3.7.3 evolution

• Genetic factors which cause phenotypic variation

  Mutation of alleles

  Random fertilisation by gametes

  Random assortment of genetic material during miosis
• Other than genetic factors, why else may phenotype vary within a species 


  Environmental influences
• Why does natural selection occur?

  Predation

  Disease

  Competition
• How does natural selection cause a change in a populations gene pool over generations?

  Organisms with advantageous characteristics are more likely to survive and pass their favourable alleles to offspring. Frequency of unfavourable alleles decreases
• Stabilising selection

  Occurs when environmental conditions stay the same. Individuals closest to the mean are favoured, and any new characteristics are selected against. Results in low diversity
• Directional selection

  Occurs when environmental conditions change. Individuals with phenotypes suited to the new conditions will survive and pass on their genes. Over time the mean of the population will move towards these characteristics
• Disruptive selection

  The opposite of stabilising selection, in that both extremes of the normal distribution are favoured over the mean. Over time, the population becomes phenotypically divided
• Speciation

  Where a population is split and isolated, there are different selective pressures on the two groups. If the genetic makeup changes to the extent the two groups can not longer interbreed, they have become separate species.
• Allopatric separation

  Separation resulting from a physical barrier, such as a river or mountain range. The environments occupied by the two groups are different, and therefore different alleles are favored
• Sympatric speciation 


  Speciation resulting from a non-physical barrier. Any changes in anatomy or behaviour may also prevent breeding
• Genetic drift

  A change in a populations allele frequencies that occurs due to chance rather than selective pressures. In other words, it is caused by 'sampling error' during reproduction
• Why does genetic drift affect small populations more than large ones? 


  The gene pool is smaller, so there are less alleles available and any changes in frequency becomes pronounced very quickly