This characteristic, known as “shattering seed heads,” makes it easier for wild grains to spread their seeds in their natural environment. However, this adaptation makes it more difficult for humans to collect grains without dropping a good portion of the harvest.
As early humans selected which heads of grain to harvest, they showed preference for wheat that naturally produced larger grains that did not shatter when collected. These improved forms were probably the result of natural spontaneous hybridization in the field. Scientists believe that einkhorn wheat crossed with a related wild grass (T. searsii), resulting in the improved emmer wheat species (T. turgidum). These natural crosses increased the number of chromosomes, or the ploidy level, in the plant. For example, diploid einkhorn wheat has 7 pairs of chromosomes, while the tetraploid emmer wheat has 17 pairs of chromosomes.
The modified forms may have initially sprouted closer to villages from seed that was accidentally spilled, but humans eventually figured out that these harvested grains could also be planted for an improved and more uniform crop. While these new forms of wheat may not have been the product of deliberate hybridization by plant breeders, the choice to primarily grow improved forms (such as emmer wheat rather than einkhorn wheat) was a form of artificial selection (Figure 9.1.2). Improved strains of wheat quickly spread from the Near East to northern Africa, southern Europe, and parts of Asia. As humans cultivated wheat in new environments, they continued to select improved forms that performed better in their region. This tradition continues to this day.
Norman Borlaug (1914 – 2009) is a notable modern wheat breeder who rigorously crossed, trialed, and selected new strains of wheat for characteristics, such as improved yield, stout forms that are less prone to lodging or falling over, as well as the ability to grow in a variety of environments (Figure 9.1.3). His developments contributed to the Green Revolution, which made use of these improved varieties, better crop management practices, and synthetic fertilizers and pesticides to increase crop yields across the globe (Raven et al., 2005).
A Brief History of Maize
While the first evidence of plant domestication is found in the Fertile Crescent, artificial selection also happened independently in the New World. Before the arrival of European settlers, maize was a staple crop for many people living in North, Central, and South America (Figure 9.1.4), but its origins are still somewhat of a mystery. Researchers believe that maize was developed in Central America sometime around 6000 BCE. Unlike progenitors of wheat and most other cereal crops, there doesn’t appear to be a wild form of maize (Zea mays sbsp. mays). There have been several hypotheses as to the origin of maize. We will explore just one of those hypotheses in this text.
Some researchers believe modern corn is the domesticated form of the wild grass teosinte (Zea mays sbsp. parviglumis). Teosinte is a very large grass that looks like a taller, multi-stemmed version of modern corn (Figure 9.1.5). However, there are many important differences between teosinte and maize, including the fact that teosinte’s kernels are basically inedible (Mann, 2005).
Teosinte produces much smaller, extremely woody grains that are held on narrow, two-rowed ears. Each ear is contained in its own husked chamber, with 5 to 12 chambers contained in one larger husk. These chambers split open and the grains shatter, effectively scattering the seed that will sprout the next season. In contrast, maize produces large, succulent grains that are held tightly on a large cob contained by a husk (Raven et al., 2005). These characteristics make maize ideal for harvesting and human consumption, but they would prevent maize from being able to scatter seed independent of human intervention. That is, maize and corn could not survive in the wild without people to harvest and plant its seeds (Mann, 2005).
With an ancestor so markedly different from the cultivated form, it is difficult to understand how exactly maize was selected from teosinte. Scientists are still trying to figure out exactly how this happened, but it is clear that the selection process for more edible grains, larger ears, and non-shattering seed heads was the result of deliberate actions over many generations (Mann, 2005). This process occurred over generations of deliberate breeding, an impressive feat even to accomplished scientists today. In fact, Dr. Nina Federoff, a geneticist at Pennsylvania State University, began her article “Prehistoric GM Corn” with the bold statement: “Corn (maize) is arguably man’s first, and perhaps his greatest, feat of genetic engineering.”
