An added advantage of using molecular sequences in taxonomy is that, according to one influential school of geneticists, the 'neutralists' (we shall meet them again in the next chapter), most of the evolutionary change that goes on at the molecular level is neutral. This means that it is not due to natural selection but is effectively random, and therefore that, except through occasional bad luck, the bugbear of convergence is not there to mislead the taxonomist. A related fact is that, as we have already seen, any one kind of molecule seems to evolve at a roughly constant rate in widely different animal groups. This means that the number of differences between comparable molecules in two animals, say between human haemoglobin and warthog haemoglobin, is a good measure of the time that has elapsed since their common ancestor lived. We have a pretty accurate 'molecular clock'. The molecular clock allows us to estimate, not just which pairs of animals have the most recent common ancestors, but also approximately when those common ancestors lived.
