Galaxy clusters show differences in their galaxy populations depending on their redshift. High redshift clusters contain more blue, star-forming spiral galaxies, while low redshift clusters are dominated by older red elliptical galaxies. As spiral galaxies fall into clusters, they undergo processes like ram pressure stripping and tidal disruption that remove their gas and quench star formation, transforming them into red elliptical galaxies over time. Observing this transition helped reveal the evolutionary path of galaxies in clusters.