Habitats and environmental pressures shape a variety of animal morphologies and behaviors over vast evolutionary time scales. These environmental pressures can sometimes produce nearly identical traits, even in completely unrelated lineages. This phenomenon, called convergent evolution, results in modern species that appear very similar despite being only distantly related.
In recent years, convergent evolution has received more attention from biologists, but such studies typically involve closely related groups. Now, recent work in Proceedings of the Royal Society B describes a robust method to better detect cases of convergent evolution across more diverse animal groups, and applies it to a large group of bony fishes called Terapontoidei. In doing so, the researchers suggest that the phenomenon may be more widespread than originally thought.
To investigate cases of convergent evolution, Aaron Davis of James Cook University and Ricardo Betancur-R of the University of Puerto Rico used a suite of statistical tools that enabled the researchers to integrate data on a broad range of traits for 88 species of Indo-Pacific fishes—traits including morphology, dietary preferences, habitat choice, and phylogeny. They were able to build a far more detailed picture of the evolutionary relationships between species than was previously possible. The species studied came from diverse habitats in temperate and tropical regions including both marine and freshwater environments.
The researchers’ combination of tools, collectively called phylogenetic comparative methods, also established a timeline over which evolutionary changes likely occurred. This better understanding of evolutionary relationships and the clarified timeline enabled Davis and Betancur-R to distinguish whether trait similarity between species resulted from common ancestry, from convergent evolution, or some combination thereof. The study represents one of the most comprehensive analyses of a large and ecologically diverse group of species analyzed to date.
“One of the neat things about this paper is that it uses a battery of new methods to address the question of convergence from different perspectives that complement each other,” says Hernán López-Fernández, associate professor of ecology and evolutionary biology at the University of Toronto and curator of fishes at the Royal Ontario Museum in Toronto, who was not involved in the research.
The analyses revealed a remarkable degree and frequency of convergence across families of fish that had been separated by at least 30 million years. Previous studies had generally examined convergence between groups separated by only a few million years. Notably, the convergence events included a magnitude of marine-to-freshwater environment species transitions that, say the authors, had not been previously observed.
The authors have provided “a really nice integrative demonstration of the role that ecology can play in shaping the phenotypes of organisms over long time scales,” says Luke Mahler, an assistant professor in the Department of Ecology and Evolutionary Biology at the University of Toronto, who was not involved in the research.
Convergence, though, doesn’t mean identical species, the authors note. These Terapontoidei traits, which primarily involved transitions from carnivorous to herbivorous behavior and from marine to freshwater environments, rarely produced identical phenotypes in different fish species. “It doesn’t have to be all or nothing. You can have similar environments shaping convergence without resulting in doppelgangers,” says Mahler. “This is an under-appreciated phenomenon.”
Going forward, Betancur-R hopes that the sensitivity provided by employing multiple statistical tools will help uncover more hidden cases of imperfect convergence. Mahler says the tools can also give us a window into the past, showing us where and when important ecological changes occurred and offering a better understanding of how the evolutionary process unfolds.