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
Ecosystem
Ecosystem
Five Kingdoms
Human Activity
Inheritance
Nutrient Cycle
Photosynthesis
Selection
Variation
Content
The Hardy-Weinberg principle
Selection and change of allele frequency
Reasons for a high incidence of a (dominant) rare
disease/allele in a population [EXAM]
Speciation
Reproductive Isolation Mechanisms
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