The mechanisms responsible for the galaxy diversification make what can be called “galactogenesis”. Understanding the details of galactogenesis should help us classify the observed objects, to build up a synthetized picture much easier to confront to tractable models.
Galaxy evolution goes through transformation processes. They are well known and can be gathered into five main categories.
This is the most basic process by which an ensemble of baryonic matter gathers together into a self-graviting object. This process is obviously strongly favored by Dark Matter Halos, but there is no real reason why it might not happen independently. This is not so important here, since we are interested not in the detailed process, which can take many forms (yes, another source of diversity), but on the general physical mechanism by which some gas, dust or stars end up into something corresponding to the definition of a galaxy. Naturally, this is the very first process in the history of the Universe that make galaxies appear. This transformation process is generally qui short with respect to the age of the Universe.
2. Secular evolution
I define secular evolution as the evolution of a galaxy in isolation. This does not prevent the galaxy to change, since at least stars are ageing anytime, transforming the global color into a redder and redder one. But this is not all, some stars do also explode, modifying the metallicity of the interstellar medium, instabilities can change their orbits, the gas may also be affected by shocks, and new stars can form from gas and dust clouds. The complexity of galaxies implies that two identical galaxies will very probably evolve differently. The differences might no be obvious to observe, would not justify the definition of two distinct classes, but this is arbitrary and depends on the level of details we are interested in. The time scale of the secular evolution is slow.
Galaxies are flying in a gravitational field populated by other galaxies. Since gravity is a force of infinite range, interactions of galaxies must be very frequent, especially when the Universe was younger and denser. Interactions are now known to have significant and even dramatic influence on many properties of galaxies. This transformation process is a huge source of diversification for two reasons. The first reason is because the impact parameters have an infinite number of possible values. The second reason is due to the complexity of the galaxies, making consequences of the same interaction to vary very much even with very similar galaxy pairs.
When a galaxy catches a baryonic object not classified as a galaxy, we call this process an accretion. It is necessarily a transformation since the galaxy, at least, has acquired more mass. But it has also acquired more gas, dust or stars, that unavoidably change the global composition of the galaxy. Of course, if the accreted material is tiny with respect to the galaxy, this transforming process can be neglected (and would remain probably undetectable).
When the accreted object is a galaxy, then we have a merger. It is minor if the accreted galaxy is smaller, and major if masses are comparable. A single minor merger or a single small accretion can be ignored, but if this transformation process is repeated, then the consequences are obvious. Indeed, this is the way massive galaxies around us are supposed to have “formed” in a purely hierarchical scenario. Galaxy growth is thus sometimes quite continuous and regular. At each step, the differences remain unnoticed, but at the end there is really a necessity to separate small galaxies (like dwarfs) from massive ones. But are only two categories enough? Probably not, even though there is no absolute number of categories, it depends on the use and on other parameters to describe galaxies.
Major mergers are a good illustration of galaxy diversification, a good case that shows that galaxy “evolution” is not linear. It is more obvious in the case of two similar spiral galaxies. The result is known to be generally a galaxy of elliptical shape. Hence, two galaxies have disappeared, have died, and a new one appeared. If we put the two spirals in the same class, then another class appeared, at least when this transforming process first occur in the Universe. From one class, we end up with two class, we have increased the diversity.
Hence, the accretion/merging transformation process is certainly very frequent and relatively quick.
These processes are the opposed phenomena to the previous one above. Sweeping affects essentially the gas by ram pressure, while ejection involve mainly stars by kinematical perturbations. The tidal effects appear during interactions or major mergers, the ejected material can later form a galaxy. We know such newly born (assembled) objects as tidal dwarfs. They are a good illustration of diversification, and of the fact that galaxy formation is not only primordial, and is really a continuous process resulting from the transformation of galaxies and more generally of baryonic entities self-graviting or not.
It must be clear now that galaxy evolution is nothing else than transformation. More than that, during any of the transforming processes, the constituents of galaxies are transmitted to the new object, with modification. Galaxy diversification proceeds through a transmission with modification scheme. This justifies to look at phylogenetic tools to study more adequately the Universe.