Causes of evolution

• Causes of evolution

  • Natural selection
  • Mutation
  • Genetic drift
  • Gene flow
• Mutation

  = Change in DNA base sequence

  • Only affects if there is also a change in the environment.
• Genetic drift

  = Change in frequency of an existing gene variant in the population due to random chance.

  • May cause alleles to disappear completely and thereby reduce genetic variation.
  • could cause initially rare alleles to become much more frequent.
• Gene flow

  = Movement of alleles between populations.

  • Immigration and emigration result in changes of allele frequencies within a population.
• Impact of small populations

  • Gene pool of a small population results in limited genetic diversity. (Not many different genes and alleles)
  • So there is little genetic variation with the population.
  • Cant adapt to change easily.
  • Selection pressures= changes in the environment, new diseases, prey, competition, human influence.
• Density dependent factors= dependent on population size and include competition, predation, parasitism, communicable disease.
• Density independent factors= affect populations of all size in the same way- climate change, natural disaster, seasonal change, human activity.
• Bottleneck event= large decrease in population size, lasts a generation or more.

  • Gene pool and genetic diversity is reduced.
• Founder effect= new colony by a few isolated individuals, small gene pool, less genetic variation. Any rare alleles carried over to the new population will occur at a higher frequency in the new population compared to the old.
• stabilising selection= allele frequencies are kept constant over generations.

  • Eg: birth weight of babies.
• Directional selection= gradual change in allele frequencies over generations.

  • There is a change in the environment and the normal phenotype is no longer the most advantageous.
  • Organism which are less common and have more extreme phenotypes are positively selected.
  • Allele frequency then shifts towards the extreme phenotypes and evolution occurs.
  • Eg: peppered and black moths
• disruptive selection= extremes are selected for and the norm selected against.