Biol 1300 Unit 2.docx

**Biol 1300 Unit 2** **DOMESTICATION OF PLANTS** **EARLY HISTORY OF PLANTS AND PEOPLE** The domestication of plants is thought to have occurred approximately 10,000 years ago. Prior to domestication, small nomadic tribes of humans led a hunter-gatherer existence, collecting food from the wild. Po...

**Biol 1300 Unit 2** **DOMESTICATION OF PLANTS** **EARLY HISTORY OF PLANTS AND PEOPLE** The domestication of plants is thought to have occurred approximately 10,000 years ago. Prior to domestication, small nomadic tribes of humans led a hunter-gatherer existence, collecting food from the wild. Populations of hunter-gatherer groups were maintained well below the carrying capacity of the environment. Because food supplies are seasonal, hunter-gathering required a nomadic existence to provide an adequate supply of food throughout the year. Humans are omnivores, meaning we can digest both plant and animal matter. Plants were crucial for early societies, providing food, medicines, and psychoactive substances. Hunter-gatherers were expert botanists-ecologists, with extensive knowledge of plant life cycles, habitat requirements, and edible parts. While little is known about past hunter-gatherer societies, some present-day groups, like the !Kung of central Africa, still exist. They spend about two days per week obtaining food, with women gathering plants and men hunting, leaving ample leisure time. Why did human populations give up a nomadic hunter-gatherer existence in favour of plant domestication and agriculture? There are many incentives to settle in one place: an abundant and reliable local food supply (e.g. estuaries, with fish and rich soil for growing plants), access to trade routes, a year-round water supply, and so forth). [Settlement leads to plant cultivation (and animal domestication), resulting in:] More reliable, stable and dependable food supplies. Maintenance of a larger human population. Greater control over the local environment (both the plants and the land). A sedentary existence, i.e. permanent settlements and dwellings. Greater efficiency of food production, leading to more free time and career specialization (civilization). [When examining the origins of agriculture, several important questions are asked by ethnobotanists and cultural anthropologists:] Where were the centers of origin of agriculture, and were they in contact? What led to the development of agriculture, and what precipitated the move away from a hunter-gatherer existence? When did societies become partly, and wholly, dependent upon agriculture? Where did crop species originate, and how have crop plants changed over time? How did human societies and cultures change with the advent of agriculture? **ORIGINS OF AGRICULTURE** **Agriculture** is defined as the tilling of land for the deliberate sowing or planting of crop plants. A major advantage of agriculture is that it ensures an adequate food supply throughout the year. Agriculture arose from the domestication of plants and was often accompanied by the domestication of herd animals as well. It is likely that agriculture arose gradually, as a slow transition from a hunter-gatherer existence. Archeological evidence suggests that the domestication of plants occurred independently in three regions of the globe, and at about the same time. By 5,000 to 7,000 years ago, agriculture was commonly practiced in Asia Minor, China-Southeast Asia, and the Americas. The earliest evidence of agricultural development is from arid regions, and particularly the Fertile Crest region of Asia Minor (present-day Iraq, Iran and eastern Turkey). Why did agriculture develop in this region? Two reasons have been proposed: The need for a reliable and adequate food supply in this relatively arid region. Wild precursor food plant species (cereal crops in particular), conducive to domestication, are native to the region. However, it is important to recognize that archaeological materials are best preserved in more arid environments. It is possible that agriculture had also developed in more humid regions, but the archeological record has been lost. **Domestication of Plants** Several anthropologists and ethnobotanists have developed theories to explain the shift from a hunter-gatherer existence to agricultural dependence: -CHILDE (1928) (Neolithic): Gordon Childe proposed that humans and herd animals were brought together during extended dry periods, perhaps around watering holes. Disturbance to the soil and vegetation in these crowded encampments favoured the establishment of \"weedy\" grass species, the precursors of some of our modern domesticated cereal crops. Childe suggested a pathway proceeding from hunter-gatherer to animal herder and finally, to plant cultivator. This theory, developed for the Asia Minor region, is termed the Neolithic Revolution. -SAUER (1952) (favourable habitats): Carl Sauer proposed that human populations first developed a sedentary existence in favourable habitats (e.g. areas with a mild climate, available edible plants, good fishing, and/or an adequate water supply). He hypothesized that less optimal areas were settled as population sizes increased, necessitating the domestication of plants to ensure an adequate year-round food supply. This theory was originally developed to explain agricultural development in the southeast Asia region. -ANDERSON (1952) (weed precursor): This theory stresses the importance of weeds as precursors to domesticated plants. Edgar Anderson proposed that plant hybridization in disturbed habitats (e.g. dump heaps) resulted in considerable and rapid genetic variation