That’s the outcome of attempting to classify the 150 million-year-old, widely distributed family of spiny fish, which includes most commercial species.
The researchers looked at 10 genes in more than 500 fish species representing most of the families of spiny-rayed fish. They used the genetic data to construct a tree, grouping related families together. They also looked at the pace of evolution — the rate at which new species formed — in different branches, and across the group as a whole.
The tree shows some interesting relationships. For example, tuna are more closely related to seahorses than to swordfish or barracuda. The oddly shaped pufferfishes are related to anglerfish, the only fishes whose bodies are wider than they are deep.
Cichlids, a family that includes about 2,000 species of freshwater fish known for brooding their young in their mouths and a favorite for studies of evolution, are related to the engineer gobies, an obscure family of just two species that live on coral reefs and raise their young in a nest.
Wainwright’s special interest is in the evolution of fish jaws. Fish have two sets of jawbones, an outer jaw and “pharyngeal jaws” in the throat that adapted to different functions. In some fish, the lower pharyngeal jaw is fused into a single solid bone that can be used to crush prey such as shellfish.
Biologists had assumed that this fused jaw had evolved once and then spread into different groups of fish. Instead, the new tree shows that this structure evolved at least six times in different groups of fish.
It just doesn’t seem like Darwin’s mechanism (natural selection acting on random mutation) should be yielding results like this. No one would have predicted it. No one did, in fact.
We really don’t know how evolution happens.