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Interactions between species are as evolutionarily malleable as the species themselves and have played a central role in the diversification and organization of life. This malleability creates complex geographic mosaics in interspecific interactions that can evolve rapidly over decades, blurring the distinction between evolutionary time and ecological time and making the study of coevolution crucial for human health and welfare.
The history of evolution and biodiversity is fundamentally a history of the evolution of species interactions. Species in pure isolation simply do not make sense. Most living organisms have evolved in ways that absolutely require them to use a combination of their own genetic machinery and that of one or more other species if they are to survive and reproduce. Indeed, most described species take this to an extreme, living symbiotically on other species as parasites, commensals, or mutualists. In tum, most organisms must devote a large share of their resources to defense. Even now, among human populations one-third of deaths are caused by infectious disease. The more we learn about the diversity of life and the structure of genomes, the more it appears that much of the evolution of biodiversity is about the manipulation of other species-to gain resources and, in tum, to avoid being manipulated.
Many of the major events in the diversification of life can be traced back to the appearance of novel species interactions (1, 2). The consequences of these events are so pervasive-and, in some cases, the genomes of the species so completely anastomosed-that it is easy to forget how central they have been to life on Earth: mitochondria and the origin of the eukaryotic cell; chloroplasts and the origin of plants; dinoflagellates and the origin of coral reefs; lichens, mycorrhizae, and rhizobia and the process of terrestrial plant succession; gut symbionts and animal digestion. Other interactions between free-living species are equally pervasive. A majority of plants would quickly become extinct without their animal pollinators (3). Even in a gene-centered view of evolution, it is the armies of gene packages we call species that wage the genetic wars and create the genetic alliances.
The Coevotutionary Framework
Despite the central importance of species interactions to the diversification of life, we still know little about how the genomes of...