and recombination, producing new and useful food plants that were then sown and harvested as crops. -BINFORD and FLANNERY (1960s) (applied botanists): These researchers hypothesized that early plant gatherers were sophisticated \"applied botanists\" who quickly learned to cultivate plants according to need. This hypothesis views the change from intensive gathering to cultivation as a simple and straightforward process: population pressures forced people into less favourable habitats (or the climate changed), making cultivation necessary. These theories are by no means mutually exclusive and are not necessarily consistent with all the available evidence from all regions. For example, it is thought that human populations in the Americas continued to wild-harvest plants and animals long after agriculture was first practiced. Agricultural development in the comparatively cool, dry climate of Asia Minor is not consistent with Sauer\'s hypothesis. **CENTERS OF AGRICULTURAL ORIGIN** The three principal centers for the origin of agriculture are outlined below. **ASIA MINOR (NEAR EAST)** This area incorporates the semiarid regions of Iran, Iraq and eastern Turkey, NOT including the Mesopotamian (Tigris and Euphrates) valleys; these valleys are thought to on have been settled later. The archeological site at Jarmo (in present-day Iraq) reveals the following developments: 10,000 years ago: wild grains were being collected. 8,750 years ago: the major cereal crop was wheat, although barley was also cultivated. Goats and sheep, and later pigs, were domesticated. Additional plants were domesticated over the next century, including peas, lentils, vetch, grape, olive, date, pears and cherries. 7,000 years ago: movement of human populations into the Tigris-Euphrates valleys, ensuring a more reliable supply of water and food. Sophisticated urban civilizations developed in these valleys by about 6,000 years ago. Agricultural practices were introduced from Asia Minor into the Balkan region of southeast Europe around 6,000 years ago. The cooler European climate led to a shift in cereal crops from wheat and barley to rye and oats. Pollen records from Danish Lake sediments show a rapid landscape transformation as forests were converted to cropland. Equipment for grinding cereal crops was developed in present-day Egypt (Nile delta of north-east Africa) about 14,000 years ago, though it's debated whether it was used for domesticated or wild cereals. Important African crops include sorghum, millet, and yams, with clear evidence of agriculture in the Sahara region by about 6,000 years ago. **CENTRAL CHINA (FAR EAST)** The earliest evidence of agriculture in central China is from the Yang-Chao site near the Hwang Ho (Yellow) River, dating back about 6,000 years. This region had a developed agrarian society with irrigated rice fields, large villages, and sophisticated social structures. While there is evidence of agriculture in other parts of eastern Asia, preservation issues in tropical environments limit our knowledge. However, Spirit Cave in Thailand shows evidence of bean and pea cultivation 9,000 years ago and rice cultivation 7,000 years ago. **CENTRAL AMERICA** The drier climate of central Mexico and Peru resulted in good preservation of archeological material. By 7,500 years ago there is evidence of agricultural development in both Central America (present-day Mexico) and South America (Peruvian Andes), although agriculture developed somewhat differently in these two regions. At Tehuancan in Mexico, agriculture developed more slowly than at Jarmo, Iraq. This slow development of agriculture is referred to as **incipient cultivation**. Developments were as follows: 9,000-7,000 years ago: mostly hunter-gatherer. 7,000 years ago: 15% cultivation; major agricultural crops included corn (maize), squash, peppers, amaranth, and avocado. 5,500 years ago: about 30% cultivation. 3,500 years ago: fully agricultural; hybrid corn, tomato, squash, bean, peppers, cotton, many fruits, domestication of dogs. 2,500 years ago: irrigation was introduced, and turkeys were domesticated. Plants native to South America were also being sown, indicating that seeds of crop plants were being traded. There is also evidence of agricultural development in wetter areas of Central and South America, but unfortunately archeological material in such areas is poorly preserved. The following sophisticated agricultural practices were about 2500 years ago: Aztec (Mexico): intensive irrigation agriculture. Mayan (Central America): selection of corn and bean cultivars. Inca (Andes, South America): potato domestication, irrigation systems. **SELECTION PRESSURES ON PLANTS** Plant characteristics result from the cumulative and synergistic effects of the genome, which evolves through natural selection. The phenotypic characteristics of wild plants are often drastically modified when humans begin to cultivate (sow and harvest) them. Although early agricultural development did not involve the deliberate and conscious selection of superior plant cultivars, the **planting-harvesting link** (what is planted aligns with what is harvested) led to rapid passive selection of agriculturally beneficial cultivars. In the wild, plant species are often prolific seed producers, and the seeds develop and mature over an extended period. Such factors ensure that at least some seeds will mature and/or germinate when environmental conditions are favorable. For illustration, consider a simplified example in which humans' plant and harvest two wild genotypes of a plant species: Genotype 1: seeds develop over an extended