populations and evolution

• Hardy-Weinberg Principle


  A mathematical model that predicts the frequencies of alleles in a population will not change from one generation to the next
• Conditions for Hardy-Weinberg Principle to apply

  • Large population
  • No immigration, emigration or natural selection
  • Random mating - all genotypes can breed with all others
• Using the Hardy-Weinberg Equations

  1. Calculate frequency of one allele if you know frequency of the other allele

  2. Calculate frequency of genotypes if you know allele frequencies

  3. Calculate if external factors are affecting allele frequency
• Flower colour example


  • Alleles R (red) and r (white), genotypes RR, Rr, rr
  • Frequency of Rr genotype is 0.27
  • Frequency of rr genotype is 0.39
• Genotype frequencies can be used to calculate phenotype frequencies
• Cystic fibrosis example


  • Current frequency of cystic fibrosis (recessive genotype) is 1 in 2500
  • Frequency of recessive allele q = 0.02
  • Frequency of carrier genotype Ff = 0.039 (3.9% of population)
• Cystic fibrosis frequency measured 50 years later


  • Frequency of recessive allele q = 0.017
  • Hardy-Weinberg principle doesn't apply, external factors affecting allele frequency
• A population is a group of organisms of the same species living in a particular area
• A gene pool is the complete range of genes present in a population
• A species is defined as a group of similar organisms that can interbreed to give fertile offspring
• Variation


  The differences that exist between individuals
• Variation within a species

  • Individuals in a population can show a wide range of different phenotypes
  • Individuals of the same species have the same genes but different alleles (versions of genes)
  • Genetic variation is introduced through mutation, meiosis, and sexual reproduction
  • Variation can also be caused by differences in the environment
• Most variation within a species is caused by a combination of genetic and environmental factors
• Only genetic variation results in evolution
• Evolution


  A change in allele frequencies over time
• Natural selection


  1. Individuals vary

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

  3. Individuals with beneficial phenotypes are more likely to survive and reproduce

  4. Beneficial alleles increase in frequency over generations
• Stabilising selection


  • Individuals with alleles for characteristics towards the middle of the range are more likely to survive and reproduce
  • Reduces the range of possible phenotypes
• Stabilising selection example


  • In a mammal population, individuals with average fur length are most likely to survive and reproduce
• Directional selection


  • Individuals with alleles for a single extreme phenotype are more likely to survive and reproduce
  • Can occur in response to environmental change
• Directional selection example


  • Cheetahs have developed increased speed through directional selection
• Disruptive selection


  • Individuals with alleles for extreme phenotypes at either end of the range are more likely to survive and reproduce
  • Characteristics towards the middle of the range are lost
• Disruptive selection example


  • In a bird population, individuals with large or small beaks are more likely to survive than those with medium-sized beaks
• Disruptive selection occurs when the environment favours more than one phenotype
• Genetic variation can be caused by genes or the environment
• Evolution is a change in allele frequencies over time
• Directional selection is where individuals with alleles for a single extreme phenotype are more likely to survive and reproduce
• Disruptive selection is where individuals with alleles for extreme phenotypes at either end of the range are more likely to survive and reproduce
• Speciation


  The development of a new species from an existing species
• Reproductive isolation


  Changes in alleles and phenotypes in some individuals prevent them from breeding successfully with individuals without these changes
• Allopatric speciation


  1. Populations become geographically separated

  2. Populations experience different selection pressures

  3. Differences in allele frequencies accumulate

  4. Individuals from different populations can no longer interbreed
• Sympatric speciation


  1. Random mutations occur within a population

  2. Individuals with different numbers of chromosomes can't reproduce sexually

  3. Polyploid organism emerges and reproduces asexually

  4. New species develops
• Ways reproductive isolation can occur


  • Seasonal
  • Mechanical
  • Behavioural
• Genetic drift


  Chance, rather than environmental factors, dictates which individuals survive, breed and pass on their alleles
• Natural selection and genetic drift

  Work alongside each other to drive evolution, but one process can drive evolution more than the other depending on the population size
• Evolution by genetic drift usually has a greater effect in smaller populations where chance has a greater influence
• In larger populations, any chance variations in allele frequency tend to even out across the whole population
• The diversity of life on Earth today is the result of speciation and evolutionary change over millions of years
• Evolution of new species

  1. One population divided

  2. New populations evolved into separate species

  3. New species divided again

  4. New populations evolved into more separate species

  5. This process repeated over a long period of time
• Speciation is the development of a new species from an existing species
• Reproductive isolation can occur when populations become geographically separated and experience different selection pressures, leading to differences in allele frequencies and the inability to interbreed