Variation: Discontinuous & Continuous (Cambridge (CIE) A Level Biology)

Variation: Discontinuous & Continuous

• The term variation refers to the differences that exist between at least two things (be it a level, presence, quantity or feature of something)

• Concerning 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 qualitative or quantitative differences

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

Discontinuous Variation Example

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

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 exists 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

Continuous Variation Example

Population variation in height: an example of continuous variation with quantitative differences

The Genetic Basis of Variation

• Discontinuous variation refers to the differences between individuals of a species where the differences are qualitative (categoric)

• Continuous variation is the differences between individuals of a species where the differences are quantitative (measurable)

• Each type of variation can be explained by genetic and/or environmental factors

Genetic basis of discontinuous variation

• This type of variation occurs solely due to genetic factors

• The environment has no direct effect

• 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

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

The additive effect of genes

• The height of a plant is controlled by two unlinked genes H / h and T / t

• The two genes have an additive effect

• The recessive alleles h and t contribute x cm to the plant's height

• The dominant alleles H and T contribute 2x cm to the plant's height

• The following genotypes will have the following phenotypes:

  • h h t t :  x + x + x + x = 4x cm

  • H H T T : 2x + 2x + 2x + 2x = 8x cm

  • H h T t : 2x + x + 2x + x = 6x cm

  • H H T t : 2x + 2x + 2x + x = 7x cm

  • H h T T : 2x + x + 2x + 2x = 7x cm

  • h h T t : x + x + 2x + x = 5x cm

  • H h t t : 2x + x + x + x = 5x cm