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Evolution: 11. Selective breeding of plants and animals, methods and their advantages and disadvantages

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Sub-index of biology notes on all aspects of EVOLUTION

(11) Selective breeding of plants and animals (artificial selection by humans!)

Introduction - methodology of selective breeding and reasons for it

Selective breeding has been practised for thousands of years producing the forerunners of our modern crops and farm animals.

Even Darwin had noticed and admired the successful breeding of livestock in agriculture and 'fancy' pigeon enthusiasts.

BUT note, this initially does not produce a new species.

In principle selective breeding of plants or animals by us humans, the basic process is quite simple.

You pick the plants and animals with the best features you want and interbreed them to get the best possible offspring - you are cross-breeding varieties to get the best outcome - the best set of genes.

The basic process of selective breeding

(i) From your existing plant/animal stock you pick those with the best characteristics you desire.

(ii) Breed your selection with each other.

(iii) Repeat the process selecting the best offspring from your initial stock from (ii).

(iv) You continue this process over several generations so that the desired trait gets stronger and stronger. The offspring should, in principle, display the desired characteristic to its full potential.

Unlike natural selection, 'in the wild', selective breeding is when we artificially pick the plants or animals to breed to keep the genes for the selected characteristic in the population.

This selective breeding develops the features that are e.g. most useful or attractive or resilient to the environment.

There is nothing new in selective breeding - for thousands of years, for their own use, people have been domesticating animals from the wild (e.g. dogs and cows) and producing useful edible grain like wheat, oats and barley from wild grasses.


Examples of selective breeding and their advantages

Crops that give the highest yield e.g. cereals or vegetables e.g.

Tall crop plants (e.g. wheat) give high grain yields but are easily damaged by rain and wind. Dwarf crop plants are better weather resistant, but give lower grain yields.

If you cross-breed the tall and the dwarf plant and then cross-breed the offspring, after several generations you get a new breed of crop variety that combines the good characteristics of the original varieties - a compromise of improved yields an weather resistance

Crops that are edible for our digestive system were bred from wild grasses thousands of years ago.

Crops that are disease resistant, without the controversy of genetically modified crops.

Farm animals give better yields e.g. milk from cows or beef from cattle.

You interbreed, through several generation, the best bulls or cows that give you the greatest volume of milk per cow or the greatest mass of meat per head of cattle.

This is not new in the 20th/21st centuries - this has been done for thousands of years e.g. domestic cows have been bred from wild cattle, domestic woolly sheep from wild sheep.

With these and other animals you may also want to breed for other good phenotypes e.g. good mothering skills, amiable temperament, successful fertility rates and good health in general.

Developing a particular breed of dog - size, colour, quality of fur, facial looks, amiable temperament.

It is believed the first domesticated dogs were first bred from wolves by people in China 16,000 years ago.

Race horses and greyhounds are bred from thoroughbred stock known for their speed in racing.

Laboratory bred animals with reared with particular preferences (e.g. different foods, testing drugs?) and the results compared e.g. their behavioural activity or brain activity.

Higher quality of flowers with bigger petals of a particular or unusual colours produced in a plant nursery.

For more on plant breeding see How to produce new varieties of plants


Disadvantages of selective breeding

Problems can arise because in selective breeding for specific characteristics, you are inevitably reducing the gene pool in the population.

This reduction in alleles results e.g. from the farmer/horticulturalist repeatedly breeding from the best animals/plants which are closely related genetically - this is known as inbreeding.

In general, for animals or plants, there is more chance of organisms inheriting harmful genetic defects from a more limited gene pool.

This is why it is inadvisable for close human relatives to interbreed - there is a much greater chance of inheriting a genetic disorder or disease.

Certain 'modern' dog breeds are quite susceptible to defects e.g. breathing problems in pugs, invertebral disc problems of the spine in corgis and dachshunds, Caesarean operations needed for the safe birth of pups.

This raises ethical issues as to whether we should breed animals with negative characteristics because of other desired characteristics.

The same ethical issue crops up if animals 'pure' bred for medical research.

Cows have been bred with larger udders to give greater milk yields, BUT, many cows have suffered from all sorts of problems e.g. lameness (due to increase in body weight), shorter lives, mastitis (the udder tissue becomes inflamed and painful) and metabolic diseases.

Due to the narrower gene pool of a given organism population, serious problems can arise if a new disease crops up, especially with crops!

selective breeding ==> reduction in forms of genes (alleles) ==> less chance of alleles being present in the population that can help the organism resist the disease.

Because all the animal/plant stock are closely related, there is not much genetic variation in the population.

So if one individual in the population is susceptible to a disease, all the others may be equally at risk and can result in the death of most, if not all, of a population of animals or plants.

For more on plant breeding see How to produce new varieties of plants

Summary of learning objectives and key words or phrases

Be able to describe and discuss selective breeding of plants and animals for particular characteristics.

Understand the methods employed and the advantages and disadvantages of a more selective gene pool.

Appreciate and be able to discus ethical issues related to selective breeding.



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