One time, astronomers believed that the morphological sequence galaxies in their classification by Hubble reflects the stages of their evolution. This belief was built, obviously, on the gradual transition of the elliptic spiral galaxies in the Hubble classification. It was believed that galaxies are born elliptical, and then slowly change their morphology, passing through the stage of the spiral, and finally become the galaxies of the wrong type. According to this logic, elliptical galaxies should be the most young and spiral - the oldest.
Classification of galaxies by Hubble (Hubble Sequence)
However, according to modern theories, this view is completely outdated. Now it is believed that all galaxies in the Universe occurred simultaneously in the form of large gas balloons, which was compressed under its own gravity. Over a period of about 100 million years this ball reaches the state of equilibrium, and its further evolution proceeds much more slowly - until the age of about 100 billion years (that is, it is still ongoing). And the shape of the galaxies associated with other factors, in particular, to the number formed in the subsoil of the stars. If for some reason the stars are formed within a short period of time and consume a lot of gas and interstellar dust, galaxy quickly exhausts all his interstellar gas and evolves into an elliptical. If the formation of stars runs very slow or quick stops, excessive gas evolves very quickly, and around the nucleus and a thin disk. The result is a disk galaxy. Conversely, if the formation of stars flows less intensively, the galaxy becomes a spiral.
Stars born at the initial stage of evolution of the galaxy, forming a halo of globular clusters and individual stars, and drive keep coming up with all new stars. In a certain sense we can say that elliptical galaxies are those with the halo dominates bilgem and disk (there is none), and spiral galaxiki is a system with a predominance of disk and bulge above the halo.
In this scenario irregular galaxies are the products of evolution of small clouds, which because of its small mass could not become elliptical or spiral galaxies.