Diet and Evolution

Top predators, like this snapper from the family Lutjanidae, showed rapid morphological evolution

Matthew D. McGee

The breadth of an animal’s diet has long been thought to influence the speed at which natural selection acts upon the species—those that eat a wide variety of foods could have more variations in body size and structure, which could lead them to evolve faster. This hypothesis, however, has been difficult to test, especially at a multi-species scale. Recently, a group of researchers used data from several fish databases to investigate the relationship between diet and evolution speed across a large sample of coral reef species from the taxon Acanthomorpha, spiny-rayed fishes.

Reefs—ecologically complex and extremely diverse—are great testing grounds for evolutionary ideas. Samuel R. Borstein, a doctoral candidate at the University of Tennessee in Knoxville, with four co-authors in the United States and Australia, used a publicly accessible database called FishBase to identify 1,545 species of reef fishes with available diet data. They categorized the species into five groups according to where they fit along a continuum from herbivory to omnivory to carnivory and used genetic data from the sequence database GenBank to construct the species’ evolutionary family tree. Fish images from databases were used to make digitized measurements of morphological traits, including length of head, jaw, and fins, and other body attributes. These physical trait data, along with the evolutionary tree, were used to estimate the fishes’ rates of evolution at each level of the food chain.

The data showed that generalist omnivores have the slowest rates of evolution; herbivores—which adhere to the narrowest diets—have the fastest rates of overall morphological evolution; and carnivorous predators at the top of the food chain have the fastest rates of evolution for specific body traits that aid in capturing prey.

Their findings support a newly coined “trophic extremes hypothesis”—that species at the top and bottom of the food web have higher evolutionary rates of trait diversification, driven by difficulty of accessing or digesting resources. “Being at these dietary extremes is not super-constraining in terms of morphological evolution, and omnivory might be more constraining than we [previously] thought,” says Borstein. One big take-away, he adds, is that “the scale at which you analyze your data—especially in evolutionary studies—matters, and patterns that might be found within a single family might disappear when you look across a whole tree.” (Nature Ecology and Evolution)