Isolation and Speciation

It is long-established that marine organisms evolve through geographic isolation. When a population is physically divided (by distance, depth, temperature, or salinity), factions cannot interbreed, and become genetically distinct over time. New species form from a single ancestral population. 
Many studies invoke geographic isolation as THE singular factor in marine speciation, but the story is likely much more complex. The ocean has no walls, and many species can drift thousands of miles as surface-dwelling larvae before developing into free-swimming adults. Perhaps marine physical barriers are more porous than once anticipated. Genes do move between geographically separate populations, with the rate of genetic exchange informing the speed and extent of reproductive isolation. 
 Understanding the patterns of genetic exchange between geographically distinct marine populations is a priority in evolutionary biology. With the rapid advancement of molecular techniques in the last 40 years, there has been a proliferation of work on speciation, but further research is needed to grasp the drivers of evolution at sea.    

(Larval Monkfish, Lophius vomerinus)
From Innovations Report

(Adult Monkfish, Lophius vomerinus).  From WWF Southern African Sustainable Seafood Initiative