Selection

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)

Speciation

• 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

Premating

• 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, �

Postmating

• 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