AQA A Level Biology

Revision Notes

7.3.5 Evolution


  • The allele frequencies (within the gene pool) of a population can change over time due to processes such as natural selection
  • When the allele frequencies of a population change sufficiently over time, the characteristics of the species will also change
  • These changes can become so great that a new species forms
  • This process is known as evolution
  • Evolution can be defined as:

The formation of new species from pre-existing species over time, as a result of changes to gene pools and allele frequencies from generation to generation

  • The formation of new species via the process of evolution has resulted in a great diversity of species on planet Earth
    • Theoretically, at the origin of life on Earth, there would have been just one single species, to begin with
    • This species evolved into separate species (i.e. new species)
    • These species would then have divided again, each forming new species once again
    • Over millions of years, evolution has led to countless numbers of these speciation events, resulting in the millions of species now present on Earth

Diversity of species, downloadable AS & A Level Biology revision notes

Evolutionary change over a long period of time has resulted in a great diversity of species

  • The evolution of a new species can take a very long time and many generations
  • For organisms with a short generation time (such as bacteria), evolution can be observed far more quickly


Alistair graduated from Oxford University in 2014 with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems and Societies.

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