• 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
• 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

Graph showing 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?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

• 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
  • 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

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 plants height
• The dominant alleles?H?and?T?contribute 2x?cm to the plants 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