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Ecosystem > Selection
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The Hardy-Weinberg principle

  • Allele frequency → p (A) + q (a) = 1 (total gene pool)
  • Genotype frequency p2 (AA) + 2pq (Aa) + q2 (aa) = 1 (gene pool)
  • Allele frequency must be constant / population must be stable [EXAM]
    • Large population
      • Prevents large swings in frequencies
      • Anomalies and chance variation less significant
    • Random mating
      • Equal chance of alleles being passed on
    • No mutations / no immigration/emmigration / no natural selection
      • Prevents addition or removal of new alleles
  • Baseline by which to judge whether allele frequency of population's gene pool has changed
  • Gene pool: all the alleles in a population

Selection and change of allele frequency

Natural selection

  • New environmental factor affects survival rate of phenotype before reproduction
    • //otherwise population may become extinct
  • Organisms better adapted to the environment survive, reproduce, pass on their alleles/genes
  • Allele frequency of the advantageous gene increases
  • Changes frequencies of alleles in gene pool / phenotype in population
  • Population becomes adapted to environment

Stabilising selection

  • Natural selection favours "average" organisms best adapted to that environment
  • Organisms with extreme forms of characteristics/mutations are selected against
  • Heaviest and lightest babies have highest mortality
    • Less likely to survive, reproduce, pass on their alleles
  • [Graph] Normal distribution curve with thinner bell-shaped curve

Directional selection

  • Natural selection favours organisms with one extreme form of a characteristic
  • Pesticide resistance (warfarin - poison used to kill rats)
    • Resistant rats / need a lot of vitamin K / stabilising selection
    • New environmental effect: warfarin / kills normal rats
    • Resistant rats survived, reproduced, pass on resistance gene
    • New population forms by directional selection
  • Antibiotic resistance (penicillin resistance)
    • Resistant bacteria / unnecessary enzymes / selected against
    • New environmental factor: penicillin / kills normal bacteria
    • Resistant bacteria survived, reproduced, passed on resistance gene
  • [Graph] bell-shaped curve shifted to the right

Disruptive selection

  • Natural selection favours organism with two extreme forms of a characteristic
  • Balanced polymorphism: equilibrium of non-carriers and carriers of a characteristic caused by natural selection
  • Sickle-cell anaemia
    • Abnormal Hb makes red blood cells sickle-shaped / stick in capillaries
    • People homozygous for this recessive allele die before reproducing
    • People heterozygous for the allele should be at a disadvantage / red blood cells can sickle during exercise / allele should be selected against and rare
    • Where malaria is found, people heterozygous for sickle-cell have an advantage (resistant) and are likely to survive, reproduce and pass on the allele; people without the allele also have an advantage, because their red cells behave normally
    • Balanced polymorphism is produced / carrier is heterozygous for sickle cell
  • [GRAPH] Acts against the mode in a range of variation producing a bimodal distribution (two new modes) / might result in two distinct forms of the species (→morphs)

Reasons for a high incidence of a (dominant) rare disease/allele in a population [EXAM]

  • Allele frequency stays constant due to
  • Common ancestor/no migration/genetic isolation/small gene pool/in-breeding
  • High probability of mating with person having the allele
  • Reproduction before symptoms of the disease are apparent
  • No survival/selective disadvantage (no elimination by natural selection)


  • Splitting of one into more species/transformation of one into a new species over time
  • Emigration/immigration moves alleles between populations
  • Changes allele frequency by genetic variation in meiosis

Reproductive Isolation Mechanisms


  • Habitat isolation / populations inhibit different local habitants within one environment
  • Temporal isolation / same environment but are reproductively active at different times
  • Behavioural isolation / two populations have different courtship patterns
  • Geographical separation / populations inhabit different continents, islands, �


  • Gametes mortality / sperm cannot reach or fertilize egg
  • Zygote mortality / fertilisation occurs, but zygote fails to develop
  • Hybrid sterility / hybrid survives (viable) but is sterile and cannot reproduce (no meiosis)
  • Hybrid inviability / F1 hybrid has reduced viability: incomplete development

Allopatric speciation (geographical isolation)

  • Physical barrier (H2O, mountains, dessert) divides a population
  • Two different environments (abiotic, biotic)
  • Natural selection
  • Genetic drift changes genotype and phenotype
  • Two populations evolve separately
  • Reproductively isolated / 2 distinct species

Sympatric speciation (reproductive isolation)

  • Genetic isolation by mutation / reproductively isolated / but inhibit same habitat
  • Drift can cause further divergence between isolated gene pools
  • Hybridisation in plants
    • Offspring produced from parents of two different species
    • Chromosomal number doubles / polyploidy
    • New species is reproductively isolated by a postmating mechanisms
  • Can only reproduce with other polyploids, backcrosses with (2n) parents are sterile