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.
Breed (verb) - in plants and animals, this is to reproduce and produce offspring.
Breed (noun) - a group of plants or animals within the same species having a distinct trait, usually brought about by artificial selection.
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:
Selective Breeding (Artificial Selection) | Natural Selection |
Only takes place with the intervention of humans | Happens naturally |
Takes less time than natural selection as only organisms with desired traits are selected for reproduction | Typically takes a very long time to occur |
| Results in populations that are better adapted for survival and to their environment |
Check out the Variation article to learn more about how we're all different organisms!
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:
Step 1 | Decide on desired characteristics, i.e. bigger flowers |
Step 2 | Select parents who display these traits so they can be bred together Most of the time, several different parents all displaying the chosen traits are selected, so siblings of the next generation don't have to breed together. |
Step 3 | Choose the best offspring that have the chosen traits to reproduce together. |
Step 4 | The process is repeated over several generations until all offspring show selected traits. |
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
There are 3 methods of selective breeding that are practised today to get the desired phenotypic characteristics, these include:
1. Crossbreeding - this involves 2 unrelated individuals being bred together.
In a golden retriever dog crossed with a poodle dog, the desired characteristics are the calm, trainable temperament of the retriever and the low-shedding coat of the poodle, resulting in a 'golden doodle' which exhibits both of these desired traits.
Figure 1 A 'golden doodle' is an example of a crossbreed.
2. Inbreeding - the breeding of very closely related relatives (like siblings) to establish a population with the desired traits. This is how 'purebred' populations are created.
3. Line breeding - a type of inbreeding but with more distantly related relatives (like cousins). This reduces the rate of 'purebred' breeds and their associated ill-health.
Advantages of Selective Breeding
A lot of the advantages of selective breeding are the same as the reasons for creating selectively bred crops and animals in the first place. It has allowed for the many advances we witness today in agriculture and farming. These benefits of selective breeding include:
- 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.
Unlike selectively bred crops, GMO crops involve more direct genetic manipulation to achieve a certain phenotype. Read our article on Genetic Engineering to learn how it's done!
One of the earliest known species of selective breeding is corn or maise. This plant exemplifies the benefits of this process as it was selectively bred from tesonite (a wild grass) over thousands of years to produce the corn we are familiar with today - a corn with larger kernel sizes and number of cobs (or ears).
Figure 2 Modern day corn has gone through selective breeding over thousands of years to produce the variety we know and love today.
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.
The risks associated with selective breeding can be shown in certain breeds of dogs. Dogs such as French bulldogs and pugs have been specifically bred to have exaggerated features so they look 'cuter'. This type of inbreeding has led to these dog breeds having breathing issues and blocked airways to achieve that 'squashed nose' effect.
Figure 3 In order to achieve the 'cute' squashed face look, pugs have gone through years of selective breeding but it comes with its downfall of health issues such as breathing difficulties.
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
Beta-carotene - a natural pigment that gives yellow and orange coloured fruit and vegetables their rich colour. It also turns into vitamin A in the human body.
Selective Breeding - Key Takeaways
- Selective breeding is the artificial selection of organisms with desired characteristics to breed together.
- The selective breeding process is repeated over several generations until all the offspring of the new breed can successfully show the chosen trait.
- Selective Breeding advantages include economic importance, fewer safety concerns, improved food quality, and well-tolerated organisms.
- Selective Breeding drawbacks include a lack of gene pool diversity leading to an increased vulnerability to genetic disorders, physical concerns, altering the natural evolutionary process and increased risk of certain diseases, pests and environmental changes.
- Examples of selective breeding include domestic dogs, Belgian blue, orange carrots, and corn/maise.
References
- Marcia Stone, Taming the Wild Carrot, BioScience, 2016
- Figure 1: Golden doodle (https://commons.wikimedia.org/wiki/File:Golden_Doodle_Standing_(HD).jpg) by Gullpavon. Licensed by CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/deed.en).
- Figure 2: Corn (https://commons.wikimedia.org/wiki/File:Klip_kukuruza_uzgojen_u_Međimurju_(Croatia).JPG) by Silverije (https://en.wikipedia.org/wiki/User:Silverije). Licensed by CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/deed.en).
- Figure 3: Pug (https://commons.wikimedia.org/wiki/File:A_PUG_dog.jpg) by Nancy Wong. Licensed by CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/deed.en).
- Figure 4: Belgian Blue (https://www.flickr.com/photos/23296189@N03/2713816649) by ERIC FORGET (https://www.flickr.com/photos/tarchamps/). Licensed by CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/deed.en).
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