Molecular genetics is a rapidly changing field with new developments almost from day to day. One interesting hypothesis that has come from everyone's ability to sequence proteins and/or genes is that of the molecular clock. This hypothesis postulates that homologous sequences of DNA and thus macro molecules evolve at a constant and invariable rate across all taxa. If true, this would enable divergence times and phylogenetic relationships to be more accurately determined. However documented variable rates of mutation, and thus evolution, have led to the question of the validity of this theory. Explanations of this variation, such as difference in generation times or metabolic rate failed to completely explain this variation. Therefore the validity of a molecular clock except in closely related species still remains controversial. (Contains 1 figure.)