Angular Momentum at collapse (nature)
Evidence: On average Spirals spin faster than Ellipticals.
Problems: Ellipticals have a large range in rotation speeds.
Random motions in the collapsing cloud (nature)
Evidence: Models with high random motion preferentially create Ellipticals.
Problems: Remains to be proven observationally.
Star Formation rate (nature? nurture?)
Evidence: An intense burst of star formation used up all Elliptical gas.
Problems: What triggers such large-scale star formation?
Galaxy Collisions (nurture)
Evidence: Ellipticals are more common in areas of galaxy collisions (clusters).
Problems: Then why are Ellipticals found in complete isolation?
Galaxy Mergers (nurture)
Evidence: Enormous Ellipticals at the centers of Galaxy Clusters. It is happening
NOW in our galaxy!
ALL these effects are occurring. This makes it difficult to determine the importance of any one effect.. It is the combination of all these effects (and perhaps others not yet identified) which give way to the amazing array of galaxy properties found in the universe.