HomeCells & MoleculesDiseasesEcosystemHuman BiologyAQA BIOL1AQA HBIO1AQA HBIO2AQA HBIO4AQA HBIO5
HBIO5 > Evolution
Welcome, Guest!
Login with Facebook | Login


  • Variation is the presence of any differences (genotypic and phenotypic), between individuals in a population, or between parents and offspring.
  • Genotypic variation can be due to:
    • Mutation
    • Random assortment during Meiosis,
    • Crossing over during Meiosis,
    • Random fertilisation of gametes.
  • Phenotypic variation can be due to:
    • Genetic Factors,
    • Environmental Factors,
    • Combination of both.
  • Variation provides organisms with a means of producing new phenotypes.
  • Variations to the structure or behaviour of an organism may confer a selective advantage to survive and reproduce


  • Evolution is the sum of changes of heritable characteristics in population.
  • It is the development of life in geological time
  • Natural selection is the process where organisms that are better adapted to their environment survive and breed.
  • Those that fail to adapt will not survive. 
  • The environment therefore exerts a selection pressure on the population.
  • Allele frequency is the commonness of the occurrence of any particular allele in a population.
  • Evolution changes allele frequency in population because:
    • Organisms with a selective advantage:
      • more likely to survive  →  reproduce  →  pass on genes to next generation.
      • This will increase the frequency of the allele that corresponds to the adaptation.
    • Organisms with a selective dis-advantage:
      • less likely to survive  →  die  → fail to pass on genes to next generation.
    • This will decrease the frequency of alleles possessed by these individuals.
  • A population is a group of individuals of a species, living close together, and able to interbreed.
  • Populations reproducing in isolation can result in the formation of new species
  • This is known as Speciation.

Allopatric Speciation:

  • Geographically separated sister species reproduce in isolation.
  • Natural and man-made barriers may arise rapidly.
  • For example: a population may be divided if a river breaks it's banks and takes a new route, or a new road may separate a population.
  • Many generations may elapse.
  • Sister species are now so different - no interbreeding is possible.

Sympatric Speciation:

  • Genetic variation occurring in same geographical location.
  • Reproductive isolation due to:
    • Temporal mechanisms
    • Behavioural mechanisms
    • Polyploidy
  • Temporal (time) mechanisms may occur when two very closely related species occupy the same habitat, differing only in the time of year that they complete their life cycles.
  • Reproductive isolation may develop within the population so that some members produce gametes at distinctly different times of the year from others.
  • Thus, two distinctive gene pools start to evolve.
  • Behavioural mechanisms results when members of a population acquire distinctive behaviour routines in their growth and development, courtship or mating process that are not matched by all individuals of the same species.
  • For example: imprinting behaviour of birds.
  • Polyploidy is a change in structure or number of chromosomes.
  • Applicable mostly to Plants.
  • This chromosomal mutation may instantly give rise to a new species.
  • Human activities have, and continue, to alter the environment of many organisms.
    • Changes selection pressures
    • May effect evolution of species.
  • Artificial Selection is an intentional selection process by humans,
  • selecting certain organisms based on their phenotype, specifically for breeding purposes,
  • In the hope offspring inherit the desired characteristics. (NOT a guarantee).
  • Usually carried out by animal and plant breeders during Selective Breeding.
  • Selective Breeding is use of artificial selection to modify and improve quality/yield
  • In plants and animals.
  • Eg. cross-fertilisation of maize variates to produce plants that:
    • mature faster,
    • larger fruit size,
    • higher yield,
    • resistant to disease.
  • Eg. All modern race horses have been artificially selected.
    • Horses are selected for breeding on basis of success in races.
    • Gene pool is constantly improved.
  • Humans are the selective agents.