Two major models describe the formation of early-type galaxies: high-z collapse and hierarchical build up through merging. In order to better understand the process of galaxy formation and evolution in the context of these two models, we study how the population of galaxies in clusters evolves in a span of z from ~0.05 to ~1.5. We first focus on studying the problem of downsizing by investigating the evolution of the population of cluster Red- Sequence galaxies (RSGs) aiming to find a normalization for this process at low z (0.05-0.19). After that, we measure the evolution of the K-band luminosity function and the Halo Occupation Distribution (in particular halo occupation number and radial galaxy distribution as parametrized by the concentration parameter) using Subaru observations of 15 of the most distant X-ray clusters known at z = 0.8-1.5 and compare results with mock clusters extracted from Millennium Simulation (MS). Our results will be discussed in comparison with the expectation of both the high-z collapse and the hierarchical galaxy formation models.