Variation and evolution

• Phenotype


  The characteristics of an organism, which result from the interaction of the genes of the organism with the environment in which it lives
• Types of variation in phenotype

  • Continuous
  • Discontinuous
• Continuous variation


  Variation within a range, includes mass and height
• Discontinuous variation


  Can only take particular values, such as gender or shoe size
• Variation in genotype


  Has an effect on variation in phenotype
• Monogenic


  Characteristics influenced by one gene only, show discontinuous variation
• Polygenic inheritance


  Several genes at different loci are involved in determining a characteristic, often gives rise to continuous variation
• Characteristics influenced by both genotype and environment


  • Height
  • Lung cancer
  • Animal hair colour
• Niche


  The role of a species within the environment
• Adaptations


  • Anatomical (physical)
  • Behavioural (changes in behaviour)
  • Physiological (processes inside an organism's body)
• Natural selection


  Fitter individuals who are better adapted to the environment survive and pass on the advantageous genes to future generations
• Evolution


  The process by which the frequency of alleles in a gene pool changes over time as a result of natural selection
• Evolution via natural selection

  1. Variety of phenotypes within a population

  2. Environmental change occurs, selection pressure changes

  3. Individuals with advantageous alleles survive and reproduce

  4. Advantageous alleles passed on to offspring

  5. Frequency of alleles in population changes, leading to evolution
• Genetic drift


  Small change in allele frequency which occurs as a result of not all individuals in a population reproducing
• Genetic bottleneck


  Rapid reduction in population size which affects population size and genetic variation in future generations
• Founder effect


  Decrease in genetic diversity which occurs when the population descends from a small number of ancestors
• Speciation


  The process by which new species arise after a population becomes separated and cannot interbreed
• Allopatric speciation


  Caused by a physical barrier, gene flow is reduced, different selection pressures lead to separate species
• Sympatric speciation


  New species evolve from a single ancestral species when inhabiting the same geographic region, e.g. due to chromosomal error
• Hardy-Weinberg Equation


  Used to estimate the frequency of alleles in a population and see if a change in allele frequency is occurring
• p


  Frequency of the dominant allele
• q


  Frequency of the recessive allele
• For a population in genetic equilibrium: p + q = 1.0 (The sum of the frequencies of both alleles is 100%)
• The three terms of the binomial expansion indicate the frequencies of the three genotypes: p2 = frequency of AA (homozygous dominant), 2pq = frequency of Aa (heterozygous), q2 = frequency of aa (homozygous recessive)