Population genetics

• Investigates how allele frequency changes within a population over time
• Gene pool = total of all genes in a population at one time
• Allele frequency = relative frequency of a particular allele in a population
• Frequency isn't linked to whether the allele is dominant or recessive
• ^ can change over time in response to different conditions

Calculating allele frequency

• P + Q = 1
• Where P is frequency of dominant allele and Q is frequency of recessive allele

The Hardy-Weinberg principle

• Mathematical relationship between the frequencies of alleles and genotypes in a theoretical population
• Theoretical population had 5 features
  • Large population
  • No mutations
  • No natural selection
  • Random mating
  • No immigration or introduction of new alleles
• P^2 + 2PQ + Q^2 = 1
• P^2 is the frequency of homozygous dominant
• 2PQ is the frequency of heterozygous
• Q^2 is the frequency of homozygous recessive
• Use this plus P+Q=1 to work out questions

Factors affecting evolution

• Mutation leads to genetic variation
• Sexual selection leads to an increase in the frequency of alleles that code for characteristics that improve mating success
• Immigration and emigration increase the gene flow to a population
• Genetic drift occurs in small populations --> appearance of a new allele will have a greater effect
• Natural selection increases the frequency of alleles that allow the organism's survival

The impact on small populations

• Small populations have smaller gene pools so cannot adapt to change as easily and are likely to become extinct
• Population size limiting factors:
  • Density-dependent factors depend on population size like competition, predation, parasitism and communicable disease
  • Density-independent factors affect populations of all sizes like climate change, natural disasters, seasonal change, human activities
• Genetic bottlenecks = large reductions in population size for at lease one generation greatly reduces gene pool and genetic variation but a beneficial mutation will have a greater impact

Founder effect

• Extreme example of genetic drift
• Smaller populations have smaller gene pools 
• Rare alleles in original population become more common in the isolated population due to the reduced gene pool

Evolutionary forces

• Normal distribution = bell-shaped curve of distribution of different variants
• Stabilising selection = norm or average is selected got and the extremes are selected against (values in middle of bell are selected) --> results in reduction of frequencies of the alleles for extreme conditions
• Directional selection = occurs when there's a big change and the normal phenotype is no longer advantageous --> organisms with more extreme phenotypes positively selected --> allele frequency switches to more extreme end of spectrum
• Disruptive selection = the extremes are selected for and the norm selected against --> very rare but in North America young male lazuli buntings are either dull brown or bright blue this is because the brown aren't seen as threatening by the adults so are left alone but the blue is seen as very threatening by the adults so they're also left alone --> intermediate or normal phenotypes were attacked by the adults