Evolution may lead to speciation

• Variations between individuals


  • Genetic and environmental factors
• Genetic factors which result in variation

  • Primary genetic factor is mutation
  • Meiosis
  • Random fertilisation
• Environmental factors which result in variation

  • Climate
  • Lifestyle
  • Diet
• Evolution


  A change in the allele frequencies in population over a large period of time/ many generations
• Natural selection


  • One way in which evolution can occur
  • Those organisms with phenotypes providing selective advantage are likely to produce more offspring and pass on their favourable alleles to the next generation
• Model answer for natural selection

  1. Variation in alleles already increase within a population - due to random mutation

  2. Individuals with alleles which provide them with selective advantages - physical or behavioural changes

  3. More likely to survive and reproduce compared to other individuals

  4. More likely to pass on favourable/ advantageous alleles to offspring

  5. Over time/ many generation frequency of allele/ characteristics will increase in population - evolution
• Differential reproductive success


  Increases the frequency of more favourable alleles within a population
• Differential reproductive success occurs because not all individuals are likely to reproduce resulting in this change in allele frequancy within a populations gene pool
• What cause natural selection to occur?

  • Selection pressures - anything which may affect an organaisms chance of survival and reproduction
  • E.g predation, disease and competition
• Stabilising selection


  • When the middle - median - trait has the selective advantage
  • So individuals with alleles towards the middle range are more likely to survive and reproduce
  • Selection occurs when there is no change in the environment
  • So the modal trait remains the same and is present in most individuals in the population
• Effect of stabilising selection

  • Reduces the range of possible phenotypes/ alleles within a population
  • Extreme traits are lost over time - reduces the possible range of phenotypes
• Directional selection


  • Where individuals with alleles for a single extreme allele are selected for - have the selective advantage
  • So individuals with alleles an extreme phenotypes are more likely to survive and reproduce
  • Selection occurs when there is a change in the environment
• Effect of directional selection


  • Allele frequency changes and more extreme trait alleles become more frequent
• Disruptive selection


  • Where individuals with alleles for extreme phenotypes at either end are selected for
  • Opposite to stabilising selection as middle trait is lost/ becomes less frequent
• Continued disruptive selection can lead to speciation
• Speciation


  The development of a new species from an existing species
• How does speciation occur?

  1. When populations of the same species become reproductively isolated/ separated

  2. This change in allele frequency causes a change in phenotype

  3. Results in the accumulation of differences in populations gene pools

  4. They can no longer interbreed to produce fertile offspring
• How might reproductive isolation/ separation occur between two populations?

  • Through a geographic isolation in the form of a physical barrier - allopatric speciation
  • Or without a physical barrier - sympatric speciation
• New species are able to arise from existing species when genetic difference lead to an inability of members of populations to interbreed and produce fertile offspring's
• Allopatric speciation


  • When populations are geographically separated
  • Leading to reproductive isolation
  • And therefore the development of a new species
• Examples of geographical barriers

  • Formation of new mountain ranges
  • New bodies of water separating land masses
• How does allopatric speciation lead to reproductive isolation?

  1. Both population experience slightly different selection pressures due to different conditions

  2. This results in different changes to allele frequencies as some alleles will be more advantageous in different populations

  3. May also be mutations that occur spontaneously and independently within each population

  4. Populations accumulate different gene pools - different phenotype frequencies

  5. Overtime, large differences between DNA of two populations

  6. Can no longer breed to produce fertile offspring's
• Sympatric speciation


  • Populations can become reproductively isolated due to difference in behaviours
  • Individuals do not need to be geographically isolated but are still unable to reproduce
• How might sympatric speciation occur?

  1. Random mutations occur within the population - affect reproductive behaviour

  2. Those individuals will not reproduce together

  3. No gene flow between the two groups still within the same population

  4. Over time, these reproductively isolated populations will accumulate different mutations

  5. DNA so different they can no longer interbreed to produce fertile offsprings

  6. Now two different species
• Examples of mutations which might change reproductive behaviours in individuals in the same population

  • Mutations which change courtship behaviours/ rituals - no longer attractive to majority of the population - behavioural mutations
  • Mutations which change the genitalia of certain individuals preventing successful mating - mechanical mutations
  • Mutations which result in individuals within the same population developing different flowering or mating seasons or becoming successfully inactive at different times of the year - seasonal mutations
• Genetic drift


  The change in allele frequency within a population between generations
• Genetic drift - example

  1. Variation shown in genotype (e.g. allele A & B)

  2. By chance, one allele B is passes on more often than allele A

  3. The number of individuals with allele B in the population increases

  4. Changes in allele frequency in 2 isolated populations could lead to reproductive isolation and speciation
• Genetic drift is only important in small populations because chance has a greater influence - smaller gene pools
• In larger populations any change variation will tend to even out across whole population