(A) the main points of Condell's theory of crop origin
Swiss botanist Condell (1806- 1893) was a creationist of a species until 1950s, but later he gradually changed his view. He is the earliest founder of crop origin research. He studied the wild relatives, history, names, languages, archaeological evidence and variation types of many crops, and thought that the main criterion for judging the origin of crops was whether there were wild species forming this crop in the distribution area of cultivated plants. His masterpiece The Origin of Cultivated Plants (1882) involves 247 species of cultivated plants, which provides a model for future generations to study the origin of crops. Although it seems that the information quoted in the book is incomplete or even wrong, his contribution to the study of crop origin is indelible. Another great contribution of Condell was the drafting of the international botanical nomenclature rules for the first time in 1867. This rule has been in use ever since.
(B) the main points of Darwin's theory of evolution
Charles Darwin, a British naturalist (1809— 1882), published his masterpiece "The Origin of Species" in 1859. In this book, he put forward the following theories related to origin and evolution: ① evolution must exist; (2) Evolution is gradual and takes thousands to millions of years; ③ The main mechanism of evolution is natural selection; The existing species come from the same primitive life. He also pointed out that the variation within species occurs randomly, and the survival and extinction of each species are determined by its ability to adapt to the environment, and the survival of the fittest.
(C) The main points of vavilov's theory of crop origin
Russian (Soviet) geneticist vavilov (N.I.Vavilov, 1887- 1943) is not only a famous scholar who studies the origin of crops, but also the founder of plant germplasm resources. During the 1920s-1930s, he organized many inspection activities on four continents, and made detailed analysis and research on the farming system, crop utilization, ethnobotany and even environmental conditions. He collected 6.5438+0.5 million germplasm resources of various crops, including some wild relatives, and studied their phenotypic diversity. Finally, vavilov put forward a set of theories about the origin of crops.
In vavilov's theory of crop origin, the most important theory is the theory of crop origin center. He proposed in the article "Origin Center of Cultivated Plants" written by 1926 that the origin center of crops can be determined by studying the types of variation, and the region with the greatest genetic diversity is the origin of crops. After entering the 1930s, vavilov constantly revised his theory, and proposed that determining the origin center of crops should not only be based on the genetic diversity of this crop, but also consider the genetic diversity of its wild relatives, and also refer to archaeological and cultural materials. After many years of updating, vavilov analyzed the geographical distribution of phenotypic genetic diversity of more than 600 species (including some wild relatives) in 1935, and published the World Origin Center of Major Cultivated Plants [мировыеочч].
The Origin Center of Cultivated Plants in vavilov
1 .China 2. India 2a. Indo-Malaya. Central Asia 4. Near East 5. Mediterranean 6. Ethiopia. Southern Mexico and Central America. South America (Peru, Ecuador, Bolivia) a. Chile 8b. Brazil and Paraguay.
These centers are usually geographically separated by deserts or mountains. They are called "primary origin centers". Wild relatives and dominant genes of crops often exist in such centers. Vavilov also found that far away from this primary origin center, sometimes there is rich genetic diversity, and there may be some variations that are not found in its primary origin center. Vavilov called this area the "secondary origin center". There are many recessive genes in the secondary origin center. Vavilov believes that the genetic diversity of secondary origin centers is due to natural selection and artificial selection of crops introduced from their primary origin centers under the condition of long-term geographical isolation.
Vavilov regards the Mediterranean coast of North Africa and the area around the Mediterranean as the Mediterranean center; Abyssinia (now Ethiopia) in Africa is one of the centers of crop origin in the world; Take Central Asia as another origin center independent of pre-Asia (Near East); China, the United States and South America are independent centers of origin. Together with China and India (Indo-Malaya), it is the origin center of the world's eight major crops advocated by vavilov.