Most of our modern corn is the product of hybridization between inbred selections. In fact, nearly all corn produced in the United States is hybrid corn. These hybrid selections produce a more uniform crop where individuals have identical growth and development characteristics, which makes mechanized harvesting easier. Hybrid corn is usually very vigorous, and strains can be selected based on their ability to grow well in a variety of climates. Most importantly, hybrid corn produces significantly higher yields than traditional corn and maize (Raven et al., 2005).
These characteristics mean that more corn can be produced per acre with less water, fertilizer, pesticides, and labor. While intensive breeding and hybridization has unquestionably led to higher yields, monoculture stands of hybrid corn are especially vulnerable to pests and diseases (Figure 9.1.6). Genetic diversity is the key to overcoming these health issues. Saving and studying landraces (Figure 9.1.7) and preserving wild relatives (like teosinte), in order to use this material for future breeding efforts, will help protect our ability to produce food for ourselves in the future (Raven et al., 2005).
A Brief History of Cole Crops
The Cole Crops (vegetables in the species Brassica oleracea) provide an excellent example of artificial selection. European kale, broccoli, Brussels sprouts, cabbage, cauliflower, collard greens, and kohlrabi, vary widely from one another in appearance Figure 9.1.8). However, they are all members of the same species that have been selected by humans over time for their different physical attributes.
- Both kale and collard greens were selected for their large, tender leaves, and these were probably the first domesticated forms of the B. oleracea. Modern varieties of these plants may have green, purple, or red leaves that may be flat or frilly. Scientists believe kale most closely resembles the wild ancestor of this species.
- Broccoli was bred for its flower buds and tender stems. Plants can form large heads or small spears that can be green, purple, or yellow in color.
- Brussels sprouts was selected for its lateral buds, and it is believed to have been bred from cabbage in Brussels, Belgium in the 16th century.
- Cabbage was developed to produce tightly wrapped leaves around a large terminal bud. Cabbages are also available in a variety of shapes (rounded or savoyed leaves) and colors (white, green, red, and purple).
- Cauliflower is believed to have been developed from broccoli by selecting for easily blanched, tender flower buds and shoots. While white cauliflower is the most familiar form, heads can also be chartreuse, orange, or purple in color.
- Kohlrabi was bred to produce an edible stem that is swollen and fleshy. Plants are available in green or purple.
While each of these crops may look quite different from one another, they are all members of the same species and are purely the product of human selection. While it’s unclear exactly when the first Brassica oleracea was domesticated, there is evidence of early versions of broccoli, cabbage, cauliflower, collards, and kale being grown by the Greeks as early as 300 BCE. The Romans brought cabbage and kale with them on their conquests of and to their settlements in northern Europe between 40 and 450 CE. Kohlrabi and Brussels sprouts were developed in northern Europe around 1500 BCE. New and improved varieties of these crops continue to be introduced in modern times (Colley et al., 2015).
Attribution and References
Attribution
Image credit: “CIMMYT- CESBatan - MEX - 06082019 - 0037.jpg” by Alfonso Cortés/CIMMYT is licensed under CC BY-NC-SA 2.0
References
Colley, M., Zystro, J., Buttala, L. A., & Siegel, S. (2015). The seed garden: The art and practice of seed saving. Seed Savers Exchange.
Diamond, J. (2002). Evolution, consequences and future of plant and animal domestication. Nature (London), 418(6898), 700–707. https://doi.org/10.1038/nature01019
Diamond, J. (2005). Guns, germs, and steel: The fates of human societies. W.W. Norton.
Fedoroff, N.V. (2003). Prehistoric GM Corn. Science, 302(5648), 1158–1159. https://doi.org/10.1126/science.1092042
Mann, C. (2005). 1491: New revelations of the Americas before Columbus. 1st ed. Knopf.
Raven, P.H., R.F. Evert, and S.E. Eichhorn. (2005). Plants and People. Biology of plants. 7th ed (pp. 475-495). W.H. Freeman and Company, Worth Publishers, New York.
Standage, T. (2005). A history of the world in 6 glasses. Walker & Co.