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Text 6.3. Artificial Selection




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Essential targets:

By the end of this text you should be able to:

· describe one example of artificial selection;

· distinguish between inbreeding and outbreeding;

· explain the meaning of hybrid vigour.

 

Pre-reading

■ In a small group discuss the current problems associated with the artificial selection and then try and answer the following questions:

1. How do you understand artificial selection?

2. Do you have any ideas about how wheat cultivation began?

3. Can you explain the difference between inbreeding and outbreeding?

Read the given text and make your essential assignments:

The cultivation of wheat

Ever since farming began in the Middle East about 10 000 years ago, humans have been breeding animals and plants selectively to produce specific desirable qualities. Wheat was probably among the first crop to be cultivated. By selective breeding over thousands of generations, wild wheat has been converted into the modern types which produce much higher yields. In selective breeding, particular individuals are chosen and allowed to breed, whereas others are prevented from breeding. This means that alleles that give characteristics favoured by humans are retained, while those that give undesirable characteristics are eliminated. Artificial selection is therefore similar to directional selection, in that selection pressure brings about a gradual change in the genotype of a group of organisms. However, in artificial selection it is humans, not environmental factors, that act as the selection pressure, gradually bringing about changes in allele frequencies.

We can only speculate as to how wheat cultivation began. Perhaps people who gathered wild seeds for food observed that seeds spilled accidentally sprouted new plants from which more seeds could be harvested. This might have encouraged them to save some seeds to sow for the following season’s crop.

Wild wheat sheds its grains as soon as they are ripe. This makes harvesting difficult. Therefore, grains were most likely to be gathered from plants that by chance retained their grains a little longer. By using this grain for the next crop, farmers would inadvertently have started the process of selective breeding.



The next stage in the cultivation of wheat would have been the deliberate selection of varieties with desirable qualities. Early farmers appear to have selected grains from plants which gave the greatest yield, and produced grain which was easy to separate from its husk. Eventually, over many generations, the variety of cultivated wheat changed. This led to the ancestor of our modern wheat, in which the grains are held so firmly that they must be removed by a separate operation after harvest. Selective breeding of wheat continues today by a combination of inbreeding and outbreeding.

Inbreeding involves breeding between closely related individuals which by chance, possess some desirable character. In wheat, desirable characters include:

· high yield

· short stem length (allowing the plant to devote more energy to the production of seeds, which have a much higher value than straw from stems)

· pest resistance (for example, to fungal moulds and rusts)

· high protein content of the grain.

Inbreeding is carried out to try and retain the desirable characters in future generations. Wheat plants are particularly suitable for selective breeding because they self pollinate naturally. They are unlikely to cross fertilise without the intervention of the plant breeder.

Inbreeding allows a farmer to produce a uniform crop which is easy to harvest and has, given certain conditions, predictable characters. However, this uniformity of characters is at the expense of genetic diversity may be reduced to such an extent that every individual has identical alleles (a condition known as complete homozygosity). Such a wheat strain cannot be changed because there are no other alleles present that could produce genetically different plants. Another problem is that if genetically identical plants are exposed to new diseases to which the plants have no resistance, all the plants may be killed.

 
Similar techniques of selective breeding have been used to develop domestic and farm animals. Although complete homozygosity has not been reached in any animals, inbreeding increases the risk of a harmful recessive allele occurring in the homozygous condition and being expressed. Because of these disadvantages, inbreeding is not carried out indefinitely. New alleles are introduced by outbreeding with other stock.

Outbreedinginvolves crossing individuals from genetically distinct strains. The offspring from such a cross are called hybrids. If the parental stocks are pure breeding, the offspring are called F1 hybrids. F1 hybrids often have characters, such as grain yield in wheat, which are superior to the characters in either parent. This phenomenon is called hybrid vigouror heterosis. Hybrid vigour probably results from an increased heterozygosity arising from the mixing of alleles. Harmful recessive alleles are less likely to be present in the homozygous condition. Hybrid vigour is also thought to result from some form of interaction between particular combinations of alleles in the hybrid. Whatever the explanation of hybrid vigour, if the descendants of F1 hybrids are continually inbred, the vigour decreases as the plant become more homozygous again.

Outbreeding depends on the availability of genetically distinct animals and plants. It is therefore important to maintainsources of genetic diversity. This may be done by maintaining seed banks of old or wild varieties of plants (the genetic diversity of wheat, rice, cabbages, and carrots is maintained in this way). Also, adults of old varieties of animals and plants with little or no commercial value may be maintained as a source of new alleles for future breeding programmes.

