4.3.7 Types of Variation

• The term variation refers to the differences that exist between two or more things
• Organisms show:
  • Genetic variation - the variation in genotypes (i.e. the genes) between species or within a species
  • Phenotypic variation - the variation in phenotypes (i.e. anything other features apart from the genes) between species or within a species e.g. variation in height, mass, shape, colour, blood type

• As described above, genetic and phenotypic variation can occur between different species or within a single species
  • Interspecific variation is the variation between species
  • Intraspecific variation is the variation within species

Interspecific variation

• Interspecific variation can be useful in identifying and classifying different species
• Even between species that look very similar to each other, there are usually forms of phenotypic variation that can help differentiate them
  • For example, Bactrian and dromedary camels are highly similar but show one very obvious variation in their phenotypes, as Bactrian camels have two humps whereas dromedaries have one
  • Other species that look very similar will have slightly different niches, occupying slightly different habitats or fulfilling slightly different roles if present within the same habitat. This can be used to help distinguish between them

• Some species have such similar phenotypes that they can be very difficult to distinguish. In these cases, there will always be some level of genetic variation between the species, so their genotypes can be used to help identify them

Intraspecific variation

• In relation to natural selection, variation refers to the differences that exist between individuals of the same species
  • This may also be referred to as intraspecific variation

• Variation observed in the phenotypes of individuals from the same species 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

Causes of variation

• Variation can be explained by genetic factors, environmental factors or a combination of the two

Causes 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

Whether earlobes are attached or free is an example of discontinuous variation that is solely due to genetic factors

Causes 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

Environmental factors

• In some cases, phenotypic variation is explained by environmental factors alone
  • For example, clones of plants with exactly the same genetic information (DNA) will grow to different heights when grown in different environmental conditions

• Different environments around the globe experience very different conditions in terms of the:
  • Length of sunlight hours (which may be seasonal)
  • Supply of nutrients (food)
  • Availability of water
  • Temperature range
  • Oxygen levels

• Changes in the factors above can affect how organisms grow and develop
  • For example, plants with a tall genotype growing in an environment that is depleted in minerals, sunlight and water will not be able to grow to their full potential size determined by genetics

• Variation in phenotype caused solely by environmental pressures or factors cannot be inherited by an organism’s offspring
  • Only alterations to the genetic component of gametes will ever be inherited

• Other examples of environmental variation include:
  • An accident may lead to scarring on the body
  • Eating too much and not leading an active lifestyle will cause weight gain
  • Being raised in a certain country will cause you to speak a certain language with a certain accent