Progress in galaxy evolution studies in the last two decades has been driven by two mutually re-enforcing developments. On the one hand, observational astronomy has been able to transit from case studies to large samples. This is true for stars in the Milky Way (the ‘era of Gaia’), but also for galaxies outside the local group, where large samples of good quality data have transformed our understanding of galaxy evolution in general (e.g., the SDSS project or the CALIFA survey). On the other hand, simulations of galaxy formation in a cosmological context are simulating larger samples of galaxies and/or treat the physics in more detail. The best possible baryonic mass resolution (for simulations encompassing an entire galaxy) is now approaching 20 Msun.
In order to constrain these new simulations and thus improve our understanding of galaxy evolution, it is paramount that observational astronomy concentrate on measuring the physical properties of gas and stars at the best possible physical spatial resolution and over a significant areal coverage. The most useful technology to do so is imaging spectroscopy. Additionally, the northern sky accessible from Calar Alto contains two objects that are uniquely suited for such studies, because they are the closest representatives of their kind: the Andromeda galaxy and the Virgo cluster of galaxies. The GAMACA science case rests on the combination of the large field of view and high angular resolution that the instrument offers, with the high spatial resolution that is only possible in the northern sky due to the physical proximity of our two targets.