Natural Selection (Cambridge (CIE) IGCSE Co-ordinated Sciences (Double Award)): Revision Note

Natural Selection

• Variation in a population can lead to natural selection:

• The process of natural selection involves the following steps:

  • Individuals in a species show variation caused by differences in genes

  • Reproduction results in many offspring

  • This leads to a struggle for survival due to competition for food and other resources

  • Individuals that are better adapted to their environment have a higher chance of survival and more chances to reproduce

  • The alleles coding for the adaptive characteristics are more likely to be passed on

  • Over many generations the advantageous alleles become more common in the population

Natural selection in moths diagram

Natural selection in peppered moths occurred when an environmental change caused the allele for dark body colour to become advantageous

Adaptation by Natural Selection: Extended

 Extended Tier Only

• Natural selection can lead to the development of antibiotic resistance in bacteria as follows:

  • There is variation in a bacterial population; some bacteria have alleles that provide resistance to an antibiotic

  • Bacteria compete for survival in their environment

  • When bacteria are exposed to an antibiotic the individuals with the resistance allele have an advantage and are more likely to survive and reproduce

  • The resistance allele is passed on to offspring

  • The antibiotic resistance allele becomes more common in the population

  • Eventually the entire population is resistant and is described as an antibiotic resistant strain of bacteria

Natural selection in bacteria can give rise to antibiotic resistance