Variation (DP IB Biology: HL)

• The ways in which organisms differ from one another is called variation
• Variation occurs between species
  • In fact, species are classified based on differences between their respective members
  • This is called interspecific variation
• Variation can occur within the same species
  • Between different individuals or groups of individuals
  • This is called intraspecific variation
  • This suggests that only one gene is involved in governing discrete variation
  • This is called monogenic inheritance

Variation can be discrete or continuous

• Discrete variation is an example of intraspecific variation
  • Individuals fall into two or more clear-cut categories with no overlap or in-between categories
  • Blood group is an example of discrete variation
    • All human blood is either group O, A, B or AB, each with a Rhesus factor (+ or -)
• This gives just 8 distinct blood groups

Worldwide A, B, O blood group distribution by percentage, 2019

(data varies regionally with ethnicity)

• The petal colour of snapdragons is a discrete variable; either red, white or pink with no in-between colours
• Discrete variation is sometimes referred to as discontinuous variation, in contrast to continuous variation

Snapdragons display 3 main petal colours: red, white and pink, determined by a single pair of codominant alleles 

This is an example of discrete variation that is solely due to genetic factors

Causes of discrete 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

• Continuous variation occurs when two or more genes affect the final characteristic
• For example, height in humans is determined by many genetic factors:
  • Bone length
  • Skeletal muscle structure
  • Ability to absorb food substances effectively
  • Hormone production
  • …As well as environmental factors like diet, exercise, prenatal nutrition, lifestyle etc
• Most characteristics are determined by more than one gene - a polygenic characteristic
• Even grouped data like shoe size appears to be discrete but in fact, peoples' feet vary continuously in size
  • Shoe size is merely a practicality for shoe manufacturers, who cannot make exactly the right-sized shoes for everybody
• Continuous variation in birth mass results in the population displaying a normal distribution (bell-shaped curve)
  • Of course, environmental factors can affect birth mass, eg. mother's diet, presence of a twin, smoking etc
• 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
• • For example, the mass or height of a human is an example of continuous variation
  • Instead for these features, a range of values exist between two extremes within which the phenotype will fall
• 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

Comparison of Continuous and Discontinuous Variation Table

Polygenic traits such as human height may also be influenced by environmental factors

• Many environmental factors can affect the intraspecific variation displayed by an organism, including
  • Diet
  • Lifestyle
  • Exercise
  • Exposure to sunlight eg. tanned skin
  • Availability of soil minerals in plants
  • Human intervention eg. pruning plants, neutering animals
  • Fashion, individual preference
  • Native language and dialect (based on where an individual is brought up)
• These traits and differences have been observed in identical twins who were unfortunate enough to have been separated at birth 
  • Not a practice condoned in the 21st century, but was once considered a valid investigative method
• Individuals displayed distinct phenotypic differences based on their diet and lifestyle differences