Fish evolution is all in the head, according to a study by Lauren Sallan and Matthew Friedman to be published in a forthcoming issue of the Proceedings of the Royal Society B.
The results of this study indicate that evolution in fishes is primarily driven by food availability and the ability to exploit it, leading to diversification of head morphology before the rest of the body.
The authors focused on two different adaptive radiations following mass extinction events in the evolutionary history of fishes: the first during the Devonian–Permian transition about 360 million years ago and the second following the end-Cretaceous extinction that wiped out the dinosaurs.
By separately analysing the head and body features of 69 species of Devonian fishes and 304 species of Cretaceous fishes using a method known as geometric morphometrics, the authors were able to analyse the change in shape through time. They found that during the post-extinction radiations, the heads of the fishes began to diversify long before the rest of their bodies did. This implied that the fish were evolving adaptations for procuring and processing food (in the jaws and head) to exploit niches left vacant.
Evolutionary biologists have proposed two models of how adaptive radiations occur. In the first model, they posit a single burst of explosive divergence followed by a long period of relative stability; the second model proposes that diversification occurs in stages, with changes in the body driven by habitat occurring before those in the head.
Although their study provides evidence for the first model, the authors are careful to generalise their results, stating that the real world is more complicated than any model.
For more information, see the paper: Sallan, LC and M Friedman (2012) Heads or tails: staged diversification in vertebrate evolutionary radiations. Proceedings of the Royal Society B: Biological Sciences doi:10.1098/rspb.2011.2454
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