evolution

• what is allele frequency?

  how often a specific allele occurs in a population
• evolution?

  the change in the characteristics of a group of organisms over time caused by changes in the frequency of alleles within a population
• what are the stages of evolution?

  • Variation based on genetic diversity
  • Change in environment and new selection pressures and or competition for resources. random mutations arise leading to new beneficial alleles
  • survival of those with beneficial alleles
  • natural selection --> stabilising, directional or disruptive
  • reproduction where those with beneficial alleles pass them on to offspring
  • evolution leading to speciation --> sympatric or allopatric
• what is a selection pressure?

  any change in environment that favours one phenotype over the other
• what are the 2 types of natural selection

  directional and stabilising
• what is stabilising selection?

  • the environment is stable so phenotypes closest to the mean (most common) are favoured
  • phenotypes at the extremes of the normal distribution are selected against
  • the mean stays the same but the number of individuals at the extremes is reduced so the range is reduced
• what is directional stabilisation?

  • when there's a change in environment, individuals with alleles for characteristics of an extreme type are more likely to survive and reproduce
  • overtime, the mean will move to the right or left of the distribution curve as these individuals are favoured
• what is genetic drift?

  Random changes in allele frequencies in a population.
• how does genetic drift work?

  • individuals within a population show variation in their genotypes
  • by chance, the allele for one genotype is passed on to the offspring more often than others so the allele frequency of that allele increases
  • if by chance the same allele is passed on more often for many generations, it can lead to evolution as the allele frequency increases
• when would genetic drift drive evolution more than natural selection?

  • if the characteristic doesn't affect survival or reproduction so can't be selected against
• genetic drift has a greater effect on smaller populations where chance has a greater influence. in larger populations, any variations in allele frequency due to chance tend to even out across the whole population
• what is genetic bottleneck and how does it affect evolution?

  an event such as a natural disaster that causes a big reduction in a population's size leading to a reduction in the gene pool

  if by chance more individuals with a certain allele survive, genetic drift will cause the allele frequency of that allele to increase
• what is the founder effect?

  when just a few organisms from a population start a new population and there are only a small number of different alleles in the initial gene pool
• how does the founder effect work and how is it affected by genetic drift

  • individuals within a population show variation in their genotypes
  • some of these individuals start a new population by chance, these individuals are mostly one particular genotype
  • without any gene flow - introduction of new alleles from outside the population the new population grow with reduced genetic variation
  • the population is small so is more influenced by genetic drift
• what is gene flow

  when alleles are transferred from one population to another by interbreeding
• what are the factors that cause evolution?

  • natural selection
  • genetic drift
  • genetic bottleneck
  • founder effect
• what are the hardy-weinberg equations for?

  • they are used to estimate the frequency of particular alleles and genotypes within populations
  • if the allele frequencies do change between generations in a large population, immigration, emigration, mutations or natural selection has happened
• what does the p stand for in the equation?

  frequency of the dominant allele in the population
• what does the q stand for?

  frequency of the recessive allele in a population
• what is p²

  the frequency of individuals with the homozygous dominant genotype in the population
• what is q²

  the frequency of individuals with the homozygous recessive genotype in the population
• what is pq?

  the frequency of individuals with the heterozygous genotype in the population
• what is the hardy-weinberg principle

  the frequency of alleles in a populations wont change from one generation to the next as long as its a large population with:

  • no imigration or emigration
  • no mutations
  • no natural selection
  • there is random mating so all possible genotypes can breed with all others