4.4 Variation and Evolution

• Species


  A population of organisms similar enough to breed and produce fertile young
• Genetic drift


  A change in allele frequency because of chance, more likely in small populations

  
• The founder effect


  When a small population has become separated from the original population and so an allele in the small population becomes more frequent in the succeeding generations
• Speciation


  Allele changes can significantly alter the phenotype of the isolated population so that they can no longer breed with the original population to give fertile young. A new species has been formed
• Speciation by natural selection

  1. Mutations in alleles lead to changes in allele frequencies

  2. Variation in phenotypes due to different alleles

  3. Many offspring are produced and there is competition for limited resources

  4. Selection pressures from the environment give some phenotypes a survival advantage and they survive long enough to breed

  5. Successful phenotypes pass on their alleles increasing their frequency in the population

  
• Natural selection


  • A mechanism proposed by Charles Darwin and Alfred Russel Wallace for evolution
  • The mechanism describes how phenotypes in a population can be maintained or changed to be optimal in the environment

  
• Selection pressures e.g. selective predation

  Can confer survival advantages to phenotypes that show camouflage or mimicry
• Mechanisms for isolation


  • Allopatric speciation
  • Sympatric speciation
  • Morphological isolation
  • Hybrid sterility
  • Behavioural isolation
  • Seasonal isolation
  • Hybrid fertility
• Allopatric speciation


  Evolution of a new species from demes isolated in different geographical locations
• Sympatric speciation


  Evolution of a new species from demes sharing the same geographical location
• Demes


  A sub-group of a population that interbreed more frequently, reducing gene flow with the rest of the population
• Types of variation


  • Heritable (can be passed on to offspring)
  • Non-heritable (cannot be passed on to offspring)
• Heritable variation


  Genetic differences caused by crossing over, independent assortment, sexual reproduction (mixing of 2 parental genotypes)
• Non-heritable variation


  • Differences caused by the environment
  • Epigenetic differences
• Types of competition and selection pressure

  • Interspecific (Between members of different species)
  • Intraspecific (Between members of the same species)
• Competition for limited resources


  The phenotype of an organism could give an advantage, and that would increase the prevalence of the alleles that code for that phenotype in the population as the organism survives to breed and pass on the successful allele
• Selection pressure


  The effect of selective agencies on the phenotypes in a population
• Selective agencies


  • Food availability
  • Breeding sites
  • Climate
  • Human impact
• Selection pressure for


  Phenotypes have an advantage in competition so the alleles that code for them are selected for
• Selection pressure against


  Phenotypes do not have an advantage and are unable to compete successfully. So, alleles that code for them are selected against
• Gene pool


  All the alleles of all the genes of all the individuals in a population of sexually reproducing organisms
• Genetic drift


  The change in allele frequency in a population by chance, most significant in small or isolated populations

  
• Allele frequency


  Population genetics is concerned with all the alleles in the gene pool and describes the proportions of alleles present at any one time

  
• Hardy Weinberg equation


  Estimates the allele frequency in a population, if a population is stable (large population, no migration, no mutation, no selection pressure) the allele frequency will stay the same from generation to generation (Hardy Weinberg equilibrium)

  
• Continuous variation


  • Characteristics have many possible values
  • Generally polygenic (being controlled by many genes)
  • Gene expression is influenced by the environment
  • Forms a normal distribution curve

  
• Discontinuous variation


  • Characteristics are discrete and have no intermediates
  • Monogenic (controlled by a single gene)
  • Gene expression not influenced by the environment
  • Usually drawn as a bar chart