6.2.13 Artificial Selection

• Artificial selection is the process by which humans choose organisms with desirable traits and selectively breed them together to enhance the expression of these desirable traits over time and over many generations
• This practice is also known as selective breeding
• Humans have been selectively breeding organisms for thousands of years, long before scientists understood the genetics behind it
• Knowledge of the alleles that contribute to the expression of the desired traits are not required as individuals are selected by their phenotypes, and not their genotypes
• As the genetics is not always understood, breeders can accidentally enhance other traits that are genetically linked to the desirable trait
  • These other traits can sometimes negatively affect the organism's health

The process of artificial selection via selective breeding

1. The population shows phenotypic variation - there are individuals with different phenotypes (i.e. different characteristics)
2. A breeder (the person carrying out the artificial selection) selects an individual with the desired phenotype
3. Another individual with the desired phenotype is selected. The two selected individuals should not be closely related to each other
4. The two selected individuals are bred together
5. The offspring produced reach maturity and are then tested for the desirable trait. Those that display the desired phenotype to the greatest degree are selected for further breeding
6. The process continues for many generations: the best individuals from the offspring are chosen for breeding until all offspring display the desirable trait

Selective breeding in animals

• Animals are selectively bred for various characteristics, including:
  • Cows, goats and sheep that produce a higher [popover id="SNOLkFinu7~t8SeY" label=yield] of milk or meat
  • Chickens that lay large eggs
  • Domestic dogs that have a gentle nature
  • Sheep with good quality wool
  • Horses with fine features and a very fast pace

• An example of an animal that has been selectively bred by humans in many ways to produce breeds with many different characteristics is the domestic dog, all breeds of which are descended from wolves

Selective breeding has produced many different breeds of domestic dog

Example 1: Artificial selection to improve milk yield in cattle

• Milk is a global food source, rich in calcium and protein (essential for growth)
• Over many years and generations, farmers have selected female cows that have the highest milk yield and crossed them with male bulls related to high yield females
• Over time this selective breeding has resulted in cows with greater milk yields, which has been of great economical benefit to farmers
• The selective breeding of cows for increased milk yield is a good example of how artificial selection (controlled by humans) does not take into account an organism's survival
• Selective breeding usually focuses on only one, or a handful of, characteristics, often to the extreme. Little thought is given to other traits important to an organism's health
  • In cows, it has been observed that selectively bred individuals are much more prone to ailments such as mastitis (inflammation of the udder), milk fever and lameness compared to those that were allowed to breed at random

Example 2: Artificial selection in racing horses

• Selective breeding has been a major part of the horseracing industry for many years. Breeders have found that horses tend to have one of the three following phenotypes:
  • Good at sprinting short distances
  • Good endurance over long distances
  • All-rounder

• If a breeder wanted to breed a horse for a sprinting event they are likely to do the following:
  • Select the fastest sprinting female horse they have
  • Select the fastest sprinting male horse they have
  • Breed the two selected horses
  • Allow their offspring to reach maturity and test their sprinting speeds to find the fastest horse (male or female)
  • The breeder could then use this horse for racing, or they could continue the process of selective breeding by breeding this horse with another horse that is fast or descended from fast-sprinters
  • Over several generations, it would be hoped that the offspring are all fast-sprinters (but remember there are biological limitations to this)

Selective breeding in plants

• Selective breeding of plants takes place in the same way as selective breeding of animals
• Plants are selectively bred by humans for development of many characteristics, including:
  • Disease resistance in food crops
  • Increased crop yield
  • Hardiness to weather conditions (eg. drought tolerance)
  • Better tasting fruits
  • Large or unusual flowers

• An example of a plant that has been selectively bred in multiple ways is wild brassica, which has given rise to cauliflower, cabbage, broccoli, brussel sprouts, kale and kohlrabi

An example of selective breeding in plants

Example 1: Artificial selection of disease-resistance in wheat and rice

• Wheat plants have been selectively bred for hundreds of years as a crop
• Wheat crops can be badly affected by fungal diseases: Fusarium is a fungus that causes “head blight” in wheat plants
• Fungal diseases are highly problematic for farmers as they destroy the wheat plant and reduce crop yield
• By using selective breeding to introduce a fungus-resistant allele from another species of wheat, the hybrid wheat plants are not susceptible to infection, and so yield increases
  • Introducing the allele into the crop population can take many generations and collaboration with researchers and plant breeders

• Rice is another crop that has been subject to large amounts of selective breeding
• Rice plants are prone to different bacterial and fungal diseases
  • Examples include “bacterial blight” and “rice blast” caused by the Magnaporthe fungus

• These diseases all reduce the yield of the crop as they damage infected plants
• Scientists are currently working hard to create varieties of rice plants that are resistant to several bacterial and fungal diseases

The importance of maintaining a resource of genetic material for use in selective breeding

• In artificial selection, it is important to maintain a resource of genetic material that includes types that are close to the original wild type (of the organism you are selectively breeding)
• This is important as it ensures that the gene pool for a particular species doesn't become too small (which can weaken the population by reducing variation)
• An example of the importance of this can be seen in the artificial selection of maize
• Maize (also known as corn) is a staple crop in many countries around the world and is grown to feed both livestock and people
  • Different hybrids of maize are constantly being created and tested for desirables traits such as resistance to pests/disease, higher yields and good growth in poor conditions

• In the past, maize plants have been heavily inbred (bred with plants with similar genotypes to their own)
• This has resulted in small and weaker maize plants that have less vigour
• This is due to inbreeding depression, which:
  • Increases the chance of harmful recessive alleles combining in an individual and being expressed in the phenotype
  • Leads to decreased growth and survivability

• A farmer can prevent inbreeding depression by outbreeding with wild type varieties of maize
  • This involves breeding individuals that are not closely related
  • Outbreeding produces taller and healthier maize plants
  • It decreases the chance of harmful recessive alleles combining in an individual and being expressed in the phenotype and leads to increased growth and survivability (known as hybrid vigour)
  • Crops of these plants have a greater yield

• Selective breeding can lead to ‘inbreeding’
• This occurs when only the ‘best’ animals or plants (which are closely related to each other) are bred together
• This results in a reduction in the gene pool – this is a reduction in the number of alleles (different versions of genes) in a population
  • This is known as inbreeding depression 

• As inbreeding limits the size of the gene pool, there is an increased chance of:
  • Organisms inheriting harmful genetic defects (as there is a higher chance of harmful recessive alleles combining in an individual and being expressed in the phenotype)
  • Organisms being vulnerable to new diseases (as there is less chance of resistant alleles being present in the reduced gene pool)

• This means that some intensive artificial selection processes have resulted in conditions and diseases that are extremely damaging or deleterious for the animals involved, which is ethically questionable
• For example, there are many dog breeds that suffer as a result of the long history of artificial selection that created the breed:
  • Breathing problems are common in Bulldogs and Pekinese because of their extremely shortened snouts
  • Very large dogs such as Sait Bernards and Great Danes often suffer from hip dysplasia
  • Golden Retrievers commonly suffer from malignant blood vessel tumours