Where does clock collide?

In evolutionary biology, the molecular clock is the observation and method that molecular differences between species accumulate roughly steadily over time, so the number of substitutions between two lineages can estimate how long ago they diverged. Zuckerkandl and Pauling's 1965 Molecules as documents of evolutionary history argued that proteins and nucleic acids record evolutionary time in their sequences. The referent is a rate of molecular change used to read elapsed time from sequences, a "clock" only by analogy, with no gears, hands, or timer pulse.[1]

Unit

The rate at which molecular substitutions accumulate over evolutionary time

Recovery means

Not applicable

Measured by

Calibrated against fossil or geological dates and read as substitutions per site per unit time

In digital electronics, the clock is a periodic signal that paces a synchronous circuit: on each edge the system's flip-flops sample their inputs together, so the clock's frequency sets how fast the machine advances through its states. Harris and Harris's Digital Design and Computer Architecture shows how clock period, skew, and timing constraints bound a processor's speed. The referent is a timing waveform that coordinates logic, measured in cycles per second, unrelated to a timekeeping machine or an evolutionary dating rate beyond the shared idea of regular beats.[2]

Unit

The frequency and period of the synchronizing signal driving a circuit

Recovery means

Not applicable

Measured by

Specified by clock frequency and period and bounded by setup, hold, and skew constraints