In this work we consider the behaviour of two distinct AGN populations: unabsorbed (low hydrogen column density) and absorbed (high column density and lower Eddington ratio). Assuming that the number of black holes (BH) remains constant with time, we follow the evolution of X-ray detected AGN, and compare the final result with the observed local distribution of BHs. We then investigate the role of the Eddington ratio, and radiative accretion efficiency, for each population, on the shape of the evolved mass function. We find that two distinct populations of AGN can evolve with a wider parameter range than that usually considered, and still be consistent with the local mass function. Taking fixed values for the accretion efficiency and Eddington ratio does not provide a full knowledge of the evolution mechanism. We also find that absorbed AGN have typically low Eddington ratios and low accretion efficiencies, while unabsorbed AGN have high Eddington ratios.