A pseudogene is a sequence of bases in the DNA that clearly resembles the sequence of a known gene, but differs from it in some crucial respect and has no function. For example some pseudogenes cannot be expressed because they lack the promotors and introns needed for transcription.
It was originally thought that pseudogenes evolve faster than any other DNA and that their evolution was completely unconstrained: they would show pure neutral evolution in the pan neutralist sense. In this case, their rate of evolution would be a direct estimate of the total mutation rate. However, the latest evidence suggests that pseudogenes evolve at the same rate as silent base changes, and it is argued that evolution is probably to some extent constrained even in pseudogenes.
The neutral theory correctly predicts that pseudogenes should evolve rapidly and it is thought that they evolve roughly five times faster than functional genetic material. Most selectionists concede that pseudogenes are an exception to natural selection and it is widely accepted that the rapid evolution of pseudogenes is almost entirely due to genetic drift.