Bill, no problem.

The speed of evolution is measured by the rate new genetic traits are accumulated. Ignoring single-celled organisms (quite chauvinistic, as they account for the vast majority of life on earth, but lets keep things simple for now), the only route we have to gain new genetic material as a species is mutation. Note that mutation would include "real" mutations (changing the 'letters' of a DNA sequence) as well as other mutagenic processes (duplications, insertions, etc).

Ergo, the fastest we can evolve as a species (i.e. acquire new genetic material) is equal to our mutation rate.

Selection slows this process - by selecting for or against a trait, you reduce the amount of genetic diversity in a species - either by removing an allele (negative selection) or by promoting the spread of an allele at the expense of others.

This gets a little more complex with single-celled organisms, as many of them are able to acquire genes from other species - one of the better examples of this is the spread of antibiotic resistance genes between unrelated species of bacteria. This horizontal gene transfer represents a very fast form of evolution, and is a mechanism that exists outside of conventional mutation. But again, selection limits even this source of new genetic material.