There is a discrepancy between the mutation rate we can measure today and the rate at which evolution is supposed to have proceeded. The former is sometimes called the genealogical mutation rate, for it is obtained by comparing individuals whom we know to be related. The latter is sometimes called the phylogenetic mutation rate. It is calculated by counting the fixed differences between two species and dividing by the estimated time since their common ancestor. Genealogical mutation rates are generally several orders of magnitude faster than phylogenetic estimates. This causes problems for the evolutionary model. For example, using the genealogical method would place Y Chromosome Adam and Mitochondrial Eve well within the biblical timeframe. The evolutionary community often uses appeals to natural selection or genetic drift to explain away the discrepancy. Population modeling software and simple statics can be used to show that these explanations do not work. The genealogical mutation rate is a serious challenge to evolutionary theory.
