Gobies of the genus Pomatoschistus (Gobiidae) occur as a species complex. Habitatsegregation of the sympatric species P. minutus and P. lozanoi makes that both species inhabitat the same environment. The processes of this speciation will be studied in space and time with genetic markers. Moreover, a recent habitat adaptation seems to have occurred in P. minutus in the Baltic Sea. We will use population dynamical and genetic traits to clarify the phenomenon.