period, and there are few seeds on the plant at any given time. Genotype 2: seeds mature almost simultaneously, so that many are present on the plant at harvest time. Genotype 2 will be over-represented in the seed harvest and will therefore be more common in the next generation (i.e. when the harvested seeds are sown the following spring). After only a few harvest generations, this differential harvesting-planting by humans will strongly favour Genotype 2 (i.e. simultaneous seed maturation). Similar selection pressures have favoured the following characteristics in cereal crops: Uniform seed maturation (as discussed above). Compression of tillering: in wild grasses, clonal axillary shoots (tillers) are produced, resulting in the seeds maturing at different times; this will be selected against. Loss of seed appendages: selection for inedible appendages that \"shatter\" (fall off) before the seed is harvested. Loss of germination inhibitors: chemical inhibitors result in seeds germinating at different times; harvesting-planting selection will favor simultaneous germination. Increase in number of florets: more florets mean more seeds, which will be selected for. Reduction in day-length sensitivity: day length triggers seed production. Loss of this dependence is very important as crop plants are transported north. Loss of shattering: seed will stay on the plant, rather than shattering (falling off) prior to harvest; this will be selected for. The seed degrades once it falls off, which is not commercially viable. Increase of food reserves (starch) in the seed: when plants are sown at high density, intraspecific competition (i.e. competition among members of the same species) occurs. A seedling (embryo) with more food can grow faster and will therefore be bigger than its neighbours and thus survive to maturity. Seeds containing proportionately more carbohydrate (\"food\" for the germling) and less protein (which is concentrated in the embryo) are therefore favoured. Selection can also affect weedy species growing with the crop: Weeds may germinate with a crop but shed their seed before harvest and thus maintain their populations. Weeds may mimic the crop plant, developing a seed whose size is like that of the crop plant, and/or seeds that mature at the same time as those of the crop plant. Such seeds cannot be easily separated at harvest time and are planted back into the field along with the preferred crop. Rye is thought to have evolved in this way; it was initially a weed of early European wheat fields. **GEOGRAPHIC ORIGIN AND SPREAD OF PLANTS** Most of our common food plants originally had very limited global distributions. The introduction of new crops often had dramatic effects on both agriculture and the human diet. Examples include the introduction of potatoes from South America to Europe; the tomato and chili pepper from South America to Europe and Asia; and sugar cane from southeast Asia to the Caribbean. **EUROPE**: The Romans introduced several Mediterranean species into northern Europe, including peas, oats, rye and many herbs. The Arabic colonization of Spain introduced rice, sugar cane, sorghum and citrus fruits. The European conquest of the Americas was a critical time for the global distribution of food crops, e.g. corn (maize)-1600s in Spain, Portugal, Italy, 1800s into central Europe; potatoes-1600s in Spain, England, 1700s in Russia, central Europe. European explorations of eastern Asia (Vasco da Gama, 1497) resulted in the introduction of rhubarb, almond, apricot, peach, coffee, black pepper and many other Asian species to Europe. **ASIA**: The Asian continent has many indigenous food plants, most notably rice. Important crops introduced from the Americas include corn (maize), papaya, pineapple, potato, sweet potato, tapioca (cassava), and the chili pepper. Coffee was introduced from north-east Africa. **AFRICA**: Food plants indigenous to the African continent include coffee, sorghum, millet, yam, cowpeas, watermelon, sesame and palm oil. Important introductions from Asia include the coconut, rice, and bananas. Mango and eggplant are from the Near East. Many root crops (e.g. cassava, sweet potatoes), as well as corn (maize) and beans, were introduced from the Americas. **AMERICAS**: Central and South America had a good staple of indigenous crops, including corn, cassava, potato, sweet potato, peanuts, tomato, chili peppers and beans. By contrast, very few crop plants are native to North America. Spanish and English tropical colonists introduced sugar cane, bananas, rice, citrus fruits, breadfruit, and coffee to and sub-tropical America. **THE GREEN REVOLUTION** **Crop breeding focuses on high yield**, at the expense of disease and pest resistance. High-yield crops are expensive, typically need more fertilizer, water (possibly requiring irrigation), herbicides, pesticides, and mechanized harvesting. These crops are also more vulnerable to weather anomalies like extremely wet or dry seasons, hail, and windstorms, which can cause lodging (crop damage). The **green revolution**, which significantly increased crop yields, has promoted technological farming. This method benefits wealthy countries with large-scale farms but is often harmful to developing nations that cannot afford the necessary machinery, pesticides, and herbicides. Additionally, crops bred for temperate regions often do not thrive in the tropical and subtropical climates and soils of developing countries. **Plant monocultures**, which involve extensive plantations of the same genotype, are common in modern farming. These crops are highly susceptible to pathogen-pest