Weeds Versus Domesticates
How does a domesticated plant differ from a wild or weedy one, how can plants become domesticated, and how can an archaeologist tell which they have? Both weedy and domesticated plants like to grow in soil that has been disturbed, whereas wild plants do not. Weedy plants possess a number of characteristics that enable them to survive on their own: they are good at dispersing their own seeds, their seeds may have dormancy or the ability to lie in the ground for many years before sprouting, different plants and sections of individual flowers mature at different rates, and overall the plants display phenotypic (morphological) plasticity or variability.
Little barley (Hordeum pusillum) on left; maygrass (Phalaris caroliniana) on right.
The inflorescences ripen from the top down at different rates on these weedy plants. As each unit containing a grain ripens in little barley, the unit breaks off and spreads the seed. Note the protective coverings on little barley grains that would have to be removed to make the grain edible.
Domesticated plants, on the other hand, are usually less good at dispersing their own seeds, their seeds do not possess dormancy and must either sprout the year they are planted or not at all, different plants and individual flowers tend to mature at the same rate, and overall the plants are more rather than less similar to each other. Finally, among most domesticated plants the desired part – whether it is a fruit, nut, seed, leaf, root, or stem – is usually larger than it is in nondomesticated plants. Alternatively, the domesticated plant may be less toxic than the nondomesticated plant. All of these morphological changes indicate genetic changes that set domesticated plants apart from weedy plants.
How can such changes come about? Nearly all of our domesticated plants were domesticated prehistorically by ancient peoples. Archaeologists believe that domestication was an unconscious process that occurred thanks to everyday interactions between peoples and plants. Let’s follow a hypothetical weedy plant through its process of domestication. One patch of this weedy plant is visited by a person who collects seed. The collector is able to gather only a portion of the genetic variability present in the weedy population, which in effect jump starts a genetic change if she then plants the collected seeds elsewhere and collects the next generation to plant again. Those weedy individuals good at dispersing their seeds escape collection. Likewise, those individuals that mature at a different time escape collection. By collecting once, the horticulturalist gathers only those seed with genetic tendencies to mature at the same time and not disperse their seeds. By harvesting the seed once again the following year from the planted collection, the horticulturalist selects for those seeds that lack dormancy. When this process is repeated year after year, the differences between the collected plants and the weedy populations become more and more pronounced: in effect, the weedy plants become weedier (those with less weedy tendencies have been collected and removed from the gene pool) and the collected plants are selected to exhibit more domesticated properties. No deliberate breeding program need be followed. However, if the horticulturalist specifically selects certain seed to plant – say, from the largest fruit – then selection will also result in a planted population that tends to have larger fruit than the original weedy population.
Although the genetic changes that set a domesticated plant apart from its weedy relatives may not be examined in the charred plant remains commonly preserved (see "What Can You Learn From Trash?"), morphological or physical characteristics that CAN be examined accompany the genetic changes. In particular, the paleoethnobotanist can track the enlargement through time of plant parts such as edible seeds. One example of this is the domestication of the oily seeded sumpweed (Iva annua variety macrocarpa). First recovered from late Middle Archaic Period sites dating to 5,000 years ago in west-central Illinois, over the next 1,000 years the mean size of sumpweed achenes (woody seeds) increased to up to ten times larger than their original size! Its progress toward domestication was matched by that of sunflower (Helianthus annuus variety macrocarpus). Whereas domesticated sunflower is still grown today, the domesticated sumpweed faded from the memory of all but archaeologists by the time of European contact (but see "Uses Change Through Time").
The loss of seed dormancy or the ability to lie in the ground for many years before sprouting may also be tracked in the archaeological record. Seeds that possess dormancy have thick seed coats that keep out moisture. Seeds lacking dormancy have thin seed coats. Domesticated chenopod (Chenopodium berlandieri subspecies jonesianum), present as early as 3,500 years ago in eastern Kentucky, has a thin seed coat with truncate seed margins (edges) compared to weedy chenopod, which has a thick seed coat and biconvex seed margins. The North American domesticated chenopod whose seeds were an important source of starch for thousands of years perhaps survived to European contact – there are one or two historic accounts that seem to describe this plant in Indian gardens – but soon after contact it went extinct, evidently to be replaced by corn in the diet. Although only weedy forms of chenopod or goosefoot survive in North America today, elsewhere other domesticated chenopods (such as quinoa, sold at health food stores) still survive. Take a look at quinoa and you will notice that the thin-coated domesticated seed is pale or white in color, whereas the thick-coated weedy seed from a plant in your yard is dark or black in color. Colors such as these are visible only in rare desiccated specimens.
Little barley (Hordeum pusillum) on left; maygrass (Phalaris caroliniana) on right.
The inflorescences ripen from the top down at different rates on these weedy plants. As each unit containing a grain ripens in little barley, the unit breaks off and spreads the seed. Note the protective coverings on little barley grains that would have to be removed to make the grain edible.
