What is the difference between natural variation and natural selection?
Variation is the raw material on which evolutionary force of natural selection can act.
Variations are mainly manifestations of small genetic differences among organisms of same species. Genetic variations add diversity to life.
Variations arise randomly due to mutation of genes. Various forms of genes are called alleles which arise due to mutation and these are inheritable. Generally variations neither help nor harm but that is not true for all variations.
In a population some variations help an organism to survive longer and produce more progeny. Such variations are adaptive variations and organisms with adaptive variations would eventually become more numerous in the population due to differential survival. This is because nature 'selects' favourable variations and allow organisms with favourable variations to survive better through generations.
Idea of natural selection of variations came from British naturalist Charles Darwin.