Types of Variation (OCR A Level Biology)

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Types of Variation

  • The term variation refers to the differences between living organisms
  • Variation can be:
    • between different species or within a single species
    • continuous or discontinuous
    • caused be genetic and/or environmental factors

Interspecific vs intraspecific variation

Interspecific variation

  • Interspecific variation is that which exists between individuals of different species
  • Interspecific variation can be useful for classifying organisms into species groups
    • Different species may show clear phenotypic variation that can help differentiate them
    • Some species have such similar phenotypes that they can be very difficult to distinguish, meaning that genetic variation must be used for classification

Intraspecific variation

  • Intraspecific variation is that which exists between individuals of the same species
    • These differences are smaller than those found between individuals of different species
  • Variation within a species allows natural selection to occur

Discontinuous vs continuous variation

Discontinuous variation

  • Discontinuous variation refers to differences that fall into discrete and distinguishable categories with no intermediates
    • E.g. there are four possible ABO blood groups in humans; a person can only have one of them
  • Discontinuous variation can be represented using a bar chart with bars that are clearly distinct from each other

Blood type graph, downloadable AS & A Level Biology revision notes

Blood type is an example of discontinuous varation

Continuous variation

  • Continuous variation refers to differences that show a range of values and can fall anywhere between two extremes
    • E.g. body mass and height are measured on a continuous scale
  • Continuous variation can be represented on a histogram with bars that touch each other, and will often show a characteristic bell-shaped curve

Height graph, downloadable AS & A Level Biology revision notes

Height is an example of continuous variation

Causes of variation

  • Variation can be caused by genetic factors, environmental factors or a combination of the two

Causes of discontinuous variation

  • This type of variation occurs solely due to genetic factors
  • The environment has no direct effect
    • Phenotype = genotype
  • At the genetic level:
    • Different genes have different effects on the phenotype
    • Different alleles at a single gene locus have a large effect on the phenotype
    • Remember diploid organisms will inherit two alleles of each gene, these alleles can be the same or different
  • A good example of this is the F8 gene that codes for the blood-clotting protein Factor VIII
    • The different alleles at the F8 gene locus dictate whether or not normal Factor VIII is produced and whether the individual has the condition haemophilia

Causes of continuous variation

  • This type of variation is caused by an interaction between genetics and the environment
  • Phenotype = genotype + environment
  • At the genetic level:
    • Different alleles at a single locus have a small effect on the phenotype
    • Different genes can have the same effect on the phenotype and these add together to have an additive effect
    • If a large number of genes have a combined effect on the phenotype they are known as polygenes

Environmental factors

  • In some cases, phenotypic variation is explained by environmental factors alone
    • For example, clones of plants with exactly the same genetic information (DNA) will grow to different heights when grown in different environmental conditions
  • Different environments around the globe experience very different conditions in terms of the:
    • Length of sunlight hours (which may be seasonal)
    • Supply of nutrients (food)
    • Availability of water
    • Temperature range
    • Oxygen levels
  • Changes in the factors above can affect how organisms grow and develop
    • For example, plants with a tall genotype growing in an environment that is depleted in minerals, sunlight and water will not be able to grow to their full potential size determined by genetics
  • Variation in phenotype caused solely by environmental pressures or factors cannot be inherited by an organism’s offspring
    • Only alterations to the genetic component of gametes will ever be inherited
  • Other examples of environmental variation include:
    • An accident may lead to scarring on the body
    • Eating too much and not leading an active lifestyle will cause weight gain
    • Being raised in a certain country will cause you to speak a certain language with a certain accent

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Alistair

Author: Alistair

Expertise: Biology & Environmental Systems and Societies

Alistair graduated from Oxford University with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems & Societies. Alistair has continued to pursue his interests in ecology and environmental science, recently gaining an MSc in Wildlife Biology & Conservation with Edinburgh Napier University.