Continuous & Discontinuous Variation (OCR A Level Biology)

• The term variation refers to the differences that exist between at least two things (be it a level, amount, quantity or feature of something)
• In relation to natural selection, variation refers to the differences that exist between individuals of a species
  • This may also be referred to as intraspecific variation

• The variation observed in the phenotypes of organisms can be due to quantitative or qualitative differences
• For this reason, variation is categorised into two groups:
  • Continuous variation
  • Discontinuous variation

• Continuous variation is the differences between individuals of a species where the differences are quantitative (measurable)
• Discontinuous variation refers to the differences between individuals of a species where the differences are qualitative (categoric)
• Each type of variation can be explained by genetic and/or environmental factors

Continuous variation

• Continuous variation occurs when there are quantitative differences in the phenotypes of individuals within a population for particular characteristics
• Quantitative differences do not fall into discrete categories like in discontinuous variation
• Instead for these features, a range of values exist between two extremes within which the phenotype will fall
  • For example, the mass or height of a human is an example of continuous variation

• The lack of categories and the presence of a range of values can be used to identify continuous variation when it is presented in a table or graph

Graph showing population variation in height: an example of continuous variation with quantitative differences

Genetic basis 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

Discontinuous variation

• Qualitative differences in the phenotypes of individuals within a population give rise to discontinuous variation
• Qualitative differences fall into discrete and distinguishable categories, usually with no intermediates (a feature can’t fall in between categories)
  • For example, there are four possible ABO blood groups in humans; a person can only have one of them

• It is easy to identify discontinuous variation when it is present in a table or graph due to the distinct categories that exist when data is plotted for particular characteristics

Graph showing population variation in blood types: an example of discontinuous variation with qualitative differences

Genetic basis 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

Comparison of Continuous and Discontinuous Variation Table