CIE IGCSE Biology

Revision Notes

18.1 Variation

Types of Variation

  • Variation is defined as differences between individuals of the same species
  • Phenotypic variation is the difference in features between individuals of the same species
  • Some of these differences are caused by differences in genes, which is genetic variation
  • Phenotypic variation can be divided into two types depending on how you are able to group the measurements:
    • Continuous Variation is when there are very many small degrees of difference for a particular characteristic between individuals and they are arranged in order and can usually be measured on a scale
    • Examples include height, mass, finger length etc. where there can be many ‘inbetween’ groups
    • Discontinuous Variation is when there are distinct differences for a characteristic
    • For example, people are either blood group A, B, AB or O; are either male or female; can either roll their tongue or not – there are no ‘inbetweens’
  • When graphs of these data are plotted, continuous variation gives smooth bell curves (a result of all the small degrees of difference), whereas discontinuous gives a ‘step – like’ shape

 

Continuous variation, IGCSE & GCSE Biology revision notesHeight is an example of continuous variation which gives rise to a smooth bell-shaped curve when plotted as a graph

 

Discontinuous variation, IGCSE & GCSE Biology revision notesBlood group is an example of discontinuous variation which gives rise to a step-shaped graph

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Phenotypic Variation

  • Phenotypic variation can be caused in two main ways:
    • It can be genetic – controlled entirely by genes
    • Or it can be environmental – caused entirely by the environment in which the organism lives

 

Genetic Variation

  • Examples of genetic variation in humans include:
    • blood group
    • eye colour
    • gender
    • ability to roll tongue whether ear lobes are free or fixed

Earlobes, IGCSE & GCSE Biology revision notesWhether earlobes are attached (lobeless) or free (lobed) is an example of genetic variation

 

Environmental Variation

  • Characteristics of all species can be affected by environmental factors such as climate, diet, accidents, culture and lifestyle
  • In this instance ‘environmental’ simply means ‘outside of the organism’ and so can include factors like climate, diet, culture, lifestyle and accidents during lifetime
  • Examples 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
    • A plant in the shade of a big tree will grow taller to reach more light

Genetic and Environmental Causes

  • Discontinuous variation is usually caused by genetic variation alone
  • Continuous features often vary because of a combination of genetic and environmental causes, for example:
    • tall parents will pass genes to their children for height
    • their children have the genetic potential to also be tall
    • however if their diet is poor then they will not grow very well
    • therefore their environment also has an impact on their height
  • Another way of looking at this is that although genes decide what characteristics we inherit, the surrounding environment will affect how these inherited characteristics develop

Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.
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