natural selection + speciation

• natural selection - the non random increase in frequency of favourable alleles and non random decrease frequency of disadvantageous alleles
• stabilising selection

  occurs in unchanging environments and helps to maintain the best adapted genotypes in the population
• directional selection

  occurs during a period of environmental change. causes a shift in value of a trait eg. during ice age average size off mammal increased as they lose less heat
• disruptive selection

  results in population becoming split into 2 distinct groups. this is because extreme versions of phenotype are selected for eg. darwins finches
• gene transfer - genes can be transferred between organisms vertically or horizontally
• horizontal gene transfer - only prokaryotes. no reproduction - bacteria can directly transfer plasmid DNA from one cell to another
• evolution - the changes that occur in organisms over many generations as a result of genomic variations
• isolation barrier - prevents gene flow between sub populations during speciation
• species - a group of organisms that can interbreed to produce fertile offspring and who do not normally breed with other groups
• allopatric speciation - geographical barrier: sea / mountains 


  • populations isolated by geographical barrier
  • different mutations arise randomly in each sub population which leads to new variation
  • natural selection favours the mutants
  • over a long period of time, natural selection increases frequency of new alleles
  • speciation has occurred. the 2 species can no longer breed to produce fertile offspring
• sympatric speciation - behavioural, ecological barrier 


  • large interbreeding population share the same niche
  • an alternative niche appears, same members of population exploit the new niche
  • 2 sub populations now exploit different resources and can no longer interbreed
  • mutants are better adapted to exploit the new resources and successfully breed
  • natural selection favours the new mutants ad eventually they cannot interbreed to produce fertile offspring