How does variation lead to evolution?
Variation provides natural selection with choices.
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.
An example of natural selection selecting variations best suited to the environment, is the peppered moths of England. There were two variations of the moth, one dark or melanic and the other white. Before the industrial revolution the white variation was dominate. During the industrial revolution the pollution favored the dark or melanic variation. The species evolved or changed to being primarily dark. When the pollution problems of the industrial revolution was solved the environment favored the white variation and the species evolved and changed back to the white variation.
Variations are necessary for changes, or evolution to occur. The amount of change is limited to the variations that exist. When Darwin proposed his theory of evolution he thought wrongly that variation in germ cells was unlimited. Gregor Medal discovered that genetic variation is generally conservative, that is that new variations are not created in normal genetics.
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.