Revision Notes

18.4 Natural Selection & Evolution

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
  • 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
  • Therefore 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 individuals with 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’


An example of natural selection

Natural selection example 1, IGCSE & GCSE Biology revision notes

Natural selection illustrated by snail shell colour

  • Within the population of snails there is variation in shell colour
  • Normal varieties of shell colours in this snail species is black or grey (as evidenced by the first picture)
  • Chance mutations lead to a small number of snails / one snail having a white shell
  • This ‘small number’ is shown in the second diagram where there are less white shelled snails than black or grey shelled snails
  • The white shelled snail(s) survive longer
  • This is the ‘survival of the fittest’, a term used to explain why some organisms succeed in the competitive struggle for survival against other members of their population
  • The reason the white shelled snail(s) survive longer is because they are better camouflaged
  • This means that they are less likely to be seen by predators and eaten
  • As they survive longer they get more opportunities to reproduce
  • And so 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


Natural selection example 2, IGCSE & GCSE Biology revision notesAnother good example of natural selection is the evolution of the peppered moths

Exam Tip

There are hundreds of thousands of examples of natural selection and you cannot possibly be familiar with all of them, however, they ALL follow the same sequence described above:

  • Based on the idea that within a species there is always variation and chance mutations, some individuals will develop a phenotype (characteristic) that gives them a survival advantage and therefore will:
    • live longer
    • breed more
    • and 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.

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  •  If the environment does not change, selection does not change
  • This will favour individuals with the same characteristics as their parents
  • If the environment changes, or a chance mutation produces a new allele, selection might now favour individuals with different characteristics or with the new allele
  • So the individuals that survive and reproduce will have a different set of alleles that they pass on to their offspring
  • Over time, this will bring about a change in the characteristics of the species – it will produce evolution
  • Evolution is defined as the change in adaptive features of a population over time as a result of natural selection
  • Natural selection results in a process of adaptation, which means that, over generations, those features that are better adapted to the environment become more common
  • This means populations of organisms become better suited to their environment
  • A good example of this is the development of antibiotic resistance by bacteria
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Antibiotic Resistance in Bacteria

  • An antibiotic is a chemical that can kill or inhibit the growth and reproduction of bacteria
  • They are extremely useful to humans as some bacteria are pathogenic and can cause life-threatening disease
  • Bacteria reproduce, on average, every 20 minutes and therefore evolution occurs in a much shorter time span
  • Like all other organisms, within a population there will be variation caused by mutation
  • A chance mutation might cause some bacteria to become resistant to an antibiotic (eg penicillin)
  • When the population is treated with this antibiotic, the resistant bacteria do not die
  • This means they can continue to reproduce with less competition from non-resistant bacteria, which are now dead
  • Therefore the genes for antibiotic resistance are passed on with a much greater frequency to the next generation
  • Over time the whole population of bacteria becomes antibiotic-resistant because the bacteria are best suited to their environment
  • This is an example of natural selection that humans have helped to develop due to overuse of antibiotics in situations where they were not really necessary, for example:
    • for treatment of non-serious infections
    • routine treatment to animals in agriculture
    • failure to finish prescribed course of antibiotics


Antibiotic_resistance, IGCSE & GCSE Biology revision notesDevelopment of antibiotic resistance in bacteria


  • Increases in the population of antibiotic-resistant bacteria cause infections and diseases which are harder to control as it is difficult to find antibiotics that certain strains of bacteria are not resistant to
  • An example of this is MRSA, a very dangerous bacterial strain that is resistant to most antibiotics
  • If someone gets infected with MRSA they cannot be treated easily
  • Adding to these difficulties, the number of new antibiotics discovered has slowed significantly

Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.

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