Once fish evolve into top predators, they evolve more slowly

Washington, July 29 (ANI): A new study has found that once fish evolve the size and speed needed to become top predators, natural selection keeps them in an evolutionary holding pattern, that is, they evolve more slowly.

An example is the largemouth bass, which is a favorite freshwater fish with an appetite for minnows.

The study found that once they evolved to eat other fish, largemouth bass and fellow fish-feeders have remained relatively unchanged compared with their insect- and snail-eating cousins.

As these fishes became top predators in aquatic ecosystems, natural selection put the breaks on evolution, according to researchers.

For largemouth bass and other species that feed primarily on fish, the researchers wanted to know how this feeding strategy affected the pace and shape of evolution.

"The question we wanted to ask was: What is the interplay between the evolution of diet and the evolution of form?" said first author David Collar, a postdoctoral researcher at Harvard University.

To find out, the researchers examined museum specimens representing 29 species of centrarchid fishes.

Using a chemical process to stain and visualize the bones, muscles, and connective tissue, they measured the fine parts of the head and mouth.

"A fish mouth is much more complicated than our own mouth," said co-author Peter Wainwright of the University of California at Davis.

"Whereas we have one bone that moves our jaw, fish actually have two dozen separately moving bones, and lots of muscles that move those bones in a coordinated fashion," he added.

By mapping these measurements onto the centrarchid family tree, together with data on what each fish eats, the researchers were able to reconstruct how diet and head shape have changed over time.

"It looks as if the variety of head shapes and sizes in centrarchids is strongly influenced by what they eat - primarily whether they eat other fish or not," said Collar.

More importantly, when they compared fish-feeders with species that eat other types of prey, the researchers found that bass and other centrarchids that feed primarily on fish have remained relatively unchanged over time.

According to the researchers, once they evolved the optimal size and shape for catching fish - roughly 20 million years ago - natural selection seems to have kept them in an evolutionary holding pattern.

"Once they achieved a morphology that was good at feeding on fish, they tended not to evolve away from that," said Wainwright. "They were already good at catching the best thing out there. Why should they diversify any more? Life was good," he added. (ANI)