Recent reconstructions of evolutionary history are often dependent on assigning divergence in terms of changes in amino acid or nucleotide sequences. For example, a comparison of cytochrome c shows 10 amino acid differences between humans and dogs, 24 differences between humans and moths, and 38 differences between humans and yeast. Such data provide no information as to the absolute times of divergence for humans, dogs, moths, and yeast. How might one calibrate the molecular clock to an absolute time clock? What problems might one encounter in such a calibration?

Verified step by step guidance

1. Understand that the molecular clock hypothesis assumes a roughly constant rate of molecular change over time, allowing us to estimate divergence times based on the number of sequence differences.
2. To calibrate the molecular clock to an absolute time scale, identify at least one pair of species for which the divergence time is known from independent sources, such as fossil records or geological events.
3. Use the known divergence time and the observed number of molecular differences between that species pair to calculate the rate of molecular change per unit time. This can be expressed as: \[\text{Rate} = \frac{\text{Number of molecular differences}}{\text{Divergence time}}\]
4. Apply this calculated rate to other species pairs by dividing their observed molecular differences by the rate to estimate their absolute divergence times: \[\text{Divergence time} = \frac{\text{Number of molecular differences}}{\text{Rate}}\]
5. Be aware of potential problems in calibration, such as variation in mutation rates among lineages, incomplete or inaccurate fossil records, horizontal gene transfer, and the assumption that molecular changes accumulate at a constant rate, which may not hold true for all genes or organisms.

Key Concepts

Molecular Clock Hypothesis

The molecular clock hypothesis proposes that genetic mutations accumulate at a relatively constant rate over time, allowing estimation of divergence times between species by comparing molecular differences. This concept is fundamental for using sequence data to infer evolutionary timelines, assuming mutation rates are steady and comparable across lineages.

Calibration of Molecular Clocks

Calibration involves linking molecular differences to absolute time by using independent data, such as fossil records or known geological events, to assign dates to specific divergence points. This step is essential to convert relative genetic distances into actual time estimates, enabling the molecular clock to reflect real evolutionary timelines.

Challenges in Molecular Clock Calibration

Calibrating molecular clocks faces challenges like variable mutation rates among lineages, incomplete or ambiguous fossil records, and assumptions about constant rates that may not hold true. These issues can lead to inaccurate time estimates and complicate the interpretation of evolutionary history.