M6:S3 Evolution

• Evolution

  A change in allele frequency
• Gene pool

  The complete range of alleles present in a population
• Allele frequency

  How often an allele occurs in a population
• Population

  A group of organisms of the same species living in a particular area
• Evolution by Natural Selection

  1. Individuals within a population vary due to different alleles

  2. Predation, disease and competition create a struggle for survival

  3. Individuals with advantageous alleles are more likely to survive and reproduce

  4. The frequency of advantageous alleles increases over generations
• Selection pressure

  Anything that affects an organism's chance of survival and reproduction
• Stabilising Selection

  1. Individuals with alleles for characteristics towards the middle of the range are more likely to survive and reproduce

  2. Reduces the range of possible phenotypes
• Directional Selection

  1. Individuals with alleles for extreme characteristics are more likely to survive and reproduce

  2. Increases the frequency of alleles for the extreme characteristics
• Evolution by Genetic Drift

  1. Chance dictates which alleles are passed on to offspring

  2. More common in smaller populations where chance has a greater influence
• Genetic bottleneck

  An event that causes a big reduction in a population's size, leading to a reduction in the gene pool
• Founder effect

  When a new population is started by a small number of individuals, leading to reduced genetic variation
• The Hardy-Weinberg principle predicts that the frequencies of alleles in a population won't change from one generation to the next
• Hardy-Weinberg equations

  Used to estimate the frequency of particular alleles and genotypes within populations
• The frequency of cystic fibrosis (genotype ff) in the UK is currently approximately 1 birth in every 2500
• The percentage of people in the UK that are cystic fibrosis carriers (Ff) can be estimated using the Hardy-Weinberg equations
• Genotype

  The genetic makeup of an organism
• Allele

  One of two or more alternative forms of a gene
• If there are two alleles for flower colour (R and r), there are three possible genotypes - RR, Rr and rr
• The frequency of genotype RR is 0.34, the frequency of genotype Rr is 0.27, so the frequency of genotype rr must be 0.39
• Estimating the percentage of cystic fibrosis carriers (Ff)

  1. Calculate q (frequency of recessive allele f)

  2. Calculate p (frequency of dominant allele F)

  3. Calculate 2pq (frequency of heterozygous genotype Ff)
• Artificial selection

  When humans select individuals in a population to breed together to get desirable traits
• Artificial selection in animals and plants

  • Modern dairy cattle
  • Bread wheat
• Effects of artificial selection

  • Reduces the gene pool
  • Can exaggerate certain traits leading to health problems
• Artificial selection can cause ethical issues, e.g. selecting traits in dogs that cause them health problems
• Speciation is the development of a new species
• Allopatric speciation

  Speciation that occurs when populations of the same species become reproductively isolated due to geographical isolation and natural selection
• Sympatric speciation

  Speciation that occurs without geographical isolation, e.g. through polyploidy
• Reproductive isolation

  Occurs due to changes in alleles and phenotypes that prevent successful breeding between populations
• Allopatric speciation is much more common than sympatric speciation