3.7.2 POPULATIONS

• Species exist as one or more populations.

  Populations change and evolve as allele frequencies change across generations.

  A population is a group of organisms of the same species occupying a particular space at a particular time that can potentially interbreed.
• Gene Pool - all the alleles in a population (all the alleles of all the genes of all the individuals in a population)
• • Allele Frequency - the proportion of a certain allele in a gene pool. Change in response to selection pressures by natural selection between and within populations.

  No. of that allele / total no. of alleles for that gene in a population.
• Hardy Weinberg Principle - Predicts that allele frequencies (of a gene in a population) will not change from generation to generation. 

  Conditions for this:

  • Large population 
  • No mutations or selection
  • Random mating within the population
  • Genetically isolated population
  • No migration
• Hardy Weinberg equation can be used to calculate the frequency of alleles, genotypes and phenotypes in a population.

  p² + 2pq + q² = 1.
• Allele frequency

  • p = frequency of dominant allele
  • q = frequency of recessive allele
  • p + q = 1
• Genotype frequency

  • Homozygous dominant (genotype) frequency = p^2
  • Homozygous recessive (genotype) frequency = q^2
  • Heterozygous (genotype) frequency = 2pq
• Phenotype frequency

  • Organism expressing recessive allele (recessive phenotype) = q^2
  • Organism expressing dominant allele (dominant phenotype) = p^2 + 2pq
• Hardy Weinberg Strategy:

  • Work out what frequency given (p, p^2, q or q^2)
  • Turn into a probability (0 to 1)
  • Usually will need to calc p and q
  • Work out what frequency is wanted 
  • Calculate (as probability)
  • Check by adding up to 1
  • Multiply to make units needed in answer (eg. % or number in population)