Types of Variation
- The term variation refers to the differences between living organisms
- Variation can be:
- between different species or within a single species
- continuous or discontinuous
- caused be genetic and/or environmental factors
Interspecific vs intraspecific variation
Interspecific variation
- Interspecific variation is that which exists between individuals of different species
- Interspecific variation can be useful for classifying organisms into species groups
- Different species may show clear phenotypic variation that can help differentiate them
- Some species have such similar phenotypes that they can be very difficult to distinguish, meaning that genetic variation must be used for classification
Intraspecific variation
- Intraspecific variation is that which exists between individuals of the same species
- These differences are smaller than those found between individuals of different species
- Variation within a species allows natural selection to occur
Discontinuous vs continuous variation
Discontinuous variation
- Discontinuous variation refers to differences that fall into discrete and distinguishable categories with no intermediates
- E.g. there are four possible ABO blood groups in humans; a person can only have one of them
- Discontinuous variation can be represented using a bar chart with bars that are clearly distinct from each other
Blood type is an example of discontinuous varation
Continuous variation
- Continuous variation refers to differences that show a range of values and can fall anywhere between two extremes
- E.g. body mass and height are measured on a continuous scale
- Continuous variation can be represented on a histogram with bars that touch each other, and will often show a characteristic bell-shaped curve
Height is an example of continuous variation
Causes of variation
- Variation can be caused 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
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