Variation (Cambridge O Level Biology)

• Variation is defined as differences between individuals of the same species
• Phenotypic variation is the difference in features between individuals of the same species

• 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
• Examples of genetic variation in humans include:
  • Blood group
  • Eye colour
  • Ability to roll tongue
  • Whether ear lobes are free or fixed

Earlobe variation diagram

Whether earlobes are attached (lobeless) or free (lobed) is an example of genetic 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
• Phenotypic variation can be divided into two types depending on how you are able to group the measurements:
  • Continuous variation
  • Discontinuous variation

Continuous Variation

• 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 intermediate groups
• 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
• When graphs of this data is plotted, continuous variation gives smooth bell curves (a result of all the small degrees of difference)

Continuous variation bell-shaped curve

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

Discontinuous Variation

• 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 intermediary values
• Seed shape and seed colour in peas are also examples of discontinuous variation
• Discontinuous variation is usually caused by genetic variation alone
• When graphs of these data are plotted it is presented on a bar chart

Bar chart of discontinuous variation

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