Timing Lineage Splits: Molecular Clocks

What are molecular clocks?

When researchers use a molecular clock, they compare stretches of DNA to mark the passage of evolutionary time. A molecular clock uses mutation rates in DNA to estimate the time that two species have been evolving independently.

Neutral Mutations as “Ticks” To understand molecular clocks, think about old-fashioned pendulum clocks. They mark time with a swinging pendulum. A molecular clock also relies on a repeating process to mark time—mutation. As you've learned, simple mutations occur all the time, causing slight changes in the sequence of DNA. Some mutations have a major positive or negative effect on an organism's phenotype. These types of mutations are under powerful pressure from natural selection.

Many mutations, however, have no effect on phenotype. These neutral mutations tend to accumulate in the DNA of different species at about the same rate. Researchers can compare such DNA sequences in two species. The comparison can reveal how many mutations have occurred independently in each group, as shown in Figure 17–18. The more differences there are between the DNA sequences of the two species, the more time has elapsed since the two species shared a common ancestor.

In Your Notebook Which kind of mutation—neutral or negative—will most likely persist in a population over time? Explain.