Types of Variation (OCR A Level Biology): Revision Note

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 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 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