The causes of variation and it's biological importance? How do my points to consider look so far?

Bottle neck and the founder effect are quite important points to consider however if to consider modern advances in genetics - as the human genome project - the concept of genetic polymorphisms might be a good point as well. The biological importance of variation is that for the organism to have wider range of adaptation towards its environment and moreover certain lethal or deleterious genes are expressed that should be recessive if there is less variation such as inbreeding.