Section 2 focused on the broadest possible picture of the assembly of stellar mass - the build-up of the stellar population, averaged over cosmologically-significant volumes. Yet, the physical processes contributing to this evolution will be much more strongly probed by studying the demographics of the galaxy populations as they evolve. This splitting of the cosmic SFR/SFH can happen in many ways: study of the evolution of the luminosity function, stellar mass function, or SFR `function', study of galaxies split by morphological type or rest-frame color, or by identification of galaxies during particular phases of their evolution (e.g., interactions). There has been a great deal of activity over the last decade towards this goal: implicit in the exploration of Figs. 1 and 2 is the construction of SFR and stellar mass functions, and a number of studies have explored the evolution of the galaxy population split by morphological type ([Brinchmann & Ellis (2000)], [Im et al.(2002)]), rest-frame color (e.g., [Lilly et al. (1996)], [Wolf et al. (2003)], [Bell et al. (2004b)]), or focusing on the role of galaxy interactions (e.g., [Le Fèvre et al. (2000)], [Patton et al. (2002)], [Conselice et al. (2003)]).
A full and fair exploration of any or all of these goals is unfortunately beyond the scope of this work. Here, I choose to focus on one particular key issue: the importance of galaxy mergers in driving galaxy evolution in the epoch since z ~ 3, and especially at z 1. Unlike the evolution of e.g., the stellar mass function or SFR function, to which both quiescent evolution and galaxy accretion can contribute, galaxy mergers (especially those at z 2) are an unmistakable hallmark of the hierarchical assembly of galaxies. Therefore, exploration of galaxy mergers directly probes one of the key features of our current cosmological model.
I will focus here on exploring the number of major galaxy mergers (traditionally defined as those with mass ratios of 3:1 or less) over the last 10 Gyr. There are three complementary approaches to exploring merger rate, all of which suffer from important systematic uncertainties: the evolution of the fraction of galaxies in close pairs, the evolving fraction of galaxies with gross morphological irregularities, and investigation of the evolution of plausible merger remnants. In this work, I will briefly discuss all three methods, highlighting areas of particular uncertainty to encourage future development in this exciting and important field.