Museum Biologists Complete Largest-Ever Tree of Life for Living Amphibians

by AMNH on


Agalychnis calcarifer

Credit: Taran Grant, AMNH

Six American Museum of Natural History biologists and 13 of their colleagues have completed the largest analysis ever of the evolutionary relationships among all living amphibians (a group that includes frogs, salamanders and caecilians--snake-like animals lacking scales). This ambitious project represents the largest analysis of its kind of any group of vertebrate animals. The result will provide biologists with a dramatically improved basis and common language for addressing questions about amphibian evolution, life histories, biodiversity, global distribution, conservation, and extinction. Data for the new amphibian tree of life included 1.8 million base-pairs of mitochondrial and nuclear DNA, extracted, amplified, and sequenced in the Museum's genomics labs, and recently published dataset of amphibian morphology. It took more than six months of computation time using the Museum's parallel computing facility to crunch the massive combined matrix of features and taxa and to assess the huge number of possibilities to arrive at the simplest, best explanation--a tree with the fewest evolutionary changes and a common ancestor for all amphibians. The new taxonomy names 33 new groups and 2 new families, and includes dozens of new groupings among amphibians.

Eleutherodactylus elegans

Credit: Taran Grant, AMNH

This 370-page study, led by Darrel R. Frost, Curator-in-Charge in the Museum's Division of Vertebrate Zoology, was published in the latest issue of the Bulletin of the American Museum of Natural History, a peer-reviewed journal. Museum coauthors include Taran Grant, post-doctoral research fellow in the Division of Vertebrate Zoology; JuliFaivovich, a former graduate student in the Division of Vertebrate Zoology who was enrolled at Columbia University; Raoul Bain, biodiversity specialist with the Museum's Center for Biodiversity and Conservation; Christopher J. Raxworthy, Associate Curator in the Division of Vertebrate Zoology; and Ward C. Wheeler, Curator in the Division of Vertebrate Zoology.

"Rather than continuing to analyze small numbers of taxa using small amounts of molecular or morphological data to generate easily digestible stories," Dr. Frost said, "we chose to build on a catalog of amphibians I'd developed over the past two decades and conduct a project that hasn't been undertaken since the 1930s in this field--a massive study of the evolutionary diversification of the amphibian tree of life. The new amphibian tree of life shows that the taxonomy up to this point has been hopelessly flawed and provides us with a new taxonomy that offers the scientific community a new starting place from which to address questions about amphibian biodiversity."

Caecilia thompsoni

Credit: Taran Grant, AMNH

Understanding the evolutionary relationships among species is one of the most important and basic requirements for both effective conservation and fundamental ecological and evolutionary research. However, prior knowledge of the evolutionary history of amphibians--a large, ancient, and ecologically important group of animals--was largely speculative, dating to studies from the late 19th and mid-20th centuries. Theories were based on remarkably little evidence and were perpetuated more by scholarly tradition than scientific corroboration. Meanwhile, the number of recognized amphibian species has grown enormously in recent years, while amphibian populations have been undergoing a massive, global decline due to many factors including habitat loss and fragmentation. One motivation for this study is to help the understanding of amphibian evolutionary history keep pace with knowledge of amphibian species diversity.

To bring the biology of amphibians into the 21st century, Dr. Frost and his colleagues assumed nothing and set out to test prior hypotheses of the relationships as severely as possible. Their study analyzed genetic and morphological data from nearly 10 percent of all 522 amphibian species targeted to provide a representative sample of the global diversity of amphibians. The result of this analysis is the first large-scale, empirically derived, rigorously analyzed, and explicitly testable theory of the evolutionary relationships of the major amphibian lineages.

Tylototriton shanjing, or Mandarin newt

Credit: Ronald Nussbaum, University of MichiganH

This study sheds new light on the question of the evolutionary origins and relationships of amphibians, suggesting countless new avenues of inquiry. For instance, the new tree shows that tree frogs are relatively primitive, or located close to the tree's base, compared with other amphibian groups, a finding that was not necessarily anticipated. The same goes for parental care--it occurs near the base of the tree, not just in more derived groups, i.e. those that have gone through relatively more evolutionary change. Also, for decades, biologists explained the considerable diversity (some 700 species) of rain frogs (genus Eleutherodactylus) as a recent radiation likely enabled by their ability to develop directly into frogs without passing through a tadpole (larval) stage. The new results indicate that the diversity of rain frogs (genus Eleutherodactylus) is relatively limited, possibly because these animals skip the tadpole stage. The inability of this group to exploit aquatic resources as tadpoles might have prevented it from invading new environments and speciating further. 

The study also marks a turning point in the field of phylogenetics or systematic biology (which studies the family-group relationships among organisms). High-speed DNA sequencing technology and advances in parallel computation and bioinformatics now make it possible to assemble and analyze massive amounts of data--datasets that were inconceivable as little as a decade ago. By applying these recent technological and theoretical advances to the problem of amphibians, this study demonstrates the feasibility of studies of this magnitude and promises to be the first of many such large-scale initiatives to understand the diversity of life on Earth.

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