Organisms that reproduce sexually do not experience genetic changes. All changes occur in offspring during reproduction
That's not strictly true but you're right in that procreation generates the most obvious changes in genes. Gene mutations can be either inherited from a parent or acquired. A hereditary mutation is a mistake that is present in the DNA of virtually all body cells. Hereditary mutations are called germline mutations because the gene change exists in the reproductive cells and can be passed from generation to generation, and the mutation is copied every time body cells divide.
Acquired mutations called somatic mutations, are changes in DNA that develop throughout a person's life. In contrast to hereditary mutations, somatic mutations arise in the DNA of individual cells; the genetic errors are passed only to direct descendants of those cells. Mutations are often the result of errors that crop up during cell division, when the cell is making a copy of itself and dividing into two and cells divide a lot. Acquired mutations can also be the byproducts of environmental stresses. Also different genes do different things, as you know, so some mutations will be neutral if they are to an unimportant gene and others will be dramatic especially if they are to a hox gene.
You've brought up some good points Mavie. Mutation rates and which genes are important to natural selection and hox genes are hot topics at the moment in biology.
A Dawkins book I would recommend is The Ancestors Tale. It's takes a journey from present day man all the way back to the simplest life forms explaining the evolutionary changes along the way.