Here's something that had never occurred to me before. We know that speciation is generally dependent on geographical separation, as happens when two populations of a species find themselves on either side of a natural divide: in valleys separated by a mountain range for instance, or on different islands. Darwin's finches in the Galapagos Islands are the classic example. So what about marine life? In the sea there are no barriers: in effect it's all one continuous environment. Does that mean that speciation is rarer in the oceans?
As Vega and Wiens point out, compared to the land, the sea is biologically depauperate: marine habitat covers 70% of the Earth’s surface but contains only 15-25% of Earth’s species. It gets worse if you count “habitable space”: since the ocean is three-dimensional, Vega and Wiens claim that it contains “90-99% of the volume of the habitable biosphere.”
They cite the lack of geographic barriers in open water as the most likely cause. And, amazingly - well, amazingly to me anyway - they show, through some complex DNA-based phylogeny studies - that the vast majority of marine fishes are actually descended from freshwater fish. All ray-finned fish, which make up around about 96% of the fish total, are descended from freshwater species.
This is so counter-intuitive. Isn't the ocean our origin; the great womb of life on earth? Well yes, certainly the first fishes evolved there, but it seems that to really get some diversity going they had to go inland and multiply in the innumerable separate freshwater rivers and lakes, before heading back out to sea again.
It's a bit more complicated than that: there may have been the odd mass extinction of marine life along the way. All fascinating stuff though.