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• Founder effect


  A type of genetic drift in which a few individuals of a species break off from the population and form a new colony. This results in smaller gene pools and an increased frequency of rare alleles.
• Genetic bottleneck


  A drastic reduction in population size leading to reduced genetic diversity within a population.
• Genetic drift


  Random variations in allele frequencies in small populations, due to mutations.
• Selection pressures


  Factors that affect an organism's ability to survive in an environment e.g. disease, prey, competitors, water availability.
• Hardy-Weinberg principle


  A mathematical formula used to calculate the allele frequencies of traits with dominant and recessive alleles in a population.
• Population genetics


  The changes in allele frequencies in a particular population.
• Gene pool


  The total number of genes (and their alleles) in a population.
• Conditions for the Hardy-Weinberg principle

  • Organisms are diploid
  • Organisms reproduce by sexual reproduction only
  • There is no overlap between generations, i.e. parents do not mate with offspring
  • Mating is random - No selective breeding occurring
  • The population is large - There are no chances of the dominant or recessive alleles randomly disappearing due to chance and reduces the sampling errors
  • There is no migration, mutation, or selection - This would mean no individuals entering the population (immigration) or leaving (emigration)
  • Allele frequencies are equal in both sexes
• The Hardy-Weinberg principle can be useful when building models and making predictions, but the assumptions listed are very rarely all present in nature
• Allele frequencies


  p2 + 2pq + q2 = 1.0 (when a gene has two alleles, a dominant (A) and recessive (a), where A = p and a = q)
• p + q = 1.0 (use when given information about allele frequency)
• Spec: 6.1.2 - (f) Hardy Weinberg (g) Isolating Mechanisms (h) Artificial Selection


  • (f) the use of the Hardy–Weinberg principle to calculate allele frequencies in populations
  • (g) the role of isolating mechanisms in the evolution of new species
  • (h)(i) the principles of artificial selection and its uses
  • (h)(ii) the ethical considerations surrounding the use of artificial selection
• Speciation


  The formation of new species due to the evolution of two reproductively separated populations. Two forms: allopatric and sympatric speciation.
• Stabilising selection


  A type of selection that favours individuals with phenotypes close to the mean (average) and selects against extreme phenotypes.
• Sympatric speciation


  A form of speciation that occurs when two populations within the same area become reproductively isolated.
• Allopatric speciation


  A form of speciation that occurs when two populations become geographically isolated due to a physical barrier.
• Genetic isolation


  Two populations of the same species can become genetically isolated when they stop reproducing and do not exchange genes in offspring production. Changes in allele frequencies between groups are not shared, leading to independent evolution and the formation of two populations that cannot interbreed.
• When genetic differences prevent members from interbreeding and producing fertile offspring, speciation occurs.
• Mechanisms of speciation


  • Allopatric
  • Sympatric
• Allopatric speciation


  A common type of speciation where populations of a species are separated by geographical barriers, such as natural bodies of water or man-made structures like motorways. This results in two populations of the same species, reproductively separated, preventing genetic exchange. If selection pressures change gene pools and allele frequencies within both populations, they may eventually diverge and form separate species. The changes in alleles and genes affect the phenotypes present in both populations, and over time, they may begin to differ physiologically, behaviorally, and morphologically.
• Sympatric speciation


  Occurs without geographical barriers, where two populations of the same species live in the same place but do not have gene flow between them. This can occur through ecological and behavioral separation. Ecological separation occurs due to differences in soil pH, which affects plant growth and flowering. A population growing in a slightly different pH may flower at a different time from another, leading to reproductive separation and genetic isolation. Behavioral separation occurs due to differences in feeding, communication, or social behaviors, such as courting behaviors. These factors can lead to the separation of populations and the resulting genetic isolation.
• Both allopatric and sympatric speciation are reliant on mutations occurring within individuals in populations. Without mutations, there are no new alleles of genes for selection to act on. The changes in genetic material caused by mutations are important as these changes are what produce the differences in physiology, behaviors and morphology between populations over many generations, eventually leading to speciation.
• Spec: 6.1.2 - (f) Hardy Weinberg (g) Isolating Mechanisms (h) Artificial Selection


  • Summary
• Selective breeding


  The process by which humans artificially select organisms with desirable characteristics and breed them to produce offspring with desirable phenotypes. Selective breeding may also be referred to as 'artificial selection'.
• Directional selection


  A type of selection that favours one extreme phenotype and selects against all other phenotypes.
• Discontinuous variation


  A type of variation that can be categorised e.g. blood group. A characteristic can only appear in discrete values. One or two genes influence discontinuous variation.
• Disruptive selection


  A type of selection that favours individuals with extreme phenotypes and selects against those with phenotypes close to the mean.
• Artificial selection


  A practice where humans choose organisms with desirable traits and selectively breed them together to enhance their expression over generations. Also known as selective breeding, has been practiced for thousands of years, without understanding the genetics behind it. Breeders select individuals based on their phenotypes, not their genotypes, which can lead to the enhancement of other genetically linked traits, which can negatively affect the organism's health. The process involves selecting an individual with the desired phenotype, breeding them together, and testing the offspring for the desired trait. The best individuals are chosen for further breeding until all offspring display the desired trait. The process continues for many generations: the best individuals from the offspring are chosen for breeding until all offspring display the desirable trait.
• Artificial selection, a process of selective breeding, can lead to inbreeding, a reduction in the gene pool, resulting in harmful genetic defects and increased vulnerability to new diseases. This is known as inbreeding depression, where only the best animals or plants are bred together. This can result in conditions and diseases that are extremely damaging or deleterious for the animals involved, making them ethically questionable. For instance, dog breeds like Bulldogs and Pekinese suffer from breathing problems due to their shortened snouts. While large dogs like Great Danes often suffer from hip dysplasia. Therefore, the ethical considerations surrounding artificial selection are crucial in ensuring the welfare of animals and promoting ethical practices in the field of genetics.
• Examples of animals and plants selectively bred

  • Domestic dog (descended from wolves)
  • Wild brassica (given rise to cauliflower, cabbage, broccoli, brussels sprouts, kale and kohlrabi)
• Epistasis


  The interaction of genes at different loci affecting the phenotypic characteristics.
• Epistatic gene


  Gene which has alleles that masks or supresses the hypostatic gene.
• Hypostatic gene


  Gene which has alleles that can be masked by epistatic gene.
• Epistasis is the interaction of genes at different loci affecting the phenotypic characteristics. The genes involved may work against each other (antagonistically) resulting in masking, or they may work together in a complementary fashion.
• Gene regulation is a form of epistasis e.g. a regulatory gene produces a regulatory protein that regulates a structural gene.
• Dominant epistasis


  Occurs when a dominant allele at one gene locus masks the expression of the alleles at a second gene locus.
• Recessive epistasis


  Occurs when a pair of recessive alleles (homozygous) at one gene locus masks the expression of the alleles at a second gene locus.
• Complimentary epistasis


  The genes work together in a complementary fashion so you need at least one dominant allele of both genes to get one phenotype and all other combinations give another phenotype.
• Complimentary epistasis usually occurs through enzyme cascades. Where 2 genes work together coding for enzymes which catalyse the production of the substrate needed for the next reaction.
• Spec: 6.1.2 (b)ii) - Epistasis


  • (b) (ii) the use of phenotypic ratios to identify linkage (autosomal and sex linkage) and epistasis.