Evolution may lead to speciation

• Give genetic factors that cause phenotypic variation within a species?

  • Mutation of alleles
  • Random fertilisation of gametes
  • Random assortment of genetic material during meiosis (crossing over)
• Other than genetic factors, why may phenotype vary within species?

  Environmental influences
• Why does natural selection occur?

  1. Predation
  2. Disease
  3. Competition
  4. All resulting in differential survival and reproduction
• How does natural selection cause a change in a population's gene pool over generation?

  Organisms with advantageous characteristics are more likely to survive and pass their favourable alleles to offspring. Frequency of favourable alleles increase over generations.
• What is 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.
• What is 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.
• What is disruptive selection?

  When both extremes of the normal distribution are favoured over the mean. Over time, the population become phenotypically divided.
• Define speciation?

  Where a population is split and isolated, there are different selective pressures on the two groups. If the genetic makeup changes so much to the extent the two groups can no longer interbreed, they have become separate species.
• What is meant by allopatric speciation?

  Speciation resulting from a physical/geographical barrier (e.g., river, mountain range). The environments occupied by the two groups are different, so different alleles are favoured.
• What is meant by sympatric speciation?

  Speciation resulting from a non-geographical barrier (occurring in the same habitat). For example, a mutation that no longer allows two organisms to produce fertile offspring (reproductive isolation). Leads to change in frequency of alleles and disruptive selection.
• Define genetic drift?

  Random change in a population's allele frequencies that occurs due to chance.