Evolution and natural selection

• Niche


  The role of a species within the environment
• Species share the same niche


  They compete with each other
• A better adapted species survives
• Natural selection


  The idea that better adapted species survive
• Many organisms have a unsustainably large number of offspring
• Fish may lay thousands of eggs which cannot all survive due to limited resources
• Reason for large number of offspring

  Greater competition within the species (intraspecific competition) so only those with the best alleles survive and reproduce
• Variation in genotypes and phenotypes within a population increases the chance that a species will survive in a changing habitat
• Evolution via natural selection

  1. Variety of phenotypes within a population

  2. Environmental change occurs, changing selection pressure

  3. Individuals with advantageous alleles survive and reproduce

  4. Advantageous alleles passed to offspring

  5. Frequency of alleles in population changes, leading to evolution
• Selection


  The process by which individuals that are better adapted to their environment are more likely to survive and breed, passing on their advantageous alleles
• Types of selection


  • Directional selection
  • Stabilising selection
  • Disruptive selection
• Directional selection


  Environmental conditions change, phenotypes best suited to new conditions are more likely to survive and reproduce, shifting the population mean
• Stabilising selection


  Phenotypes with successful characteristics are preserved, those of greater diversity are reduced, as individuals closest to the mean are favoured
• Disruptive selection


  Both extremes of the normal distribution are favoured over the mean
• Speciation


  The process by which new species arise after a population becomes separated and cannot interbreed
• Allopatric speciation


  Caused by a physical barrier, leading to reproductive isolation and different selection pressures
• Sympatric speciation


  New species evolve from a single ancestral species when inhabiting the same geographic region, e.g. due to chromosomal errors or mutations