CIE A Level Biology复习笔记17.2.7 Examples of Artificial Selection

• 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

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