Variation with a Species (Edexcel IGCSE Biology): Revision Note

Types of Variation

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

• Variation can be caused by:

  • differences in genes brought about by random fertilisation of gametes

  • environmental factors

  • a combination of both

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

  • Discontinuous variation is controlled by a single gene (monogenic) and is not polygenic

  • Examples: blood group, ability to roll tongue, PTC tasting

• Continuous variation is caused by a combination of genetic and environmental factors, with the genetic component often being polygenic (controlled by several genes), for example height:

  • tall parents will pass on 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 environment affects how these inherited characteristics develop