Ecology branches into the tree of life
An August 2012 supplementary issue of Ecology explores the interface of ecology and phylogenetics.
By Liza Lester, ESA communications officer
Lebensbaum (Tree of Life): Detail from Gustav Klimt’s 1910/11 drawing for the immense dining room frieze at Stoclet Palace, in Brussels. Watercolor and pencil. Österreichisches Museum für angewandte Kunst, Vienna.
NATURALISTS of the late 19th century tended to holistic interpretations of the natural environment and its evolutionary history. In the decades after Darwin, the new understanding of the relatedness of organisms to each other mixed indiscriminately with the study of relationships of organisms to their living and physical environments. Theories of natural selection and inheritance sprang from observations of communities of animals, plants and microorganisms – and, in turn, informed ideas of how communities may have been shaped by the climate and landscapes of their earthly residence.
“Ecology drives evolution, evolution drives ecology, that’s how Darwin saw the world,” said University of Minnesota ecologist Jeannine Cavender-Bares. But it is possible to zoom in on one viewpoint, to focus only on the interactions of living organisms and their environment, or only on the history of life, the derivation of species from common ancestors, and their adaptations to environmental pressures. That is what biological science did for much of the 20th century.
“We partitioned the processes we were looking at into more tractable components. There are benefits to doing that, but at the expense of understanding how ecological and evolutionary processes reinforce each other.”
Cavender-Bares is chief editor of a supplementary issue of ESA’s journal Ecology dedicated to bridging that gap in methodology and perspective. It showcases work at the interface of ecology and phylogenetics, a field of biology that works to infer the evolutionary history of relationships among organisms. “Integrating Ecology and Phylogenetics” went online in August, and is open access.
“If you start with Darwin — always a good place to start! — natural selection is fundamentally an ecological process,” said David Ackerly, one of Cavender-Bares’ co-editors for the supplementary issue. “Chapter 3 of the On the Origin of Species  is really a textbook in ecology.”
“As ecology became a more quantitative science, it was just more tractable not to have to consider all of evolutionary history. But it’s become tractable again,” said Cavender-Bares. She and co-editors Ackerly and Kenneth Kozak pushed forward the supplementary issue not only to showcase available technology, but to make the case for incorporating phylogenetic research questions and concepts into ecological studies.
“Ecologists are thinking about history more, thinking about contingency and context, and not seeing ecological systems so much as systems in equilibrium,” said Ackerly. He trained in ecology as a graduate student, and phylogenetics as a postdoc, and keeps a foot in both disciplines in his current work at UC Berkeley.
To understand why ecology and evolutionary biology grew apart, you have to think about the theoretical and experimental tools available to scientists in the 1920s and 30s, Ackerly told me, speeding through a cliff notes history of biology.
One sector of biology began to think a lot about genes, inheritance, and the developmental blueprints of organisms. Laboratory investigations into the genetics of brewers yeast, fruit flies, and mustard seeds moved along well enough outside the environmental and historical context of the natural world of bugs and weeds. The elegant mathematics of population genetics applied nicely to abstract theory, micro-evolutionary models, and applied problems of agriculture and animal husbandry. This work proceeded just fine, and was certainly simplified, outside the broader context of ecology.
Meanwhile, naturalists began applying concepts of physics to living systems, adding a firm grounding of new analytical tools to a field that had been largely observational. Theories about the flow of energy through systems ignored the specifics of history in favor of more generalizable rules. The tools of genetics were not, in any case, easily applicable to large, complex systems. Ecologists did not worry about the legacy of ancient events and relationships that were not discernible from the forms of living organisms.
The advent of computers and DNA sequencing opened new windows into the deep past. It completely changed the field of phylogenetics, revolutionizing construction of trees of decent from common ancestors based on comparisons of living species. Before DNA, biologists inferred such relationships from physical similarities, comparing attributes like skeletons, seeds, and the shape and construction of cells. Today, biologists mostly rely on DNA sequence comparisons, which have the benefit of being quantitative and accessible to scientists who have not spent thirty years in the intimate study of a specific family of blackberries or dung beetles.
The rapid gains in DNA sequencing technology over the last decade have been drawing concepts of ecology and phylogenetics back into close theoretical quarters, lending physical context to evolutionary studies, and historical context to ecological analysis.
Historical context has shaped the modern constituencies of ecological communities, says Cavender-Bares. She and co-author Peter Reich see the imprint of ancient adaptations to very different, ancient environments in the selection of species that live in an Oak savanna that burns yearly, compared to a neighboring forest that does not. Species that survive the frequent burns are more closely related to each other than to the species that populate savanna that never burns, and the divergence is inferred to have occurred between 80 and 140 million years ago. In the “Phylogenetics” supplementary issue, several research papers join Cavender-Bares and Reich in witnessing the legacy of ancient adaptations in living clusters of related species.
In contrast, a series of papers describing tree-insect interactions show how more recent selection can overwrite ancient adaptations. Some experimental work also demonstrates that variability in the manifestation of genetic encoding can mask an underlying evolutionary legacy – communities of species that do not look alike may actually be related.
The last paper in the issue presents a 30-year experiment at Minnesota’s Cedar Creek Long Term Ecological Research Station showing that diverse evolutionary relationships matter more to ecosystem stability than number of species.
“Phylogenetic diversity begets ecosystem diversity,” said Cavender-Bares. “This is why we should care,” she added. “I would say that this work is really critical in understanding the processes that govern the changes in diversity within communities,” said Cavender-Bares. “These questions become critical if we want to sustain ecosystems and ecosystem services in the face of major perturbations in Earth’s systems.”
The National Center for Ecological Analysis and Synthesis and Long Term Ecological Research Network contributed under-writing to this supplementary issue of Ecology. The issue is open access. Read it online: Vol. 93, Phylogenetics. August 2012.