The molecular-clock model of evolutionary relationships is based on the assumption that changes in macromolecule sequences occur at a relatively constant rate.

This means that:

* Mutations accumulate at a steady pace over time.

* The number of differences between two sequences is proportional to the time since they diverged from a common ancestor.

This allows scientists to use the rate of molecular evolution to estimate the time of divergence between different species or lineages.

However, it's important to note that the molecular clock is not perfect. The rate of mutation can vary depending on factors like:

* The specific gene or protein being studied.

* The organism's generation time.

* The selective pressures acting on the organism.

Therefore, the molecular clock model is often used in conjunction with other methods to estimate evolutionary relationships and divergence times.