It is not a difficult concept, but very few people -- the majority of biologists included -- have a satisfactory grasp of it.

The Pleistocene time is one event, marked by the appearance of the mammoth and mastodon, wild horses, woolly rhinoceros and saber-tooth cats; also by the Ice Age. Relative dating contrasts human evolution with these events.

Neanderthal humans had died out by the end of the Pleistocene, while modern man (Homo Sapien) dominated.

The English moth, Biston betularia, is a frequently cited example of observed evolution. The frequency of the dark morph increased in the years following.

[evolution: a change in the gene pool] In this moth there are two color morphs, light and dark. By 1898, the 95% of the moths in Manchester and other highly industrialized areas were of the dark type. The moth population changed from mostly light colored moths to mostly dark colored moths.

If you know when the top or bottom layers were formed - you know just about when was the middle one formed.

This is "relative dating", but it doesn't give you an exact age. Relative dispersion, sometimes called the coefficient of variation, is the result of dividing the st. by the mean, hence it is dimensionless (it may also be presented as a percentage).

The moths' color was primarily determined by a single gene.

[gene: a hereditary unit] So, the change in frequency of dark colored moths represented a change in the gene pool.

Relative fossil dating of ancient species relies upon stratigraphy.