Variation

Variation:  the presence of differences between two individuals. 



Intraspecific variation: variation between individuals of the same species. 
This is due to genetic and environmental variation. 
eg eye colour in humans.

Interspecific variation: Variation between different species. This variation is used to determine if two organisms are a different species. 

Click on the image below to look at a high definition version of this picture.

Continuous and discontinuous variation. 

Continuous variation: there are two extremes and a full range of intermediate values between those two extremes. The highest number of individuals are clustered close to the mean value. 

eg Height

Continuous variation is often influenced by more than one gene but can also be influenced by the environment.  

eg: skin colour in humans

Examples of continuous variation

• Height in humans.
• Length of leaves.
• Number of flagella on bacterium. 

Use a histogram (bars touching) to plot this data. 

Discontinuous variation:There two or more distinct categories with no intermediate values. 

There is usually only one gene that influences the phenotype and it is not influenced by the environment. 

eg: 

We plot discontinuous data on a bar chart - the bars do not touch. 

Causes of variation

Genetic: The genes we inherit from our parents - the combination of different alleles. 

With the exception of identical twins or clones, no two members of a species have exactly the same set of alleles. Of course species have the same genes, but many genes have different alleles and the probability that two individuals have the same alleles for each gene is extremely low. 

Environmental: Variation caused by a response to environmental factors such as light intensity. 

eg a persons skin can become lighter or darker if exposed to sunlight.  

eg: A tree can grow to a certain height but if animals nibble it, the leaves lost will not be able to contribute to growth via photosynthesis so the plant will not grow as tall as it could. 

eg Cloned crops can grow taller than other identical crops if they are given more fertiliser. 

Environment and genetic influences are usually combined to affect an organism. 

• Humans height is controlled by many genes, but as better food has become available, the mean of height has increased. 

• Genes can be turned on and off by hormones that are in the environment of the cell. 

• Chemicals or physical changes in the environment can also influence genes that are turned on and off. 

Himalayan Rabbit