Natural Selection (Cambridge (CIE) IGCSE Biology): Revision Note

Natural selection

• In any environment, the individuals that have the best adaptive features (opens in a new tab) 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

• 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

Adaptation by natural selection: Extended

• 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 is how evolution occurs

• This means populations of organisms become better suited to their environment

• A good example of this is the development of antibiotic resistance by bacteria