Artificial selection
Joanna George-Johnson, Contributor

Artificial selection, sometimes called selective breeding, is when humans only breed from selected individuals in order to preserve and accentuate their desirable characteristics. Breeders, farmers and others have used this technique for many centuries.

The process

Farmer Brown is a dairy farmer and is known for his very creamy milk. The milk produced by his cows has become so popular that he is not able to fill his orders anymore. This is because his cows don't produce enough milk each day to meet the demand. He asks his veterinarian to give him some advice on how to solve this problem.

He was looking for a cow that produced large amounts of creamy milk. The veterinarian suggested that he could solve his problem by selective breeding. He needed to identify Friesian cows that produce the most milk and Jersey cows that produce the creamiest milk and breed only with them. By mating the two types of cows, Brown was able to produce offspring which had an enhanced version of the characteristic.

Over several years he followed this breeding programme until he got the desired result, i.e. a cow that produced a large amount of creamy milk.

Farmers of today look for desired characteristics, i.e. creamier milk, tastier meat, disease-resistant crops etc., in their animals or plants, and use them to breed. They examine the natural population of a specific plant or animal and select those individuals who have desired characteristics. The animals or plants with wanted attributes are then bred in the hope that the offspring will show the desired characteristic. Farm animals that are bred in this way include dairy and beef cattle, pigs, sheep and poultry.

Farmers then use the offspring from these parents to breed again. In this way they are eventually able to breed a new variety of plant or animal with the best characteristics.

Selective breeding is used extensively to produce new plant varieties that:

• Grow faster
• Are better adapted to the climate in which they live
• Have a better taste
• Are disease resistant
• Are bigger and stronger
• Yield higher quality food (eg foods that contain more natural fibre)
• Are frost resistant, so they can be planted earlier
• Are sweeter, have a better colour and/or texture (eg. fruits)
• Have a long shelf life for supermarkets and overseas shipping.

The expanding gene pool

The knowledge of genetics has resulted in many scientific breakthroughs. Humans have learnt to actively manipulate the phenotype of offspring by selectively breeding animals and plants.

Breeders of animals and plants in today's world are looking to produce organisms that will possess desirable characteristics, such as high crop yields, resistance to disease, high growth rate and many other phenotypical characteristics that will benefit the organism and species in the long term.

This is usually done by crossing two members of the same species which possess dominant genes for particular alleles, such as long life and quick metabolism in one organism crossed with another organism possessing genes for fast growth and high yield. Since both these organisms have dominant genes for these desirable characteristics, when they are crossed they will produce at least some offspring that will show all of these desirable characteristics. When such a cross occurs, the offspring is termed a hybrid, produced from two genetically dissimilar parents, who usually produce offspring with more desirable qualities.

Breeders continuously track which characteristics are possessed by each organism so when the breeding season comes, they can selectively breed the organisms to produce more favourable qualities in the offspring.

The offspring will become heterozygous, meaning the allele for each characteristic will possess one dominant and one recessive gene. Most professional breeders have a true breeding cross (ie AAbb with AAbb) so that they will produce a gene bank of these qualities that can be crossed with aaBB to produce heterozygous offspring. This way the dominant features are retained in the first breeding group and can be passed on to offspring in the second instance.

This process of selecting parents is called artificial selection and poses no threat to nature from man manipulating the course of nature.

It has allowed our species to increase the efficiency of the animals and plants we breed. Increasing milk yield from cows by continuously breeding selected cows with one another to produce hybrids is just one example of this.

Genetic engineering

Do not confuse selective breeding with genetic engineering. Genetic engineering has to do with the manipulation of the genes within an organism. Sometimes even using genes from an organism in a completely different specie in another organism to get the desired trait.

However, while it is an advantage both to the species and to humans to produce these desirable qualities that may benefit the organisms in question, continuous in-breeding and selective breeding of particular genes carries with it the risk of losing some of the other genes from the gene pool altogether, which is irreversible.

In the long term, it is more advantageous for organisms to remain heterozygous for the following reasons:

• Genetic diversity is essential in the case of widespread disease. New virus strains can possibly wipe out a species, and genetic diversity means that some organisms with immunity will allow the species to survive.
• Although in-breeding produces desirable characteristics in some offspring, others bear the recessive genes and are not as suited to their environment as others in the species, meaning they are prone to dying prematurely.
• Genetic diversity in the long term is reduced, because many organisms of the same genome are breeding with each other constantly. In normal circumstances, this process would be random, and would produce more variable offspring.

With the above facts in hand, breeders need to produce more heterozygous offspring to ensure the long term welfare of the species they are breeding and their livelihood. The most important thing here is to preserve the genetic diversity of a species, and preferably keep the gene pool of a species as diverse as possible.

Humans have realised the above dangers, and instead of harnessing and exhausting nature's reserves, we have learnt to preserve their genetic information for their long-term survival and our own well-being. One species becoming extinct can knock the balance of an ecosystem and have a detrimental knock on (domino) effect.

With this in mind, humans have gene banks to preserve the genetic information in the case of extinction and nurture species that are at dangerously low population levels. Ironically, the technology and industry that has damaged natural habitats and species well-being is now capable of bringing them back from extinction, genetic engineering. We will go into this in our next lesson.

The dog

One very familiar example of artificial breeding is dogs. Fossil evidence shows that all dogs (scientific name: canis familiaris) have a common ancestor - the wild grey wolf (canis lupus).

Over 14,000 years, humans have designed some 400 breeds of domestic dog, artificially selecting the looks and behaviour of each breed.

Questions

1. Outline the process of selectively breeding a variety of pigs for high meat content but low fat.

2. Give two consequences of extended selective breeding.

3. State what is being done to resolve the possible problems that come with extended selective breeding.

Joanna George-Johnson teaches at Ardenne High School.