 

■ Glossary of essential terms for you to know:

English term Russian equivalent
to breed разводить
to produce производить
desirable желаемый
inbreeding родственное спаривание
outbreeding неродственное спаривание
crop (syn. yield, harvest) урожай
to bring about осуществлять; вызывать
to retain сохранять
artificial искусcтвенный
wheat пшеница
wild дикий
to gather собирать
cultivation обработка, возделывание
grain (syn. seed) зерно
ripe спелый
to sprout дать ростки
to sow сеять
stem стебель
predictable предсказуемый
to reduce (syn. decrease) уменьшать
to allow позволять
to carry out выполнять
to fertilise оплодотворять
suitable подходящий
resistance сопротивление
domestic animals домашние животные
to involve вовлекать
distinct различный
strain (syn. stock) род, происхождение
harmful вредный
disease болезнь
vigour сила
interaction взаимодействие
descendant потомок
to maintain поддерживать, сохранять
diversity (syn. variety) различие, разнообразие
to depend on зависить от
therefore поэтому, следовательно
adult взрослый
to pollinate опылять

 

■ Your Essential Assignments

I. Quick check:

1. Which type of natural selection does artificial selection resemble?

2. What effect does: a) inbreeding; b) outbreeding have on the genetic diversity of a population?

3. Give two possible explanations of hybrid vigour in plants produced by a cross between two different strains of pure-breeding plants.

 

II. Fill in the missing words:

Term (verb) Noun Adjective
suit ....... .......
resist ....... .......
interact ....... .......
value ....... .......
cultivate ....... .......
desire ....... .......

III. Use monolingual English dictionary and write down what could the words given below mean:

breeding, desirable, seed, cultivation, stem, crop.

IV. Match these words with their definitions:

decrease A. the seeds of crops
modern B. living in natural state, not changed or controlled by humans
famine C. the preparation and use of land for growing crops
yield D. to go down to a lower level
grain E. happening because someone has made it happen and not as a part of a natural process
hybrid F. physical and mental energy
7. domestic animal G. breeding between closely related individuals
8. wild H. time belonging to the present time
9. cultivation I. to plant seeds on a piece of ground
artificial J. to produce crops, profits
vigour K. a thing, place activity, etc. that you get something from
12. pollinate L. an animal or plant produced from parents of different breeds or types
13. inbreeding M. an animal lives on a farm or in someone’s home
14. sow N. no food for a long time and many people or animals die
15. source O. to make a flower or plant produce seeds by giving it pollen

V. Find English equivalents to the following word combinations:

Russian term English equivalent
1. разводить животных и растения  
2. постепенное изменение в генотипе  
3. возделывание пшеницы  
4. процесс отборного размножения  
5. тесно связанные между собой особи  
6. сохранить желаемые характеристики  
7. опыляться естественным путем  
8. происходящий в гомозиготных условиях  
9. гибридная сила  
10. зависит от  
11. особые комбинации  
12. присутствуют в гомозиготных условиях  
13. дикие разновидности растений  

 

VI. Give Russian equivalents to the following English terms:

English term Russian equivalent
to produce specific desirable qualities  
selective breeding  
artificial selection  
to sow seeds  
pest resistance  
genetically different plants  
to be exposed to new diseases  
have no resistance  
to develop domestic and farm animals  
genetically distinct strains  
the mixing of alleles  
the descendants of hybrids  
to maintain sources of genetic diversity  
future breeding programmes  

 

VII. Find synonyms among the pool of words:

Pool of words Synonyms
1)1.harvest /2.stock /3.crop /4.yield /5.strain /6.breed  
2) 1.seed /2.vigour /3.strength /4.grain  
3)1.retain /2.sow /3.decrease /4.keep /5.plant /6. reduce  
4) 1.distinct /2.diversity /3.different /4.variety  

VIII. Answer the following questions. Use all information given before:

1. How long have humans been breeding animals and plants selectively to produce specific desirable qualities?

2. What does selective breeding mean?

3. What type of natural selection is artificial selection similar to?

4. Describe how wheat cultivation began.

5. What does inbreeding involve?

6. Why is inbreeding carried out?

7. What does outbreeding involve?

8. How is the offspring from outbreeding called?

9. What is called hybrid vigour?

 

IX. Match the sentence halves. Make complete sentences:




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