1300 species, 2400 genes, 21 museums, and 40 years

December 10, 2020
Spectacled Tyrant, Hymenops perspicillatus. Brazil. Image courtesy of C. Albano

Tropical regions contain many of the world's species and scientists consider them hotspots due to their immense biological diversity. However, due to limited sampling our knowledge of tropical diversity remains incomplete, making it difficult for researchers to answer the fundamental questions of the mechanisms that drive and maintain diversity.

In a paper published December 10 in Science, an international team of scientists has produced the first complete, species-level phylogeny of a major group of tropical birds known as the suboscine passerines. Passerines are the largest order of birds and among the most diverse orders of terrestrial vertebrates. The suboscine group includes more than 1,306 species and in the Neotropics they make up roughly one-third of the total avian population.

The multinational team of scientists co-led by Gustavo A. Bravo, The Department of Organismic and Evolutionary Biology, Harvard University, and Michael Harvey, the Department of Biological Sciences, The University of Texas, El Paso, assembled a large data set of 1,940 suboscine genetic samples, which represented 1,287 (98.5%) of the 1,306 suboscine species. The samples were obtained from specimens housed in 21 museums around the world. Museum scientists began collecting genetic samples approximately 40 years ago, making it possible for the team to pursue this project.

Using genetic markers scattered across the entire genome the team described the evolutionary history of and created a framework for further questions about mechanisms that drive species diversity.

The project began in 2012 when institutions from North and South America united on the quest to build a species-level phylogeny of the suboscines using genomic data; Bravo and Harvey were postdocs working on the project at the time. The project became increasingly urgent as these same tropical regions that shelter and produce such diversity came under rapidly intensifying human pressure and decline.

The study also revealed a paradox of suboscine diversity - new species actually form faster in areas researchers refer to as coldspots than hotspots. Coldspots are not actually cold in temperature but are places like deserts and mountaintops that are less populated allowing species the opportunity for room to evolve. In contrast, hotspots are the result of the gradual accumulation of species over a long period of time. This paradox is a relatively new concept suggested by recent studies comparing tropical versus temperate regions. This study is the first to uncover the paradox using a complete species-level phylogeny of a group that mostly occurs in the tropics.

"This means that conservation efforts to save the rapidly changing tropical landscape need to focus not only on the species-rich Amazon but also on regions that are less diverse, but contribute disproportionally to the generation of new species," said one of the senior authors, Robb Brumfield, Louisiana State University. "For instance, the wind-swept, cold puna of the Andes Mountains."

"The paradox of diversity in suboscines was revealed because of the dozens of natural history museums and museum researchers working for decades in tough conditions on shoestring budgets to document the rapidly disappearing diversity of the tropics," said Harvey. "These birds represent an incredible amount of diversity sampled; roughly one out of every three species of birds in the American tropics."

Bravo agreed, "We were only able to do this project because of the hard work of numerous local scientists who have dedicated their lives to studying and preserving this diversity." Samples came from vouchered museum specimens, some that dated as far back as the late eighteen and early nineteen hundreds, allowing the inclusion of rare, endangered and extinct species.

Today, eight years after the project began, the team has grown to mimic the diversity of their study's subject. Notably, these types of collecting trips and research teams are increasingly led by ornithologists from groups underrepresented in the sciences, including Latinx and women researchers. Many of the researchers involved in the study are from Latin America (Colombia, Brazil, Uruguay, and Venezuela), and recent teams fielded by the renowned Museum of Natural Science at Louisiana State University to obtain samples like those used in this study were women-led efforts. "This paper marks not only a change in our understanding of evolution in the tropics, but also an acknowledgement and valuation of the diversity of culture, expertise, and perspective in the field of ornithology" says one of the senior authors Liz Derryberry, University of Tennessee.