Homologous law in variation is also an important part of vavilov's theoretical system of crop origin. According to this theory, similar variations can be found in different crops in the same geographical area. That is to say, if a specific trait or phenotype is found in one crop in a certain area, the same trait or phenotype can also be found in another crop in that area. Hawkes( 1983) thinks that this phenomenon should be more accurately described as "similar series rule" because different gene loci may be related to it. Kupzov( 1959) regards this phenomenon as that similar mutations may occur at the same site in different species, or that different adaptive gene systems have evolved to produce similar phenotypes. The research results of genomics also support this theory.
In addition, vavilov put forward the concepts of "primary crops" and "secondary crops". "Original crops" refer to those ancient crops that have been cultivated for a long time, such as wheat, barley, rice, soybean, flax and cotton. "Secondary crops" refer to those crops that are weeds in the field at first and then slowly cultivated, such as rye, oats, tomatoes and so on. Vavilov has made important conclusions on the significance of local varieties, the significance of exotic and exotic materials, and the introduction theory.
After vavilov's "Eight Origin Centers of Crops" was put forward, other researchers revised the theory. Among these researchers, Zhukovsky, a student from vavilov, is the most influential. In 1975, he put forward the theory of mega-centers, and thought that there were 12 mega-centers, covering almost the whole world, except Brazil, southern Argentina, Canada, northern Siberia and some marginal countries. Zhukov also put forward the concept of "micro-center" of wild species that are genetically similar to cultivated species. He pointed out that there are differences in distribution between wild species and cultivated species. The distribution of wild species is very narrow, while the distribution of cultivated species is very wide and varied. He also put forward the concept of "primary gene megacenter", and thought that the primary origin center in vavilov was narrow, and the area where cultivated species spread was called "secondary gene megacenter".
(D) The main points of Harlan's crop origin theory
Harlan, an American geneticist, pointed out that the center of crop origin mentioned by vavilov is an area with a long history of agricultural development and local civilization, based on the fact that the geographical region of crop variation is closely related to the geographical region of human history. However, when the researchers analyzed different crops one by one, they found that many crops did not originate from the origin center referred to by vavilov, and even some crops did not have diversity centers.
Take the Near East as an example, there is indeed a small area that has domesticated a large number of animals and plants, which can be considered as one of the centers of crop origin; But in Africa, things are different. There are plant domestication activities everywhere in sub-Saharan Africa and north of the equator, so it is difficult to call such a large area "center", so Harlan calls this area "non-center". He believes that there are similar situations in other regions. For example, the north of China must be a center, while Southeast Asia and the South Pacific can be called "pan-regions"; Central America is certainly a center, while South America can be called a "pan-region". Based on the above considerations, Harlan (197 1) put forward his "central and pan-regional theory of crop origin". However, some later studies challenged this theory. For example, it is found that there are also plant domestication activities in the flank areas of the Near East center, including the Caucasus, the Balkans and Ethiopia; In China, because different Neolithic cultures have formed in different parts of the country, Harlan said that the northern center of China should actually be much bigger; Some areas outside Central America (including Mississippi Valley, Arizona and northeastern Mexico) also have independent domestication of plants. So Harlan (1992) finally gave up his previous theory and thought it was unnecessary to talk about the origin center.
Harlan (1992) classified the evolution types of crops into the following categories according to the temporal and spatial factors of crop evolution:
1. Native crops refer to those crops that have been domesticated and cultivated in an area and rarely spread in the future. Such as the arm grass originated in Guinea, the potbellied grass in Ethiopia, the crabgrass in West Africa, the green bristlegrass in ancient Mexico and the millet in Mexico.
2. Semi-native crops refer to those crops that originated in a region but spread moderately. For example, Eragrostic tef and Guizotia abyssinica originated in Ethiopia (also planted in some parts of India), Oryza glaberrima in central Niger and so on.
3. Monocentric crops refer to those crops that originated in a region but spread widely without secondary diversity centers. Such as coffee, rubber, etc. This kind of crops are often new industrial raw material crops.
