while DGs should have experienced delayed SF [@noe07], this evolutionary dichotomy is observed as downsizing [@cow96]. Nevertheless, the assumption that all DGs were affected in the re-ionization era and in the same way would request overlapping Stroemgren bubbles in an almost uniformly ionized Universe. This, however, must be questioned and is contrasted by the existence and amplification of cosmological density structures [@par10].

Another possible paths of DG origin is the formation of SF density enhancements in the tidal tails of merger galaxies [@duc07]. Since they should be free of DM and it is not yet well understood whether their SF acts in self-regulation the survival probability of these galaxy-like entities needs to be explored [@rec07b].

 Dwarf irregular galaxies and their extreme evolutionary stages {#dIrrs}
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dIrrs are characterized by large gas fractions, ongoing SF, and low metallicities. That dIrrs contain the same or slightly higher gas fractions than giant spiral galaxies and mostly suffer the same SF efficiency, but appear with lower metallicity $Z$ than spirals, cannot be explained by simple evolutionary models. When gas is consumed by astration but replenished partly by metal-enhanced stellar mass loss, the general analytical derivation relates the element enrichment $Z(t)$ with the logarithm of 