AQA AS Biology

Revision Notes

4.4.3 Natural Selection

Natural Selection

  • Genetic variation exists within populations due to the presence of different alleles
  • There is differential reproductive success between the organisms with different alleles of the same gene
  • Under certain environmental conditions, individuals with certain alleles will have an increased chance of survival and reproduction
  • New alleles can arise in populations through random mutation
  • Natural selection can cause the frequency of alleles in a population to change over time

Principles of Natural Selection

  • Random mutation can produce new alleles of a gene
  • Many mutations are harmful or neutral but, under certain environmental conditions, the new alleles may benefit their possessor, leading to an increased chance of survival and increased reproductive success
  • The advantageous allele is passed onto the next generation
  • As a result, over several generations, the new allele will increase in frequency in the population

Example of Natural selection in Rabbits

  • Variation in fur colour exists within rabbit populations
  • At a single gene locus, normal brown fur is produced by a dominant allele whereas white fur is produced by a recessive allele in a homozygous individual
  • Rabbits have natural predators like foxes which act as a selection pressure
  • Rabbits with a white coat do not camouflage as well as rabbits with brown fur, meaning predators are more likely to see white rabbits when hunting
  • As a result, rabbits with white fur are less likely to survive than rabbits with brown fur
  • Therefore, the rabbits with brown fur have a selection advantage, so they are more likely to survive to reproductive age and be able to pass on their alleles to their offspring
  • Over many generations, the frequency of alleles for brown fur will increase and the frequency of alleles for white fur will decrease

White and brown rabbits, downloadable AS & A Level Biology revision notes

Image showing selective pressures acting on a rabbit population for one generation. Predation by foxes causes the frequency of brown fur alleles in rabbits to increase and the frequency of white fur alleles in rabbits to decrease.

Exam Tip

Exams often ask questions about how the process of natural selection occurs for a certain species. The principles of natural selection described above are always the same! You just need to edit the details to make them specific to the question.

Author: Lára

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.
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