CIE A Level Biology (9700) 2019-2021

Revision Notes

17.2.5 Artificial Selection

Selective Breeding in Livestock

  • 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 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
  • Examples of artificial selection include:
    • Increased milk yield from cattle
    • Faster racehorses
    • Disease-resistant crops
  • There are always biological limitations to how extreme a trait can become in an organism

Principles of selective breeding

  • 1. The population shows phenotypic variation – there are individuals with different phenotypes / traits
  • 2. Breeder 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

Improving 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

Selective Breeding in Crops

Disease-resistance in wheat & rice

  • Selective breeding can be used in a multitude of ways for crop improvement
  • 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

Using alleles for gibberellins

  • Gibberellins are a group of plant hormones that are involved in regulating a variety of developmental processes
  • Gibberellins are involved in stem elongation, germination and flowering amongst other things
  • Dwarf varieties of desirable crop (eg. wheat) plants can have mutant alleles for gibberellin synthesis incorporated into them
  • These hybrids synthesise more gibberellins, resulting in an upregulation of plant development and growth
  • As a result, a greater proportion of energy (stored in biomass) is put into grain; this increases yield

Inbreeding & hybridization in maize

  • Maize (also known as corn) is a staple crop in many countries around the world; it is grown to feed both livestock and people
  • 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 inbreeding depression which:
    • Increases the chance of harmful recessive alleles combining in an individual and being expressed in the phenotype
    • Increases homozygosity in individuals (paired alleles at loci are identical)
    • Leads to decreased growth and survivability
  • A farmer can prevent inbreeding depression by outbreeding
    • 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
    • Increases heterozygosity (paired alleles at loci are different)
    • Leads to increased growth and survivability (known as hybrid vigour)
    • Crops of these plants have a greater yield
  • Uniformity is important when growing a crop:
    • If outbreeding is carried out completely randomly, it can produce too much variation between plants within one field
    • A farmer needs the plants to ripen at the same time and be of a similar height; the more variation there is, the less likely this is
  • In order to achieve heterozygosity and uniformity, farmers buy sets of homozygous seeds from specialised companies and cross them to produce an F1 generation
  • 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

Exam Tip

Selective breeding can be used to enhance a single desired trait but it can also be used to combine several desired traits together in a single individual. A lot of this type of selective breeding is seen in plants. Farmers are constantly trying to breed plants with a high yield, disease resistance and the ability to grow in poor soil.

Author: Lára

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.
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