outbreaks due to the lack of genetic resistance. A notable example is the Irish potato famine (1845-1846), the entire Irish potato crop was derived from a single plant imported from England. This monoculture potato crop was devastated by the potato blight, a contact species leading to massive famine in Ireland and parts of Scotland. Crop breeding for higher yields creates a "vicious cycle." When a newly developed resistant crop is widely planted, pests or pathogens eventually evolve to attack it. This necessitates the development of a new resistant cultivar, starting the cycle anew. This scenario is common in modern agriculture. **PLANT BREEDING: GENETIC MODIFICATION** The green revolution was a direct result of developmental advances in crop breeding, together with increased soil fertility, the control of pests and pathogens, and farm mechanization. Crop breeding methods include: **GENETIC CROSSING AND BACK-CROSSING**: Genetic crossing and back-crossing are the standard methods for developing new plant cultivars and maintaining hybrid vigor in crops like corn. This labor-intensive process requires patience. Once developed, favored cultivars like apple cultivars can be easily cloned through methods such as grafting (joining plant tissues by cutting parts and putting them together, common method for roses), root or shoot cuttings (cutting of parts of a plant like its stem and planting it), and tissue culture (like a petri dish for growing cells and specific parts). Unlike animals, plants are easily cloned, making propagation straightforward. In the 1960s, the University of Manitoba conducted a notable plant breeding program where researchers successfully crossed wheat and rye to create a hybrid crop called triticale. Triticale combines the winter resistance of rye with the higher yields of wheat, has high protein content, and grows better on poorer soils and in colder areas than wheat. This hybrid was developed using tissue culture and artificial chromosome doubling. **ARTIFICAL DOUBLING OF CHROMOSOME NUMBER**: **Colchicine**, a natural alkaloid from the Colchicum crocus (a member of the lily family (Liliaceae)), is used to induce polyploidy (doubling of chromosomes) in plants. Unlike animals (which is not beneficial), polyploids are common in plants and often result in larger, more robust plants or plant parts. Sometimes called \"monster plants.\" By doubling the number of chromosomes, polyploid plants have more genetic material to work with, which can lead to new traits and variations. Many modern agricultural crops like cereals are polyploids. **GENETIC MUTATION**: Genetic mutations can occur spontaneously or be induced artificially through irradiation. Once a useful mutation is identified, it can be easily propagated or cloned. For example, Brussels sprouts are a natural mutation of cabbage first noted in Belgium and later propagated as an economically important vegetable. Mutability, the susceptibility to genetic mutation, is crucial for crop development. Many cereals, cucurbits (Pumpkin or Cucurbitaceae family), and brassicas (members of the Mustard or Brassicaceae family) exhibit high mutability. **GENETIC ENGINEERING**: Genetic engineering of crops involves adding genetic information from another organism (e.g., plant, animal, bacteria, or virus) into a crop plant. Over the last decade, genetically engineered (GE) crops\* have become more common, with crops like canola, soybean, and corn being engineered for resistance to pests, pathogens, and herbicides. However, consumer resistance is strong in Europe, leading to bans or strict regulations. Slowly North America is also opposing these crops. Critics argue that GE crops could escape into natural habitats or hybridize with native plants, potentially harming ecosystems. GE crops are unnatural, so consumers are worried for their safety and health. Additionally, GE seeds are more expensive, making them less accessible to farmers in developing nations. More research is needed to address these concerns. \*=misleadingly termed \"genetically modified crops.\" Genetic modification encompasses a broad range of methods used to alter the genetic composition of plants and animals. These methods include traditional breeding techniques like selection, hybridization, and induced mutation. Conversely, genetic engineering specifically refers to crops that have been altered using modern biotechnology techniques. Genetic engineering involves the direct manipulation of an organism's genome (introduction of a transgene), often by introducing a targeted change in a plant, animal, or microbial gene sequence. Generally, GM uses existing genes whereas GE adds new genetic material. **GENETIC DIVERSITY** The development of new crop varieties, new crops, and medicinal drugs is dependent on the availability of a diverse genetic pool. Unfortunately, many of the older and less productive plant varieties and cultivars have disappeared as newer ones were developed. The genetic variation present in older varieties may prove useful in modern breeding programs. For example, older apple cultivars are now actively sought out by agricultural geneticists interested in developing new apple varieties. The loss of tropical rain forests and other natural ecosystems worldwide is also of great concern. These regions harbour a huge genetic resource base. It has been estimated, for example, that the rain forests contain nearly three-quarters of all species on earth. Many tropical plants (many of which have not even been described) could prove to be of immeasurable importance as foods or medicinal drugs (especially pharmaceutically).