 The next question to ask is what the expected value of this distribution is, that is, what the mean time to coalescence is? The answer turns out to be approximately \(2N\). Thus on average, a pair of randomly picked genes at a given locus will coalesce after \(2N\) generations, where \(N\) is population size. However, there is considerable variability about this mean, meaning that there is a significant chance that coalescence will occur much quicker, or much slower, than this. By building on this simple analysis, coalescent theory allows a range of more complicated questions to be answered, involving, for example, multiple alleles, sub-divided populations, populations that change in size over time, plus other deviations from the assumptions of the basic Wright-Fisher model. For example, coalescent theory yields a straightforward calculation of how many generations back we must go, on average, to find the MRCA of a number of different genes at a locus (Otto and Day 2007, ch. 13).