Genetic variation - variation due to the genes and alleles an individual possesses

Environmental variation - variation caused by the environment in which an organism lives

1. Mutations - changes to genes and chromosomes that may be passed on to the next generation
2. Meiosis - new combinations of alleles are present in the gametes formed, produced by independent assortment of chromosomes and crossing over between chromatids 
3. Random fertilisation - in sexual reproduction, random fertilisation of gametes produces new combinations of alleles in the zygote
4. Random mating

1. Light
2. Nutrient and food availability
3. Temperature
4. Rainfall
5. Soil conditions
6. pH

Environmental influences can also affect the way an organism's genes are expressed.

It is hard to distinguish between the effects of genetic and environmental influences on variation as many genetic and environmental influences combine to produce differences between individual.

Polygenes are different genes at different loci that all contribute to a particular aspect of phenotype.

Their individual effects on a phenotype are too small to observe, but they can act together to produce observable variation.

This combined effect of multiple genes is common in continuous variation.