4. Oligocentric crops refer to those crops that originated in a region but are widely distributed and have one or more secondary diversity centers. For example, all crops originated in the Near East (including barley, wheat, oats, flax, peas, lentils, chickpeas, etc. ).
5. Non-central crops refer to those crops that have been domesticated in a large area, at least the center is not obvious or irregular. Such as sorghum, common kidney beans, rape (rape) and so on.
1992 Harlan continued to adhere to the "origin of diffusion" put forward by him in his masterpiece Crops and Humans (second edition) many years ago. It means that the origin of crops can spread in time and space. Even if a crop is domesticated in a limited area, it will change in the process of spreading from the origin center. People in different areas may give this crop very different selection pressures, so that the crop formed after arriving in a specific area will be completely different from its original wild ancestors in ecology and morphology. He gave an example of corn, which was first domesticated in southern Mexico and then spread in all directions from the origin center. When Europeans arrived in America, corn had been planted in vast areas from southern Canada to southern Argentina, and a corn variety with its own characteristics had been formed in each cultivated area. Interestingly, in some large areas, such as North America, there are only ethnic minorities, and the types are relatively single; However, in some much smaller areas, including southern Mexico, Guatemala, parts of Colombia and Peru, there are many races, some of which are very diverse, and many races completely different from their origin centers have been found in Peru.
(5) The main points of Hawkes' theory of crop origin.
Hawkes (1983) thinks that the origin center of crops should be distinguished from the origin of agriculture, and thus puts forward a new theory of the origin center of crops. In this theory, the place where agriculture originated is called the core center, and the area where crops spread from the core center and form rich types is called the diversity area (Table 0- 1).
The core center and diversity area of cultivated plants
Core Center and Diversity Region of Cultivated Plants (continued)-1
At the same time, Hawkes used "small centers" to describe places where only a few crops originated.
Core Center Diversity Area Peripheral Center VII. Ethiopia VIII. West Africa C. Southern Mexico (southern Tehuacan) VII. Central America. America, Canada. Caribbean D. Central to southern Peru (Andean region, East Andes slope, coastal areas) ⅹ. Northern Andean region (Venezuela to Bolivia) 7. Southern Chile.
(six) the basic method to determine the center of origin of crops
How to determine the origin of specific cultivated plants is the central topic of crop origin research. Condell first proposed that as long as the wild ancestor of this cultivated plant is found, it can be considered as the place where it was first domesticated. But the problems are as follows: ① It is often difficult to determine whether the plants in a certain area are really wild types, because they may escape from cultivated types; ② Some crops (such as broad beans) have not found wild ancestors in nature; ③ The growing place of wild type is not necessarily the origin of cultivated plants. For example, there are many wild tomato varieties in Peru, but other evidence shows that cultivated tomatoes may have originated in Mexico. (4) With the development of science and technology, it is found that the previously recognized wild ancestors have nothing to do with cultivated plants. For example, in history, wild potatoes grown in Chile, Uruguay and Mexico were considered as the wild ancestors of cultivated potatoes, but later it was found that they were not closely related to cultivated potatoes. So we must be cautious in the research process.
In addition, when studying the origin of crops, we need to be cautious about historical evidence and linguistic evidence. Because the domestication of most crops appeared before the appearance of characters, later historical records often came from folklore or mythology, and were often handed down in the wrong way. For example, the Romans believed that peaches came from Persia, and because peaches were found in Persia, they were named persica in Latin. In fact, peaches were first domesticated in China and then spread to Persia in Roman times. The Latin name of Xiaomi is Setaria italica, and there is a similar situation.
Therefore, when studying the origin of crops, we should take botany, genetics and archaeological evidence as the main basis, that is, we should pay special attention to the diversity of crops themselves, their wild ancestors and archaeological evidence. Historical and linguistic evidence is only a supplementary and auxiliary basis.