Variation (Edexcel GCSE Biology)

Types of Variation

• Variation is defined as differences between individuals of the same species

• 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 ‘in-between 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 ‘in-betweens'

• 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

Height is an example of continuous variation which gives rise to a smooth bell-shaped curve when plotted as a graph

Blood group is an example of discontinuous variation which gives rise to a step-shaped graph

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

• Meiosis creates genetic variation between the gametes produced by an individual

• This means each gamete carries substantially different alleles

• During fertilization, any male gamete can fuse with any female gamete to form a zygote

• This random fusion of gametes at fertilization creates genetic variation between zygotes as each will have a unique combination of alleles

• Zygotes eventually grow and develop into adults

• Examples of genetic variation in humans include:

  • Blood group

  • Eye colour

  • Gender

  • Ability to roll tongue

  • Whether ear lobes are free or fixed

Whether 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