Evolution and speciation

• Gene pool


  All of the alleles of all the genes of all the individuals in a population at a given time.
• Allelic frequency


  The number of times an allele occurs within the gene pool
• Hardy-Weinberg principle 


  Used to calculate the frequency of alleles of a particular gene in a population.
• Hardy Weinberg equation assumptions

  1. No mutations
  2. No flow of alleles into or out of the population (emigration or immigration)
  3. No selection - alleles are equally as likely to be passed onto the next generation
  4. Large population
  5. Mating within a population is random
• H-W equation 1: allele frequency

  p + q = 1.0
• H-W equation 2: Genotype frequency

  P² + 2pq + q² = 1.0
• Variation


  The differences that exist between living organisms.
• Evolution due to natural selection depends upon

  1. Organisms producing more offspring than can be supported by available resources e.g. food, light and space.
  2. There is genetic variability within population of all species.
  3. A variety of phenotypes that selection operates against.
• Selection pressures


  Environmental factors that alter the frequency of alleles in a population
• Examples of selection pressures

  1. Predation
  2. Competition
  3. Disease
  4. Supply of food
  5. Nesting sites
  6. Human influence
• Selection

  • Process by which organisms that are better adapted to their environment survive and breed.
• Stabilising selection

  • Stabilising selection is natural selection that keeps allele frequencies relatively constant over generations
  • This means things stay as they are unless there is a change in the environment
  • A classic example of stabilising selection can be seen in human birth weights
  • Very-low and very-high birth weights are selected against leading to the maintenance of the intermediate birth weights

  
• Directional selection

  • Directional selection is natural selection that produces a gradual change in allele frequencies over several generations
  • This usually happens when there is a change in environment / selection pressures or a new allele has appeared in the population that is advantageous.
  • For example: A recent finding has shown that climate change is having an effect on fish size in certain habitats
  • The increase in temperature is selecting for a smaller body size and against a larger body size

  
• Disruptive selection

  • Disruptive selection is natural selection that maintains high frequencies of two different sets of alleles
  • individuals with intermediate phenotypes or alleles are selected against, extreme phenotypes are favoured.
  • Disruptive selection causes polymorphism: the continued existence of two or more distinct phenotypes in species
  • For example birds on the galapagos with different beak sizes as more seeds to eat were available for those birds with either short of long beaks.

  
• Allele Frequency is affected by selection (Stabilising, Disruptive and Directional)
• Speciation


  The evolutionary process leading to the formation of a new species

  • Populations first need to be reproductively separated from each other so gene pools can diverge.
  • Isolated populations develop different characteristics and eventually will no longer be able to breed with each other rather than being reproductively isolated.
• Adaptive radiation


  Adaptive radiation is the evolutionary process by which many species originate from one species in an area and radiate to different species
• Genetic drift

  Genetic drift describes change in allele frequencies in the gene pool of a population (evolution) due purely to chance events and not selection pressures.
• Main stages of speciation

  1. Large interbreeding population of one species exists - sharing the same gene pool

  2. Barrier (isolating mechanism) separates the population into 2 separate breeding groups - reproductive separation

  3. Random mutations in each subpopulation gives rise to variation

  4. Different selection pressures act on each subpopulation depending on local conditions. Natural selection favours individuals that better exploit their environment

  5. Over a very long time, stages 3+4 causes the two gene pools to become so altered the groups become genetically distinct and reproductively isolated

  6. If the original barrier is removed the two subpopulations are no longer able to breed (as their chromosomes are no longer able to form homologous pairs) and 2 separate distinct species are formed
• 2 types of speciation

  1. Allopatric speciation - geographical isolation that separates a population. e.g. water creating a barrier to cross.
  2. Sympatric speciation - reproductive isolation preventing individuals breeding with each other. e.g. behavioural or hybrid sterility.