Variation

• Variation in biology


  • The differences between members of different species - Interspecific variation is used to classify different species
  • The differences between members of the same species - Intraspecific variation, this variation is the basis of natural selection
• Intraspecific variation


  Any differences between individuals in a population or between parents and their offspring
• Factors influencing intraspecific variation(variation within a species)

  • Genetic factors
  • Environmental factors
• Two kinds of Intraspecific variation

  • Discontinuous variation
  • Continuous variation
• GENETIC CAUSES OF VARIATION

  • Due to differences in the alleles that organisms of the same species inherit from their parents
  • In sexually reproducing organisms, this genetic variation derives from meiosis and random fertilisation
  • Mutations involving errors in DNA replication, occurs in both sexually and asexually reproducing individuals
  • These were Darwin’s 3rd and 4th observations which lead to his theory of evolution by natural selection
• Environmental Causes of Variation

  • Due to the environmental effects on the expression of the genes during an individual’s lifetime
  • Not caused by differences in DNA and so cannot be INHERITED
  • E.g.
  • Availability of food – animals
  • Light, water, mineral ions in the soil - plants
  • Presence of antibiotics and pesticides in environment-microorganisms and animals
  • Intellectual stimulation and learning – human intelligence
  • Good nutrition in utero and after birth needed for development of brain and nervous system - mammals
• Discontinuous Variation: Individuals fall into distinct groups/categories, there are no intermediates, usually controlled by one or a few genes → MONOGENIC (expression of the phenotype is not usually affected by the environment) 
• Discontinuous Variation: Qualitative - differences between individuals cannot be measured - and is usually represented graphically using a bar chart; examples:

  • ABO blood group
  • Tongue-rolling ability
  • Resistance to pesticides
  • Resistance to antibiotics
• Continuous Variation: Individuals fall anywhere between TWO EXTREMES, there are many intermediates and a RANGE of values, controlled by two or more genes; some characteristics are controlled by MANY genes, called POLYGENES (so its POLYGENIC)
• Continuous Variation: Each gene provides an additive component to the phenotype, expression of the phenotype is STRONGLY affected by THE ENVIRONMENT
• Continuous Variation: QUANTITATIVE - differences between individuals can be measured, and usually represented graphically using a HISTOGRAM, with a large sample size a Normal Distribution Curve can be obtained; examples include: HEIGHT IN HUMANS, SKIN COLOUR, EYE COLOUR, and LEAF LENGTH
• Importance of Variation: Natural selection (Charles Darwin)

  • Involves environmental selection pressures selecting those individuals of a population that are best adapted to survival
  • These individuals will survive to breed and pass on their desirable adaptive features
  • Natural selection depends upon genetic variation within the population
• Importance of Variation: Artificial Selection/Selective Breeding

  • Has been carried out by humans since agriculture began
  • Humans have selected animals and plants with desired feature to breed from
  • Artificial Selection depends upon genetic variation within each population