Chapters
In this article, we will discuss the principles of selective breeding, i.e., artificial selection. Moreover, we will also outline the examples of selective breeding like the introduction of disease resistance to varieties of rice and wheat, inbreeding and hybridization to produce vigorous, uniform varieties of maize, and improving the milk yield of dairy cattle.
What is Artificial Selection?
Artificial selection refers to a process by which humans select organisms with favorable characteristics and selectively breed them together to improve the expression of favorable characteristics over time and several generations. This practice is named selective breeding
The practice of selective breeding is thousands of years old. At that time, humans did not comprehend the genetics behind it. The information about the alleles which participate in the expression of favourable characteristics is not needed because individuals are selected by their phenotypes rather than their genotypes. There is a chance that the breeders accidentally improve other characteristics that are genetically connected to favourable traits because they do not always understand genetics.
Some of the examples of artificial selections are given below:
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- Improved milk yield from cattle
- Disease-resistant crops
- Faster racehorses
Principles of Selective Breeding
Some of the principles of selective breeding are mentioned below:
- The population depicts phenotypic variation which means that there are individuals with varying phenotypes or traits
- An individual with a favoured phenotype is chosen by the breeder
- Another individual is selected who possesses the desired phenotype. The two chosen individuals do not share a close relationship
- The two chosen individuals are bred together
- As a result of breeding, offspring are produced. This offspring is tested for desirable characteristics when it reaches maturity. Those who exhibit the favourable phenotype to the greatest degree are chosen for further breeding.
- The process goes on for several generations. The best individuals from the offspring are selected for breeding until all offspring exhibit the desirable characteristic.
Artificial Selection in Racing Horses
- Selective breeding traditionally has been a fundamental element of the horseracing industry for several years. Breeders discovered that the horses tend to have one of the following three phenotypes:
- Better at sprinting short distances
- Better endurance over long distances
- All-rounder
- A breeder will follow the following process if we want to breed a horse for a sprinting event:
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- Choose the fastest sprinting female horse available
- Choose the fastest sprinting male horse available
- Once chosen, the two selected horses are bred together
- Enabled their produced offspring to reach maturity and test their sprinting speeds to find the fastest horse which can be either male or female
- After that, the breeders have two choices. They can either use the horse for racing or carry on the selective breeding process by breeding this horse with another horse that is fast or descended from fast-sprinters
- Over time and many generations, breeders can hope for offspring that are all fast sprinters (Remember that the biological limitations do exist)
In the next section of the article, we will give the examples of selective breeding like the introduction of disease resistance to varieties of rice and wheat, inbreeding and hybridization to produce vigorous, uniform varieties of maize, and enhancing the milk yield of dairy cattle.
Examples of Selective Breeding
We already know that selective breeding or artificial selection refers to a process by which humans select individuals with desired characteristics to reproduce to produce the offspring that possesses the desired characteristics too.
The majority of selective breeding is done to enhance the yield of a sellable product, rather than the survival of the organism. This is the main difference between artificial and natural selection.
As a result of artificial selection, the organisms produced can be poorly adapted to their environments. Until the breeders fully comprehend the genetic mechanism behind a characteristic, there is a high probability of accidentally improving other characteristics. Some examples of selective breeding in the agriculture and livestock sectors are given below:
- Production of Disease-resistant wheat and rice varieties
- Hybridization in maize
- Enhanced milk yield in cattle
Disease Resistance in Wheat and Rice
Selective Breeding in Wheat
- For hundreds of years, wheat plants were selectively bred
- Traditionally, fungal diseases badly affected wheat crops. For instance, Fusarium is a fungus that leads to “head blight in wheat plants
- Farmers are extremely worried about these fungal diseases because they destroy the wheat plant and lead to crop yield reduction
- The hybrid wheat plants are produced through selective breeding and are not susceptible to fungal disease. This is achieved by introducing a fungus-resistant allele from another wheat species. Consequently, the yield of the wheat crop increases.
Selective Breeding in Rice
- Another crop that has been subjected to a huge amount of selective breeding is rice.
- Various bacterial and fungal diseases can easily attack rice plants. For instance, Magnaporthe fungus causes bacterial blight and rice blast in these plants.
- These diseases damage the rice plants and lead to crop reduction
- Scientists are trying hard to create varieties of rice plants that are resistant to many fungal and bacterial diseases
Inbreeding & Hybridization in Maize
- Maize also referred to as corn is a crop that is grown in several countries around the world. It is grown to feed humans as well as livestock
- Traditionally maize plants have been heavily inbred which resulted in small and weaker maize plants that possessed less vigour
- This is referred to as inbreeding depression which can result in:
- Increased probability of dangerous recessive alleles combining in an individual and being expressed in the phenotype
- Enhances homozygosity in individuals
- Results in decreased growth and survivability
- Inbreeding depression can be replaced by outbreeding which offers the following benefits:
- It includes breeding individuals that are not closely related, hence it results in the production of taller and healthier maize plants
- It reduces the probability of dangerous recessive alleles combining in an individual and being expressed in a phenotype
- It enhances heterozygosity
- It results in enhanced growth and survivability, referred to as hybrid vigour
- It leads to increased crop yield
- When growing a crop, uniformity should be kept in mind because:
- If outbreeding occur completely randomly, then there is a chance that it can produce a lot of variation between plants within a single field
- A farmer requires the plants to ripen at the same time and be of the same height. If there is more variation, there is a less probability of this happening
- For attaining heterozygosity and uniformity, the farmers buy homozygous seeds sets from specialized companies and cross them to produce the F1 generation
- Varying maize hybrids are constantly being created and tested for desirable characteristics like resistance to diseases/pests, higher yields, and better growth in poor conditions
Enhancing Milk Yield in Cattle
- Milk is rich in essential nutrients like calcium and protein which are vital for growth
- Over several years and generations, farmers have chosen female cows that produced the highest milk and crossed them with male bulls that were related to high yield cows
- Over time and many generations, this selective breeding led to increased milk yields in cows
- The selective breeding of cows with enhanced milk yield is an excellent example of how selective breeding, i.e. artificial selection does not consider an organism’s survival
- Artificial selection usually has one and only one goal, i.e. to produce offspring with the desired trait, often to the extreme. A little consideration is given to other characteristics that are important for the organism’s health
- The selectively bred cows are more prone to diseases like mastitis, milk fever, and lameness
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