THE molecular clock that marks the progress of evolution has been ticking
faster than we thought.
Evolutionary biologists use the rate at which random mutations occur in the
genome to measure the time elapsed since species diverged. They mostly assume
that each mutation affects just one nucleotide.
But now an international team of researchers has investigated double
substitutions—where two nucleotides are affected— and discovered
that they occur at about one fiftieth the rate of single mutations (
Science, vol 287, p 1283). This means that some species could be slightly
younger than we thought. “People have tended to ignore…
