2. History of Plant Breeding
2.1. PLANT DOMESTICATION
The domestication of the plant was man’s crowning achievement. It allowed us to develop into the complex global society that we are today. We are arguably more dependent on those same crop species domesticated by early man up to 10,000 years ago than we ever have been. Whether early man joyfully embraced the new technology of agriculture is debatable, but once it caught on it spread quickly across continents. The hunter-gatherer systems of old were notoriously land inefficient. It took large acreages to support these early humans, and so agriculture arose from necessity and allowed more people to survive on fewer acres.
The process of man’s conversion to agricultural systems was spurred along by the warming of the earth and the scarcity of large land mammals previously hunted for food and clothing. The plants used to fill the void were less selected than they were chanced upon. Traits that made domestication possible were controlled by few genes. These traits were fixed quickly and we are left with those same original domesticated crops from antiquity. The crops have certainly evolved, but not as much as they did during those first centuries.
With domestication came some negative aspects such as reduced genetic diversity. The genetic bottle neck effect seen in modern crops is a product of man’s selection for desirable agronomic traits. Unfortunately modern crops are often susceptible to disease, insects, and abiotic stresses. To find resistance genes it is often necessary to go back to their wild ancestors and close relatives. This is can be problematic due to the setback in yields attained when crossing to wild relatives, but it necessary for advancement of the crop. An understanding of crop domestication can help the plant breeder in her pursuit of the next best plant.
Early humans lived as hunter gatherers, victims of the wax and wane of the ecosystem in which they inhabited. For those that lived in grassland systems, a nomadic existence, following the plants and animals they fed upon was necessary. Tropical forest dwellers or those that lived in ecosystems where food was available year-round could build more permanent homes. They all depended on that which sprang forth from the ground naturally for their sustenance however, which meant that their fates were not necessarily their own to choose. Food availability depended on what the ecosystem could provide, and searching for that food required a great deal of early man’s time and energies. Survival was a full-time job. This inherent lack of control over their fates changed roughly 5,000 to 10,000 years ago with the domestication of the plant species that would become the first agricultural crops (Smith and Pluciennik, 1995). The change did not occur abruptly (Anderson, 1956), and certainly did not resemble what we today would call agriculture for quite some time.
The domestication of the plant was arguably the single most important technological advance in our history, and allowed us to develop into the highly complex civilization we have become. As technologically advanced as we might be, we are still as dependent on plants as we have ever been. It could be argued, that with the current population and rate of growth, we are more dependent on these crops than ever. There were 6.1 billion humans on earth in 2000, and current population estimates for 2050 range from 7.4 billion to 10.6 billion (UN, 2004). Not only is that a lot of mouths to feed, but homes for 7 to 10 billion people covers large amounts of land. Much of that same land will be needed for food and fiber production.
It is interesting that the crops we grow globally today, to feed an ever growing society, in most cases were the same species our ancestors originally domesticated thousands of years ago. The beginnings of agriculture and plant domestication occurred at different times and places, with different plant species, for different societies around the globe (Flannery, 1973). It appears that some societies did this independently of each other, and for other societies the technology was introduced. An in-depth review of the archaeological evidence is beyond the scope of this chapter however, a discussion of plant domestication is impossible without an archaeological perspective.
2.1.1 MAN’S DOMESTICATION OF THE PLANT
The most likely model of man’s transition to food production from hunting and gathering is one that includes several explanatory variables and probably occurred gradually in several stages (Ford, 1985; Harris, 1989; Redding, 1988). The exact mode of action is contentiously debated; however, Redding (1988) provides a useful generalized method. The proposed model involves the hunter-gatherer population for a given environment reaching the carrying capacity of the land and using methods to side-step the carrying capacity, either by avoidance or by directly increasing the capacity. The inhabitants of the over-populated environment would have dealt with the lack of resources by 1) emigration, 2) reducing reproductive rate, 3) diversification, or 4) storage (Redding, 1988).
It is necessary to explain these methods clarified by Redding (1988), as they are tantamount to the evolution of agriculture. Emigration to a new environment with more available plants and animals to hunt is probably the easiest and most common method ancestral man used to escape the limited carrying capacity of a given environ. Reducing the reproductive rate would have held the population level at the environment’s carrying capacity for a longer period. Diversification involved finding new sources of nutrition like a novel food plant or devising new technology to process an available resource not yet utilized for food, such as a mortar and pestle. To decrease the uncertainty of the food supply, humans would have had to broaden their food choices and devise new methods to exploit novel sources (Flannery et al., 1969; Stiner et al., 2000).
