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