New Scientist
Image: mattk1979
Family histories don’t come much more bizarre. Three-quarters of the fish in the sea can trace their origins back to a freshwater ancestor. The finding highlights how important rivers and lakes are as a source of new species, just as that supply is under threat from disappearing freshwater habitats.
Fish first evolved in the sea. The oceans have been teeming with them for almost half a billion years, so there is no reason to doubt that the fish living there today did all their evolving in salt water – until you take a closer look at their family tree.
Greta Vega and John Wiens at Stony Brook University in New York noticed something peculiar while studying the evolutionary tree of ray-finned fish, a mega-group comprising 96 per cent of all freshwater and marine fish species on the planet.
They realised that all the fossils belonging to the ancestral group that gave rise to ray-fins some 300 million years ago – known as the polypteriformes – came from freshwater deposits. In fact, according to Vega and Wiens’s tree, the ray-fins may not have taken to the sea in large numbers until about 170 million years ago. Their descendants now make up three-quarters of all marine fish (see diagram).
We’ve seen this kind of topsy-turvy evolution before. Most whales, dolphins and porpoises, live in the sea, but like the ray-finned fish, they all evolved in rivers. Read more on newscientist.com…
