There are any number of essays that work to explain the evolutionary process. An exceptionally well done essay recently appeared on Richard Dawkins’ website by retired evolutionary biologist, Carl Bajema. This essay cogently describes the relationship of the random or undirected processes of genetic variation and the environmentally directed processes of sexual and natural selection. Check out this essay next time you are preparing your evolution lessons. It will really help you zero in on the the important points. From the essay:
The modern version of Darwinian selection theory states that the adaptive evolution is a two step process that occurs every generation—(1) the production of blind genetic variation and (2) the selective multiplication and recombination of genes by the ecologically directed processes of natural and sexual selection. That is, Natural/sexual selection is the set of environmentally directed adaptive processes that are superimposed on the undirected processes that cause genetic mutations. Natural/Sexual selection selectively multiplies adaptive genes and selectively recombines them each generation.
You’ll find the entire essay at Jon Wilkens’ Evolving Thoughts Blog.