Storage could be considered a form of diversification, as it in many cases involves development of technology. It is most likely the source from which agriculture evolved. Food storage could include capturing more animals than a group could consume immediately, and tying them up for later consumption. It could also include the storage and carrying of edible seeds to eat later. The storage and transport of seeds could have easily led to planting the seeds for later harvest the next time the hunting-gathering group camped in the same area (Redding, 1988).
Any number of likely scenarios exists for the small leap from simply carrying around a few extra seeds for a later snack, to the conscious effort of saving some seeds to plant in a favorite tribal camping ground. The small leap might have been spurred along by the rapidly changing climactic conditions of the late Pleistocene and early Holocene (Richerson et al., 2001). As the earth warmed and glaciers receded the large land mammals became less numerous. As the preferred nutrition of our hunter-gatherer forebears declined in numbers, human populations simultaneously increased. This increase in human population and decrease of such a vital resource caused the early humans to search for new resources and develop new behaviors (Binford and Binford, 1968; Flannery et al., 1969).
2.1.2 THE SPREAD OF AGRICULTURE'
It is commonly held that agriculture arose at as many as nine different locations scattered around the globe, independently of each other. These agricultural origins essentially mirror Vavilov’s originally proposed “centers of origin.” These centers of origin are the places where Vavilov suggested the currently cultivated crop species were originally domesticated from the wild type (Vavilov, 1926). It is from these centers that domesticated plants and agriculture first spread. The spread of this new technology would have been a slow process. It is unlikely that wholly nomadic peoples converted to sedentary agricultural systems over-night. It is more likely that the transition was a slow one, involving both methods together for quite some time before finally settling down into a wholly agrarian existence (Smith, 2001a; Smith, 2001b).
These new methods and new plants would have spread faster going east or west across the globe from their origin. This east/west spread was easier due largely to plant adaptability to climate (Diamond, 1997). As was found to be the case in Africa, the spread north or south was more difficult due to climate adaptation (Marshall and Hildebrand, 2002). Cohen et al. (1984) suggest that the beginnings of agriculture would have resulted in decreased fitness for adherents to the new method. This could very well be the case for groups who were forced to switch from an entirely hunter-gatherer existence to an entirely agricultural existence due to lack of prey animals or some catastrophe. The agricultural outputs would have struggled to catch up with necessity. In an experiment to test the difficulty of harvesting wild grains by hand, one researcher went to a naturally occurring stand of wild wheat in Turkey. He demonstrated that a person could easily harvest a year’s supply of grain in just a couple of weeks using nothing but their hands, and considerably more grain with a hand-held sickle made of flint (Harlan, 1967). So, given a shortfall in the productivity of a certain environ, it would have been quite easy to harvest sufficient food from the plant-scape of one’s environment. The limiting factor would be knowledge of which plants to taste or eat. Once the knowledge hurdle was crossed, the idea would have spread quickly within and without camps.
The domestication of the plant and the subsequent development of agriculture allowed people to set down permanent roots and develop the rich cultures that led to our existence. With agriculture came the production of excess food and sedentary villages that were hitherto unobtainable. The excess of food, and the decrease in time required to spend foraging, lead to a division of labor, the development of such things as art and science, and gave birth to modern civilization (Diamond, 2002). Population growth, thought to be a contributing factor to the development of agriculture, was also a consequence of agriculture’s increased sedentism (Lee, 1980). Fortunately, more people could be sustained by a smaller land area with agriculture than before.
2.1.5 CONSEQUENCES OF DOMESTICATION
Few plant species, of the thousands of possibilities, were ever domesticated for food, fiber, or other human use. In the immensely popular book, “Guns, Germs and Steel: The Fates of Human Societies”, Diamond (1997) cites a simple explanation for the domestication of this small percentage of available species. His basic hypothesis is that these species were used for their ease of breeding for those traits that made them useful plants. That is, the traits which made some plants desirable to the early plant breeders/domesticators were controlled by few genes (Diamond, 1997). This idea is supported by a great deal of molecular work discussed later in this paper. This is interesting, and answers some very important questions. An example of this simple inheritance of important agricultural traits is the shattering system in wheat and barley. The mechanism by which wheat and barley scatter their seeds at maturity is controlled by a single gene. When man selected for the non-shattering type wheat, the trait was fixed quickly and easily, making the crop preferable to others that might have been candidates (Zohary and Hopf, 1988). It is obvious that our early ancestors would have preferred these cereals to all others simply because the grain stayed on the plant longer, and so the harvest window was longer than others.
