(4) Types of Selection and Adaptation

• Selection Pressure :

  • environmental factors that affect the chance of survival of an organism
  • advantage to surviving a change
• selection pressures have different effects on the allele frequencies of a population through natural selection
• 3 types of selection :

  • stabalising
  • directional
  • disruptive
• Stabalising Selection :

  • natural selection keeps allele frequencies relatively constant over generations
  • if environmental conditions remain stable, individuals closest to the mean are favoured
• Example of Stabalising Selection :

  • (left) - very low clutch, potential of no surviving offspring
  • (peak) - mid clutch size favoured (closest to the mean)
  • (right) - very large clutch, malnourished, competition
• Directional Selection :

  • natural selection produces a gradual change in allele frequencies over several generations
  • if environmental conditions change, the individuals with phenotypes that are best suited to the new conditions are more likely to survive and reproduce
  • passing on the beneficial alleles
• directional selection - the average shifts one way, towards one extreme
• Directional Selection

  Example (1) :

  • an increase in surrounding temperature favours smaller body size/ larger SA:V
  • Global warming
  • warmer seas
  • increases metabolism of fish
  • increase O2 requirements
  • decreases O2 in the sea
  • smaller fish gain selective advantage
  • survive + reproduce
  • smaller fish frequencies in warmer oceans

  the population shifted in one direction
• Directional Selection

  Example 2 :

  • black bears increase in size during glacial periods
  • decrease during interglacial periods
• Directional Selection

  Example 3 :

  • a mutation occurred in an allele of a bacteria
  • the allele coded for a new protein that worked as an enzyme to break down the antibiotic penicillin before it is able to kill the bacteria
  • this provided an advantage to the bacteria which survived, reproduced and formed a population of antibiotic resistant bacteria
  • the population shifted in one direction
• Disruptive selection = natural selection in which extreme values for a trait are favoured over intermediate values
• Disruptive Selection

  Example 1 :

  • male bright lazuli bunting
  • only yearling males that are brightly coloured or dull are able to establish territories and breed
  • males with intermediate plumage do not mate
  • two different traits which are useful
• Disruptive Selection

  Example 1 explanantion :

  • the populations divide shifting in either direction away from the intermediate
  • this type of selection is most likely to cause evolution by natural selection
  • the members in the 'intermediate' being the common ancestor the diverging species

  this drives speciation
• Disruptive selection

  Example 2 :

  • short tails of a chipmunk keep predators from catching individuals on the ground
  • long tails are good for balance in the trees
  • medium tails have no advantage
• Directional Selection

  Example 3 :

  • wolly mammoth ancestor - Gomphotherium
  • environmental changes
  • mutations cause variation
  • Hot - elephant
  • cold - wolly mammoth
• natural selection results in species that are better adapted to the environment that they live in
• 3 different adaptations :

  • anatomical
  • physiological
  • behavioural
• Anatomical :

  • structural features
  • eg big ears, big paws, big teeth, white fur
• Physiological :

  • process
  • metabolism of fat
  • concentrated urine
  • lowers metabolism when hibernating
• Behavioural :

  • prides
  • pack hunting
  • aggressive
  • dominance alpha
• Cactus :

  • Spines : anatomical - deters herbivores
  • Thick fleshy stem : anatomical - stores water
  • wide spread root system : anatomical - supports and collects water
  • Tap roots : anatomical - find water + stability
• Hedgehog :

  • Spikes : anatomical - deters predators
  • hibernate : physiological + hibernate : behavioural
  • ears have low frequency to hear ants : physiological
  • roll into ball : behavioural
  • protruding snout access insects : anatomical
  • sharp claws/ digging : anatomical/ behavioural