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We analyze a suite of 33 cosmological simulations of the evolution of MilkyWay-mass galaxies in low-density environments. Our sample spans a broad rangeof Hubble types at z=0, from nearly bulgeless disks to bulge-dominatedgalaxies. Despite the fact that a large fraction of the bulge is typically inplace by z=1, we find no significant correlation between the morphology at z=1and at z=0. The z=1 progenitors of disk galaxies span a range of morphologies,including smooth disks, unstable disks, interacting galaxies andbulge-dominated systems. By z=0.5, spiral arms and bars are largely in placeand the progenitor morphology is correlated with the final morphology. We nextfocus on late-type galaxies with a bulge-to-total ratio B/T<0.3 at z=0. Theseshow a correlation between B/T at z=0 and the mass ratio of the largest mergerat z<2, as well as with the gas accretion rate at z>1. We find that thegalaxies with the lowest B/T tend to have a quiet baryon input history, with nomajor mergers at z<2, and with a low and constant gas accretion rate that keepsa stable angular-momentum direction. More violent merger or gas accretionhistories lead to galaxies with more prominent bulges. Most disk galaxies havea bulge Sersic index n<2. The galaxies with the highest bulge Sersic index tendto have histories of intense gas accretion and disk instability rather thanactive mergers.

A diversity of progenitors and histories for isolated spiral galaxies