CIE A Level Biology (9700) exams from 2022

Revision Notes

17.2.7 Examples of Artificial Selection

Examples of Selective Breeding

  • Selective breeding (or artificial selection) is the process by which humans choose individuals with desired traits to reproduce, with the aim of producing offspring with the desired traits also
  • Most selective breeding is done with the aim of increasing the yield of a sellable product
  • It is not done with the organism’s survival in mind, and unlike natural selection, it can lead to organisms that are poorly adapted to their environments
  • Unless the genetic mechanism behind a trait is fully understood, is highly likely that other traits could also be accidentally enhanced
  • Some examples of selective breeding in agriculture and livestock include:
    • Disease-resistance in wheat and rice varieties
    • Hybridization in maize
    • Milk yield in cattle

Disease-resistance in wheat & 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

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

In selective breeding, selection pressure is applied by humans who desire certain traits in animals or plants – this is why it’s described as artificial selection.

In natural selection, the environment applies selection pressure on populations / species – but not to achieve a desirable outcome. Selection pressures in natural selection are simply driven by the environment in which organisms live and which features within a population or species are best suited (adapted) to that environment.

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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