Edexcel IGCSE Biology: Double Science 复习笔记 3.2.7 Natural Selection

Edexcel IGCSE Biology: Double Science 复习笔记 3.2.7 Natural Selection

Darwin's Theory of Evolution by Natural Selection



  • Charles Darwin proposed the theory of evolution by natural selection
    • He came up with this theory as a result of observations from a round-the-world expedition, years of experimentation and his knowledge of geology and fossils


  • Evolution can be defined as the change in the frequency of a phenotype in a population over many generations
  • Darwin’s theory, very simply, is:
    • Individuals in a species show a wide range of variation caused by differences in genes
    • Individuals with characteristics most suited to the environment have a higher chance of survival and more chances to reproduce
    • Therefore these characteristics are passed to their offspring at a higher rate than those with characteristics less suited to survival
    • Over many generations, these beneficial characteristics become more common in the population and the species changes (the species evolves)


  • This idea of natural selection became known as ‘survival of the fittest’
  • Darwin published his ideas in his famous book, On the Origin of Species (1859)




The acceptance of evolution

  • The theory of evolution by natural selection was only gradually accepted because:
    • There was much controversy surrounding these revolutionary new ideas
    • The theory challenged the idea that God made all the animals and plants that live on Earth
    • There was insufficient evidence at the time the theory was published to convince many scientists
    • The mechanism of inheritance and variation was not known until 50 years after the theory was published
    • The theory of evolution by natural selection developed over time and from information gathered by many scientists





Examples of evolution by natural selection





Natural selection illustrated by snail shell colour




Another good example of natural selection is the evolution of the peppered moths



Exam Tip

There are many examples of natural selection but they ALL follow the same sequence described above:

  • Within a species, there is always variation and chance mutation
  • Some individuals will develop a phenotype (characteristic) that gives them a survival advantage and this allows them to:
    • live longer
    • breed more
    • be more likely to pass their genes on


  • Repeated over generations, the ‘mutated’ phenotype will become the norm


Remember, it is the concept you have to understand, not the specific example. 

Antibiotic Resistance


  • Antibiotics are chemical substances made by certain fungi or bacteria that affect the working of bacterial cells, either by disrupting their structure or function or by preventing them from reproducing
  • Antibiotics are effective against bacteria but not against viruses
  • Antibiotics target processes and structures that are specific to bacterial (prokaryotic) cells; as such they do not generally harm animal cells





How antibiotics work




  • The use of antibiotics has increased exponentially since they were first introduced in the 1930s
  • In that time they have saved millions of lives




The introduction of antibiotics has had one of the largest impacts on global health, shown by this example in the USA




  • However, since their discovery and widespread use, antibiotics have been overused and antibiotic resistance has developed in many different types of bacterial species
    • Bacteria, like all organisms, have random mutations in their DNA
    • One of these mutations may give them resistance to an antibiotic
    • If an organism is infected with bacteria and some of them have resistance, they are likely to survive treatment with antibiotics
    • The population of the resistant bacteria will increase
    • If the resistant strain is causing a serious infection then another antibiotic will be needed
    • A strain of Staphylococcus aureus has developed resistance to a powerful antibiotic methicillin, this is known as MRSA (Methicillin resistant Staphylococcus aureus)
    • MRSA can infect wounds and is difficult to treat without antibiotics






Bacteria evolve rapidly as they reproduce quickly and acquire random mutations – some of which confer resistance




Preventing resistant bacteria

  • To reduce the number of bacteria that are becoming resistant to antibiotics:
    • Doctors need to avoid the overuse of antibiotics, prescribing them only when needed – they may test the bacteria first to make sure that they prescribe the correct antibiotic
    • Antibiotics shouldn't be used in non-serious infections that the immune system will ‘clear up’
    • Antibiotics shouldn't be used for viral infections
    • Patients need to finish the whole course of antibiotics so that all the bacteria are killed and none are left to mutate to resistant strains
    • Antibiotics use should be reduced in industries such as agriculture – controls are now in place to limit their use in farming





Reducing the spread of resistant strains

  • Good hygiene practices such as handwashing and the use of hand sanitisers have reduced the rates of resistant strains of bacteria, such as MRSA, in hospitals
  • The isolation of infected patients to prevent the spread of resistant strains, in particular in surgical wards where MRSA can infect surgical wounds



Antibiotics do not affect viruses

  • Viruses cannot be treated with antibiotics
  • This is because antibiotics work by disrupting cell functions such as respiration, or breaking down the structure of the cell in some way
  • However, viruses do not carry out any cell functions and do not have cell walls, cell membranes or any cell organelles as viruses infect and utilise the machinery of animal cells to reproduce, which are not affected by antibiotics.
  • Therefore the action of antibiotics do not affect them