Selection & Speciation

• Causes of variation (sexual reproduction):

  • Mutations
  • Meiosis
  • Random fertilisation
• Variation predominantly caused by genetic factors are discontinuous (categoric data)
• Mutations can:

  • Confer a selective advantage
  • Be neutral
  • Confer a selective disadvantage
• Environmental factors influence an organisms phenotype:

  • Temperature
  • Light intensity
  • Humidity
• Characteristics caused by more than one gene can cause continuous variation e.g. height or mass
• Types of stabilisation:

  • Stabilising selection
  • Directional selection
  • Disruptive selection
• Stabilising selection - selection against both extremes of a trait, so that the mean of the trait is maintained
• Directional selection - selection for one extreme of a phenotype, leading to a change in the frequency of that phenotype
• Disruptive selection - selection for both extremes of a trait, resulting in two new phenotypes
• Speciation - evolution of a new species from existing species
• Speciation:

  • Two populations undergo reproductive isolation
  • Prevents interbreeding
  • Becomes distinct via an accumulation of mutations and different selection pressures
• Genetic drift (bottleneck) - the random increase or decrease in allele frequencies in a population due to a reduction in the number of individuals
• Gene flow (founder effect) - change in allele frequency due to migration of fertile individuals
• Allopatric speciation - when a population splits from a parent population due to geographic isolation
• Sympatric speciation - when a population splits into two separate species, but remain in the same geographical area
• Allopatric speciation (geographic isolation):

  • Physical barrier prevents interbreeding
  • Isolated population subject to selection pressures
  • Variation due to mutations
  • Eventually unable to interbreed so reproductively isolated
  • Separate species with its own gene pool
• Types of isolation:

  • Temporal (breed at different times of year)
  • Ecological (different habitats within the same area)
  • Behavioral (different courtships)
  • Mechanical (anatomical differences)
  • Gametic (incompatibility between gametes)
  • Hybrid (organisms interbreed but offspring infertile)
• Explain how succession results in a wide variety of species (2):

  1. Increased biodiversity
  2. Provides more niches or food sources
• Suggest how speciation could have occured (4):

  1. Variation due to mutations
  2. Allopatric speciation
  3. Reproductive isolation
  4. Species in different areas subject to different selection pressures
  5. Different alleles are passed on
  6. Change in allele frequency
  7. Species cannot interbreed with original species to produce fertile offspring
• Explain how human birth mass is affected by stabilising selection:

  • Most likely to be transferred to a special care unit if very below average
  • Extreme mass babies less likely to survive so less likely to pass on alleles
  • Extreme mass at birth decreases in frequency
• How animal behaviour can lead to sympatric speciation:

  • Reproductive isolation and NOT geographical isolation
  • Change in allele frequency
  • Cannot interbreed to produce fertile offspring