Selective Breeding

Farmers have been adjusting the traits of their crops and livestock for thousands of years. Ever since agriculture has been a thing, way before the idea of evolution was discovered and certainly before the understanding of genetics. This process of picking the desired characteristics in plants or animals is known as selective breeding and has made modern animal and plant species nearly unrecognisable from their wild ancestors. These 'farmed organisms' are becoming tastier, bigger or just more good looking, but it is not all positive. Selective breeding can come with health issues and other unintentional downsides.

Selective Breeding Definition

Selective breeding is artificially selecting certain members of a group of animals or plants to breed together, this is why it is also referred to as artificial selection. The individuals selected from a mixed population often have particularly desirable or useful characteristics that breeders or farmers want, usually for human benefit.

Variation amongst species happens due to mutations in genes or chromosomes. For there to be a whole new breed of the same species, evolution in the form of natural selection would have to take place. Humans intervene in this process, helping to speed things up. However, don't be fooled, selective breeding can still be a slow and long journey. Take a look at the table below, comparing natural selection and selective breeding:

Process of Selective Breeding

With selective breeding, it is essential to understand that the process doesn't stop after finding the two parents with the desired traits. As you know, with genetic inheritance, not all offspring will show the selected characteristics. Therefore, it is imperative that the offspring who do have the characteristics are selected and bred together. This process is repeated several times over many successive generations until the new breed will reliably show the desired traits in ALL children. The main steps involved in selective breeding can be summarised as follows:

Selective breeding can be used to select a whole variety of different features. The desired characteristics can be selected for either appearance or usefulness.

• In plants, the desired characteristics can be:

  • Increased crop yield

  • Disease resistance, particularly in food crops

  • Tolerance to harsher weather conditions

  • Tasty fruits and vegetables

  • Larger, brighter, or unusual flowers

• In animals, the desired characteristics can be:

  • To produce larger quantities of milk or meat or eggs

  • Having a gentle nature, particularly in domestic dogs and farm animals

  • Good quality wool or fur

  • Fine features or fast pace

• Being economically important - new varieties can allow for more benefits to farmers, such as a higher yield.
• Fewer safety concerns - no DNA tampering occurs like with GMO (genetically modified) foods, as selective breeding can allow for the natural evolutionary process to take place, albeit manipulated.
• Influencing plants or animals to grow in lands that were not suitable for farming - like in areas that are arid and dry.
• Improving food quality
• Selecting animals that cannot cause harm - like farm cows without horns.

Disadvantages of Selective Breeding

There are many problems or disadvantages associated with selective breeding. Many of which are associated with a lack of gene pool diversity. Future generations of selectively bred organisms will show less and less variation, they will show the same phenotypic traits and therefore will all share the same genes. This can pose problems in selective breeding such as:

• Being prone to rare genetic disorders - selecting the good traits can also unknowingly select bad traits
• Leading to attack by certain diseases, pests or environmental changes - lack of genetic variation means that all individuals are vulnerable as there is less chance of resistant alleles in a reduced gene pool.
• Creating physical problems in certain species - like large udders in milking cows which can be heavy and uncomfortable to the animal
• Altering the evolution of species - human intervention in selective breeding to enhance a specific trait can cause the loss of other genes/alleles which can be difficult to get back.

Selective Breeding Examples

Selective breeding has been around since the start of practices like agriculture. Farmers and breeders have been trying to achieve higher quality, higher-yielding and better-looking crops and animals for millennia. Domestic dogs are a great example of both the ups and downs of selective breeding, many modern breeds, like the golden doodle and pug, are completely unrecognisable from their wild wolf ancestors. When looking at the agriculture industry, many examples of selective breeding can be pulled. Take a look at a couple below.

Belgian Blue cows

This is a breed of cattle that has been selectively bred over the last 50 years to produce a cow that can maximise meat production. Using the selective breeding technique of inbreeding, an autosomal gene mutation has been successfully passed on to create this modern breed. This naturally occurring mutation in the Belgian Blues, known as "double muscling", means that the gene which usually inhibits muscle production is turned off, there is no limit to the muscle mass this cow can create.

As you can imagine, it causes some health issues such as an enlarged tongue making it difficult for calves to suckle; underdeveloped heart and lungs, which are 10-15% smaller in comparison to other cow breeds; bone and joint issues due to the sheer weight of the extra muscle; and reproductive issues. Belgian Blues do raise many ethical concerns, is it worth the welfare of the animal just to have a leaner, more muscular meat?

Figure 4 Due to decades of selective breeding, Belgian Blue cows have grown into a very muscular breed allowing for higher meat production.

Carrots

The modern orange carrot that many of us are familiar with was not always this way. During the 17th century, wild carrots typically came in a variety of shades ranging from white to yellow to purple. They were also quite bitter in comparison to the sweeter, orange carrot of today.

Dutch farmers wanted to pay tribute to the prince of Holland, William of Orange, so they started to selectively breed wild yellow carrots which had higher amounts of beta-carotene. Over generations, the brightly orange domesticated carrot was created and unexpectedly, proved to be more popular, tastier and healthier than the original wild carrots.¹