Causes of Variation

Variation is the differences that exist bewtween individuals.

There are two types:

Interspecific

This is the variation that exists between different species. For example, horses vary from ducks, which vary from mice. 

Intraspecific

This is the differences that occur within a species. For example, the number of eyes on peacock feathers or the length of a giraffe's neck. 

Intraspecific variation is caused by genetic and environmental factors. 

Although individuals of the same species may appear similar, no two individuals are exactly alike. 

Variation can be caused by both genetic and environmental.

All the members of a species have the same genes. But individuals within a species can have different versions of those genes, which are called alleles.

The alleles of an organism make up its genotype. Different genotypes result in varitaion in the phenotype, the characteristics displayed by an organism.

Eaxmples of variation in humans caused by genetic factors include eye colour and blood type. You inherit your genes from your parents.

This means genetic variation is inherited. 

The appearance (phenotype) of an individual is also affected by the environment , for example:

• Plant growth is affected  by the amount of minerals such as nitrate and phosphate available in the soil. 
• Fur colour of the Himalayan rabbit is affected by temperature. Most of the rabbits' fur is white except the ears, feet and tail, which are black. The black colour only develops in temperatures below 25 degrees. If a patch of their white fur is shaved and a cold pad is applied to the shaved area, the hair that grows back will be black. 
• Identical twins are genetically identical, they have the same alleles, so any differences between them is due to the environment. For example, they may have had different illnesses that affected their development or, if they grew up in different areas they may have different accents. 

In any group of individuals there's a lot of variation. It's not always clear whether the variation is caused by genes, the environment or both. Scientists draw conclusions based on the information they have until new evidence comes along to change that.

For example: Overeating 

Overeating was thought to be caused only by environmental factors, like an increased availability of food in developed countries. 

It was later discovered that food consumption increase brain dopamine levels in animals.Once enough dopamine was released people would stop eating.

Researchers discovered that people ith one particular allele had 30% fewer dopamine receptors. 

They found that people with this particular allele were more likely to overeat, they wouldn't stop eating when dopamine levels increased.

Based on this evidence, scientists now think overeating has both genetics and environmental factors.