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
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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
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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
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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
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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)
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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)
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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
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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, …
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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
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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
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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
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