How does variation lead to natural selection?

Natural selection can not create variations it can only eliminate the existing variations that are not as well adapted to the changing environment as others. Natural selection acts like a racket. Natural selection preserves variations that are best suited to environment. Variations are known to create changes within a species. These changes are also known as adaptive evolution or micro evolution.

The variations required to create changes from one species to another species have never been empirically observed. These changes are known more correctly as Darwinian evolution or macro evolution.

Variations is the driving force in evolution or change. Natural selection can only act on existing variations. So for evolution of any kind to occur there must be variations.

a tetrad is the foursome during meiosis made by two homologous chromosomes that have each already replicated into a pair of sister chromatids.

b brown mice / white sand dunes