AQA GCSE Biology

Revision Notes

6.2.2 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’

Examples 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 notes

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

Evolution 

  • 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
  • If two populations of one species become so different in phenotype that they can no longer interbreed to produce fertile offspring, they have formed two new species

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.

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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