Variation and Selection

• Variation 


  Living organisms that belong to the same species resemble each other, but usually differ in a number of ways
• Genetic Variation 


  can be genetic due to differences in DNA caused by:

  • variation as a result of sexual reproduction
  • mutation-random changes in the number of chromosomes or structure of a gene
• Environmental Variation 


  due to the environment or lifestyle
• Combination of both 


  for example, you have genes for a particular height but actual height reached depends on health and diet
• Continuous variation

  • gradual change in a feature with no clearly distinct groups/across a population
  • e.g height/length
  • often both genetic and environmental
  • usually displayed by a histogram
• Discontinuous Variation

  • individuals can be placed into distinct groups with no overlap
  • e.g tongue rolling, hand dominance, eye colour, blood group
  • often genetic
  • displayed by a bar chart
• Natural Selection 


  the process in which the better adapted individuals survive and pass on their genes to their offspring
• Process of Natural Selection

  1. The phenotypes in any population vary
  2. Some are better adapted than others
  3. Better adapted individuals are more likely to survive and pass on their genes
• When is natural selection important?


  its very important when there is competition for resources. this is because being better/less adapted can make a difference. individuals who are better adapted may succeed e.g in finding food
• 3 key elements on Natural Selection

  1. Difference between phenotypes-e.g some grey squirrels can run faster than others and escape predators
  2. Different Survival- e.g faster survives, slower gets caught
  3. Different reproductive success-fastest squirrels are able to pass on their gene to the next generation
• Antibiotic Resistance


  The resistant phenotypes are not killed by antibiotics and so survive, but the non-resistant bacteria are killed by antibiotics. The resistant bacteria are then able to survive and pass their resistant gene on the next generation
• Antibiotic Resistance Definition 


  an antibiotic resistant bacterium cannot be killed by at least one type of antibiotic
• Antibiotic Resistance In Bacteria 


  has different phenotypes, differential survival and differential reproductive success
• AR


  The use of the antibiotic doesn’t cause the bacteria to become resistant as it’s already resistant due to mutations. The use of antibiotics create conditions in which resistant bacteria are better adapted than non-resistant ones
• Charles Darwin 


  The scientist who first explained the idea of natural selection
• Link between Natural Selection and Evolution

  • Charles Darwin used his theory of natural selection to explain the process of evolution
  • natural selection can explain how species have changed overtime due to evolution
  • this happens because certain features in species are favoured
  • eventually the species may be very different from how it started out
  • evolution is a continuing process but natural selection is always happening and all species change very gradually over a longer time period
  • evolution can result in development of new species
• Evolution 


  a continuing process of natural selection that leads to gradual changes in organisms overtime which may lead to the formation of new species
• Why not everyone accepts the evolution theory:

  • contradicts some religious beliefs
  • very long timescale involved means its very difficult to see evolution happening
• Extinction


  sometimes entire species may not be well enough adapted to survive on a changing world and can no longer survive-they may become extinct e.g mammoths and dinosaur
• Reasons for Extinction

  • climate change
  • hunting by humans
  • habitat destruction

  Currently mountain gorillas and species of large cat are endangered
• Selective Breeding 


  the selection and subsequent breeding of organisms chosen by humans for their desirable properties

  people have controlled selection in crops and domestic animals by deliberately selecting particular characteristics that are of use
• Traits selected include

  • increased crop yield or quantity
  • appearance
  • hardiness
  • disease resistance
  • longer shelf life
• Selective Breeding of Wheat 


  Wheat is a cereal that has been bred over many years to produce:

  • shorter stalk length-less likely to suffer wind damage and is easier to harvest due to uniform size
  • larger head of grain-higher yield
• Selective breeding of plants with short stalk and larger heads

  1. Seeds from this cross produce plants of next generation
  2. Next generation plants are examined and only plants with shorter stalks and large heads of grain are selected
  3. The selected plants are crossed pollinated to produce a new generation
  4. Only plants with desired qualities are selected and interbred. By this stage plants are becoming more similar
  5. Eventually after many generations all the plants will be short with large heads of grain