Domesticated plants, on the other hand, are usually less good at dispersing their own seeds, their seeds do not possess dormancy and must either sprout the year they are planted or not at all, different plants and individual flowers tend to mature at the same rate, and overall the plants are more rather than less similar to each other. Finally, among most domesticated plants the desired part – whether it is a fruit, nut, seed, leaf, root, or stem – is usually larger than it is in nondomesticated plants. Alternatively, the domesticated plant may be less toxic than the nondomesticated plant. All of these morphological changes indicate genetic changes that set domesticated plants apart from weedy plants.
How can such changes come about? Nearly all of our domesticated plants were domesticated prehistorically by ancient peoples. Archaeologists believe that domestication was an unconscious process that occurred thanks to everyday interactions between peoples and plants. Let’s follow a hypothetical weedy plant through its process of domestication. One patch of this weedy plant is visited by a person who collects seed. The collector is able to gather only a portion of the genetic variability present in the weedy population, which in effect jump starts a genetic change if she then plants the collected seeds elsewhere and collects the next generation to plant again. Those weedy individuals good at dispersing their seeds escape collection. Likewise, those individuals that mature at a different time escape collection. By collecting once, the horticulturalist gathers only those seed with genetic tendencies to mature at the same time and not disperse their seeds. By harvesting the seed once again the following year from the planted collection, the horticulturalist selects for those seeds that lack dormancy. When this process is repeated year after year, the differences between the collected plants and the weedy populations become more and more pronounced: in effect, the weedy plants become weedier (those with less weedy tendencies have been collected and removed from the gene pool) and the collected plants are selected to exhibit more domesticated properties. No deliberate breeding program need be followed. However, if the horticulturalist specifically selects certain seed to plant – say, from the largest fruit – then selection will also result in a planted population that tends to have larger fruit than the original weedy population.
Although the genetic changes that set a domesticated plant apart from its weedy relatives may not be examined in the charred plant remains commonly preserved (see "What Can You Learn From Trash?"), morphological or physical characteristics that CAN be examined accompany the genetic changes. In particular, the paleoethnobotanist can track the enlargement through time of plant parts such as edible seeds. One example of this is the domestication of the oily seeded sumpweed (Iva annua variety macrocarpa). First recovered from late Middle Archaic Period sites dating to 5,000 years ago in west-central Illinois, over the next 1,000 years the mean size of sumpweed achenes (woody seeds) increased to up to ten times larger than their original size! Its progress toward domestication was matched by that of sunflower (Helianthus annuus variety macrocarpus). Whereas domesticated sunflower is still grown today, the domesticated sumpweed faded from the memory of all but archaeologists by the time of European contact (but see "Uses Change Through Time").
The loss of seed dormancy or the ability to lie in the ground for many years before sprouting may also be tracked in the archaeological record. Seeds that possess dormancy have thick seed coats that keep out moisture. Seeds lacking dormancy have thin seed coats. Domesticated chenopod (Chenopodium berlandieri subspecies jonesianum), present as early as 3,500 years ago in eastern Kentucky, has a thin seed coat with truncate seed margins (edges) compared to weedy chenopod, which has a thick seed coat and biconvex seed margins. The North American domesticated chenopod whose seeds were an important source of starch for thousands of years perhaps survived to European contact – there are one or two historic accounts that seem to describe this plant in Indian gardens – but soon after contact it went extinct, evidently to be replaced by corn in the diet. Although only weedy forms of chenopod or goosefoot survive in North America today, elsewhere other domesticated chenopods (such as quinoa, sold at health food stores) still survive. Take a look at quinoa and you will notice that the thin-coated domesticated seed is pale or white in color, whereas the thick-coated weedy seed from a plant in your yard is dark or black in color. Colors such as these are visible only in rare desiccated specimens.
Chenopod (Chenopodium berlandieri) has both edible leaves and seeds
An SEM photograph of a charred, domesticated chenopod seed. Note the thin seed coat visible at the upper edge of the seed and the accompanying truncate-margined shape to the seed. Domestication frequently results in a thin seed coat that does not allow the seed to lie dormant for years in the ground before sprouting.
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Those plants that show no morphological or physical changes yet occur in quantities, associated with known domesticates, or in geographic regions where they would not naturally grow, are considered to have been at least cultivated if not domesticated. Maygrass (Phalaris caroliniana) is one example of a cultivated plant valued for its starchy seeds prior to the rise in importance of maize. Native to the coastal plain and lower piedmont along the eastern seaboard, maygrass was an important item in the diet of Indians who lived in the midcontinent far away from its native habitat.
So far, no morphological (physical) changes have been found on archaeological maygrass that would indicate it was domesticated rather than simply cultivated. Nevertheless, it was an important source of starch prior to dependence on maize.
© Gail E. Wagner, 2014. The views and opinions expressed in this page are strictly those of the page author. The contents of the page have not been reviewed or approved by the University of South Carolina. Page last updated 13 Sept. 2014.