Types of variation

Genetic vs environmental

Causes of variation

• Genetic variation

  • caused by differences in the genetic material inherited from the parents

• environmental variation

  • caused by the environment in which the organism lives

Genetic causes of variation

• different species have different genes

  • leads to interspecific variation

• individuals within a population have different combinations of alleles

  • eg eye colour, height, flower colour, wing span

  • leads to intraspecific variation

• Mutation during DNA replication

  • leads to the development of different alleles

• Meiosis

  • chromosomes from parents are divided randomly between gametes causing variation in any offspring due to the versions of the genes they inherit

    • independent assortment and crossing over

• random fertilisation and chance

  • meeting of gametes during sexual reproduction

Environmental causes of variation

• Caused by differences in the environment

  • climate

  • rainfall

  • soil pH

  • light intensity

• plants are usually more affected than animals

Representing variation graphically

• Discontinuous variation

  • variation in which each organism belongs to one of a few clearly defined groups with no intermediate values

    • discreet groups or phenotypes

  • variation caused by genes with little or no environmental influence is discontinuous

    • eg blood group in man is controlled by one gene which has 3 alleles (A, B and O)

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• continuous variation

  • variation in which there is a continuous range of values between two extremes

    • there is a full range of intermediate phenotypes between the extremes

  • may be caused by genes or the environment or both

    • eg height

  • not controlled by just one gene but many

    • polygenic

  • data often collected in a frequency table

  • when a frequency graph is plotted a normal distribution is the expected result

    • mean, mode and median are the same

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