6.2.2 Selective Breeding of Food Products

• Selective breeding or artificial selection means to select individuals with desirable characteristics and breed them together
• The process doesn't stop there though because it's likely that not all of the offspring will show the characteristics you want so offspring that do show the desired characteristics are selected and bred together
• This process has to be repeated for many successive generations before you can definitely say you have a new breed that will reliably show those selected characteristics in all offspring
• Artificial selection has obvious parallels with natural selection
  • Both essentially restrict which individuals can mate
  • In natural selection, selection pressures prevent certain individuals from mating
    • An individual cannot mate if it's dead, sick or somehow unable to compete for a mate
  • In artificial selection, humans segregate the individuals who are allowed to mate from those that aren't
    • In livestock, the latter group face early slaughter whereas the mating population are kept alive and well looked after

Natural Selection vs Artificial Selection Table

Selective breeding of animals

• Individuals with the desirable characteristics are bred together
  • Often several different parents all with the desired characteristics are chosen so siblings do not have to be bred together in the next generation
• Offspring that show the desired characteristics are selected and bred together
• This process is repeated for many successive generations
• Animals are commonly selectively bred for various characteristics, including:
  • Cows, goats and sheep that produce lots of milk or meat
  • Chickens that lay large eggs
  • Domestic dogs that have a gentle nature
  • Sheep with good quality wool
  • Horses with fine features and that can run fast
• Whilst not a livestock animal, a good example is an animal that has been selectively bred by humans in many ways to produce breeds with many different characteristics
• This is the domestic dog (Canis familiaris), all breeds of which are descended from wolves

Selective breeding has produced many different breeds of domestic dog (Canis familiaris)

• Selective breeding of plants takes place in the same way as selective breeding of animals
• Plants are selectively bred by humans for the development of many characteristics, including:
  
  • Disease resistance in food crops
  • Increased crop yield
  • Hardiness to weather conditions (e.g. drought tolerance)
  • Better tasting fruits
  • Large or unusual flowers
• An example of a plant that has been selectively bred in multiple ways is wild brassica
  • Wild brassica has given rise to cauliflower, cabbage, broccoli, Brussel sprouts, kale and kohlrabi

An example of selective breeding in plants to produce several vegetable varieties

Problems with selective breeding

• Selective breeding can lead to ‘inbreeding’
• This occurs when only the ‘best’ animals or plants (which are closely related to each other) are bred together
• This results in a reduction in the gene pool – this is a reduction in the number of alleles (different versions of genes) in a population
• As inbreeding limits the size of the gene pool, there is an increased chance of:
  
  • Organisms inheriting harmful genetic defects
  • Organisms being vulnerable to new diseases
    • There is less chance of resistant alleles being present in the reduced gene pool