CIE A Level Biology复习笔记17.1.2 Variation: Discontinuous & Continuous

• 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