Natural Selection

• In any environment, the individuals that have the best adaptive features are the ones most likely to survive and reproduce

  • This results in natural selection
• Individuals in a species show a range of variation caused by differences in genes
• When organisms reproduce, they produce more offspring than the environment is able to support (1)

  • This leads to competition for food and other resources which results in a ‘struggle for survival’
  • Individuals with characteristics most suited to the environment have a higher chance of survival and more chances to reproduce
• When organisms reproduce, they produce more offspring than the environment is able to support (2)

  • The alleles resulting in these characteristics are passed to their offspring at a higher rate than those with characteristics less suited to survival
  • This means that in the next generation, there will be a greater number of individualswith the better adapted variations in characteristics
• This theory of natural selection was put forward by Charles Darwin and became known as ‘survival of the fittest’
• Example of natural selection (1)

  • Within the population of snails there is variation in shell colour
  • Normal varieties of shell colours in this snail species is black or grey
  • Chance mutations lead to a small number of snails / one snail having a white shell
  • The white shelled snails survive longer
  • ‘survival of the fittest’ - explains why some organisms succeed in the competitive struggle for survival against other members of their population
• Example of natural selection (2)

  • The reason the white shelled snails survive longer is because they are better camouflaged
  • less likely to be seen by predators and eaten
  • they survive longer they get more opportunities to reproduce
  • the allele for white shells is passed onto offspring more frequently than the alleles for black or grey shells
  • Over generations, this is repeated until the majority of snails in the population have white shells
• Examples of natural selection

  A) variation

  B) colour

  C) mutation

  D) phenotype

  E) advantage

  F) longer

  G) reproduction

  H) phenotype

  I) common

  
• Natural selection requires variation and a lot of organisms for natural selection to happen

  • survival of the fittest
• When exposed to antibiotics, most bacteria dies quickly but some may have mutations that make them less susceptible

  • less competition
• Some hawks have variation that gives them better eyesight than others

  • those hawks have many offspring who will all compete for mice but those with bad eyesight die first due to their bad eye sight
  • the remaining hawk reproduce, passing down good alleles, leading to the adaptation of good eyesight
• How to answer antibiotics natural selection question

  • mutation creates a resistance gene as it causes a change in the DNA
  • mutations can be caused by ionising radiation or chemicals
  • the mutation causes variation where the bacteria can survive antibiotics
  • natural selection causes those without the mutation to die and the bacteria with resistance reproduce offspring
  • the offspring pass on this resistance to more bacteria through quick reproduction
• How to avoid antibiotic resistance:

  • only use essential antibiotics
  • complete full course of prescribed antibiotics
  • isolation of patients with infections
  • improved healthcare