The molecular clock

The molecular clock uses evolutionary changes in proteins and DNA to measure the passage of time. Yet molecular evolution is clocklike only to a first approximation. Uncertainties arise because of variation in rates of molecular evolution, because of difficulty in calibrating clocks, and because we measure molecular changes only indirectly. Statistical methods now cope with all of these uncertainties. As these methods have matured and molecular data sets have increased in size, the molecular clock has grown increasingly reliable.