It seems that crop species were not necessarily selected, but serendipitously discovered because they did not need much tinkering to become valuable food sources and agricultural models. The leap from useless weed to valuable food source was short and relatively easy. Almonds provide another example of simple inheritance of beneficial traits. The wild progenitors of almonds contained bitter chemicals to fend off predators, however, the mutation that makes the distasteful compounds absent is a single gene system and as such was easy to select. Oak tree acorns, on the other hand, have similar distasteful compounds within them, but the trait is a polygenic trait, making selection difficult, especially for the unwitting plant breeders of antiquity, which might explain why oak trees have never been domesticated (Diamond, 1997).
Among the cultivated species, a certain set of traits exist that are common to nearly all of them. It was originally postulated by Charles Darwin that differences seen in cultivated plants from their wild relatives was due to selection pressures by early man (Darwin, 1859; Darwin, 1868). These are the traits that make the plants productive and beneficial to human society. This group of traits is commonly referred to as “domestication syndrome,” first proposed by Hammer (1984) and later expounded upon by Harlan (1992). The domestication syndrome traits imbued the crop plants with uniformity, predictability, and high productivity (Table 1). There are several traits involved or contributing that include short stature (rice, wheat), large fruit with tasty flesh (tomatoes, apples), non-shattering (rice, wheat, sorghum), reduced seed dormancy (common beans), and reduced feeding deterrents (virtually all). These traits are summarized by Frary and Doganlar (2003), and in the summary there is included a discussion on how few genes control each of these critical traits.
There is no better example of how these traits can come together to produce something truly nutritive, than corn and its progenitor teosinte. Teosinte is a weedy looking grass with a small seed head made up of only two rows of several small, hard seeds. It looks as though it came out of a gardener’s nightmare. But somehow this wild and bushy bunch grass became the robust, single stalked behemoth we now know as field corn, whose constituents are used as ingredients in a plethora of food products consumed heartily by Americans daily.
The domestication traits would have been immensely important to the early agriculturalist, and rapidly fixed within their germplasm. It has also been shown by several researchers that many of these domestication traits are clustered near each other on the chromosome, and so are often closely linked (Cai and Morishima, 2002; Khavkin and Coe, 1997; Koinange et al., 1996; Poncet et al., 2000; Xiong et al., 1999). This clustering of domestication traits along the genome, and the small number of genes controlling these traits, suggest that the jump from wild, weedy progenitor might have occurred quite quickly, perhaps in as little as 100 years (Frary and Doganlar, 2003). This is of course hard to prove without archaeological evidence.
2.1.6 THE BOTTLENECK CONCEPT
It is evident that genetic diversity of our crops is much lower than that of their wild relatives. Early farmers would have noticed these few mutants or deviants that exhibited a beneficial trait, kept them and planted them over and over again. This reduction of genetic diversity gave rise to what is known as the genetic bottleneck due to domestication. The bottleneck effect of domestication on the genetic diversity of crops was most likely due to small founder populations of the plants and strong selection pressures imposed upon these populations (Iqbal et al., 2001; Tanksley and McCouch, 1997).
Plant domestication was arguably the single most important advancement in the history of mankind. Once developed, agriculture spread across the globe like wild-fire. Our early ancestors unwittingly selected for traits that were easily fixed within the crop species, and as a product of their selection pressures, we now have reduced genetic diversity within our crops. Genetic bottlenecks have reduced the base of breeding materials available to the modern-day plant breeder. Fortunately, however, we know of this short-coming, and have tools to combat it. We know the centers of origin for most crops and have the wild relatives to use as sources of diversity. There is much that can still be achieved through plant breeding of the crops and genetic resources we have. As easy as it may be to complain and dwell on the lack of genetic diversity within our crops due to domestication. A discussion of plant domestication would be insufficient if it was not mentioned that our ancestors did an amazing job as amateur plant breeders. For people who never had the opportunity to attend a plant breeding class, much less learn how to tie a shoe, they did quite a service for us. It is truly amazing that the crops domesticated thousands of years ago are still with us today feeding 6 billion individuals.
2.2.Plant Breeding before Mendel
2.4.Plant Breeding in the 20th Century
2.4.4.Watson & Crick
2.4.5.The Green Revolution
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