Stars form in embedded star clusters which play a key role in determining the properties of a galaxys stellar population. Physical mechanisms discussed in this paper are runaway stars shot out from young clusters, binary-star disruption in clusters, gas blow-out from clusters and the origin of thick galactic disks. I emphasise that the SNIa rate per low-mass star depends on the star-clusters formed in a galaxy and I discuss the IGIMF theory. Based on the IGIMF theory, the re-calibrated Halpha-luminosity--SFR relation implies dwarf irregular galaxies to have the same gas-depletion time-scale as major disk galaxies, suggesting a major change in our understanding of dwarf-galaxy evolution. The IGIMF-theory also naturally leads to the observed radial Halpha cutoff in disk galaxies without a radial star-formation cutoff. It emerges that the thorough understanding of the physics and distribution of star clusters may be leading to a major paradigm shift in our understanding of galaxy evolution.
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