Fluctuate fits the model which has a single population which has been growing (or shrinking) according to an exponential growth law. It estimates 4Nu and g, where N is the effective population size, u is the neutral mutation rate per site, and g is the growth rate of the population. Fluctuate forms part of the Lamarc (Likelihood Analysis with Metropolis Algorithm using Random Coalescence) suite. See: http://evolution.genetics.washington.edu/lamarc.html