Evolution & Speciation

• Variations in the alleles of genes for members of the same species

  • Due to environmental influences
  • Due to genetic factors
• Random fertilisation


  Gametes carrying different alleles join together randomly
• Meiosis


  1. Nuclear division that creates gametes

  2. Alleles assorted in the gametes at random
• Mutation


  Mutation of an allele can lead to the creation of a new allele which can then be passed to the next generation
• Evolution via natural selection

  • Variety of phenotypes within a population
  • Environmental change occurs
  • Selection pressure changes
  • Individuals with advantageous alleles survive and reproduce
  • Advantageous alleles passed on to offspring
  • Frequency of alleles in a population changes over time
• Selection


  Process by which individuals that are better adapted to their environment are more likely to survive and breed
• Directional selection


  • Environmental conditions change
  • Phenotypes best suited to new conditions are more likely to survive, breed and produce offspring
  • Mean of the population moves in the direction of these individuals
• Stabilising selection


  • Phenotypes with successful characteristics are preserved
  • Individuals closest to the mean are favoured as they have alleles that have given them the survival advantage
• Disruptive selection


  • Both extremes of the normal distribution are favoured over the mean
• Reproductive separation


  • Accumulation of differences in gene pools
  • New species arise when genetic differences lead to inability to interbreed and produce fertile offspring
• Allopatric speciation


  • Caused by a physical barrier
  • Two groups become separated and reproductively isolated
  • Gene flow is reduced
  • Each group experiences different selection pressure
  • Over time, frequency of alleles changes through natural selection
  • Two parts of the population can no longer interbreed and become separate species
• Sympatric speciation


  • New species evolve from a single ancestral species when inhabiting the same geographic region
  • e.g. from mutation in courtship behaviour or chromosomal error during cell division
• Genetic drift


  • Change in the frequency of different alleles within the population as a result of chance
  • More important in small populations as chances of an allele being lost or fixed are much higher