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Science at the Museum: Building the Trees of Life

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To walk the fourth floor of the Museum — peering at the jagged “teeth” of armored fish Dunkleosteus, ducking under the 23-foot wingspan of the flying reptile Pteranodon, studying the long curved tusks of the elephant relative Mammuthus — is, in a sense, to walk the tree of life.

Each branching point represents the arrival of an evolutionary innovation — jaws, water-tight eggs, hooves, respectively — that unites one group of animals and distinguishes them from lineages that lack the feature. Known as synapomorphies, or shared traits derived from a common ancestor, these are the tracks of evolution.

Scientists have used trees to order life since before Charles Darwin first scribbled a spiky diagram in his notebook. In the 1950s, German biologist Willi Hennig formally proposed that trees of life should reflect evolutionary relationships among organisms, founding cladistics: a method for grouping organisms into ancestor-descendent clades, from the Greek word for “branch,” based on shared, derived features. But it took a Museum scientist, ichthyologist Gareth Nelson, to disseminate the idea among English-language biologists. Together with students and colleagues at the Museum — including another ichthyologist, Donn Rosen, paleontologists Eugene Gaffney and Niles Eldredge, ornithologist Joel Cracraft, and invertebrate specialists Norman Platnick and Randall T. Schuh – Nelson steadfastly argued the case for cladistics as the tool to test classification during academic talks, in research papers, and even on napkins over meals.

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So began the “cladistics wars” of the 1960s and 1970s that pitted those who build trees with clades against biologists who favored competing schools of taxonomy: phenetics, which does not use evolutionary relationships and instead relies on an expert to determine groupings using observable traits; and evolutionary systematics, which considers evolutionary relatedness in a less rigorous way.

“The Museum was the hub of the taxonomy universe during the 1970s and 1980s,” says Curator Ward Wheeler. “Cladistics changed everything and brought true hypothesis testing that maps evolutionary paths onto the Tree of Life.”

Seminal work that applied cladistics to classification during this time included Gaffney’s paper on fossil turtles, which was published by the Museum.

“I was a grad student in Arizona when Gene Gaffney’s 1975 Bulletin of the American Museum of Natural History was published,” says Curator Darrel Frost, a herpetologist. “It blew everything out of the water.”

Cladistics is now the commonly used taxonomic system, and many Museum scientists have spent their careers using evolutionary relationships to refine the Tree of Life, from trunk to the twigs. While cladistics was initially limited to analyzing morphological, or observable, characteristics, two recent technological leaps — DNA sequencing and the power of supercomputing — have allowed scientists to produce ever-more intricate and testable cladograms, with significant applications that include drug development and conservation initiatives.

“We couldn’t design drugs or vaccines for pathogens without understanding the relationships of organisms and the evolutionary process that produced the tree,” says Curator Rob DeSalle.

New streams of funding from federal initiatives such as the Assembling the Tree of Life (AToL) project from the National Science Foundation, secured in part by the efforts of Museum scientists, is furthering these research efforts.

“Building trees is now a big enterprise,” says Cracraft. “It is also spawning a huge amount of work in comparative behavior, community evolution, and comparative biology. The ink was barely dry on our tree for South American birds last year when ecologists asked us for help to address key questions in environmental science.”

For more about research at the Museum, visit amnh.org/science.

A version of this story originally appeared in the Winter issue of Rotunda, the magazine for Museum Members.

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