Natural Selection (AQA A Level Biology): Revision Note

Natural Selection

Selection pressures

• Environmental factors affect an individual's chance of survival

  • They are said to act as selection pressures

• These factors can be:

  • Biotic (living) factors, including predation, competition and disease

  • Abiotic (non-living) factors, including light, temperature, water and soil pH

Example: Environmental pressures on African lions

Survival and reproduction

• In any species population, genetic variation means some individuals will have phenotypes that make them better adapted to survive

  • This means they have a selective advantage

• Those with a selective advantage are likely to produce more offspring and pass on their favourable alleles to the next generation

  • Alleles that are favourable (provide a selective advantage) will increase in frequency in a gene pool

  • Alleles that are not favourable (don't provide a selective advantage) will decrease in frequency in a gene pool

• This differential survival and reproduction is the basis of natural selection

  • Read more detailed notes on natural selection from Unit 4

Natural selection and evolution

• Over generations, differential reproductive success and natural selection, leads to an increase in advantageous allele (and phenotype) frequencies

• When allele frequencies change significantly over time, the characteristics of the population change — if the changes are great enough, a new species may form

  • 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