Abstract

abstract: Coevolution is one of the major processes organizing the earth's biodiversity. The need to understand coevolution as an ongoing process has grown as ecological concerns have risen over the dynamics of rapidly changing biological communities, the conservation of genetic diversity, and the population biology of diseases. The biggest current challenge is to understand how coevolution operates across broad geographic landscapes, linking local ecological processes with phylogeographic patterns. The geographic mosaic theory of coevolution provides a framework for asking how coevolution continually reshapes interactions across different spatial and temporal scales. It produces specific hypotheses on how geographically structured coevolution differs from coevolution at the local scale. It also provides a framework for understanding how local maladaptation can result from coevolution and why coevolved interactions may rarely produce long lists of coevolved traits that become fixed within species. Long-term field studies of the same interaction across multiple communities and spatially structured mathematical models are together beginning to show that coevolution may be a more important ongoing process than had been indicated by earlier empirical and theoretical studies lacking a geographic perspective. Keywords: coevolution, geographic mosaic, local adaptation, maladaptation, metapopulation, mutualism. As we learn more about the organization of the earth's biodiversity, the central role of the coevolutionary process is becoming more evident. It is now clear that some of the most important events in the history of life have resulted directly from coevolutionary events. Among the most obvious are the origin of the eukaryotic cell through coevolution with symbionts that became mitochrondria, the origin of plants through the symbiosis that produced chloroplasts, and the development of the gut systems of many animal species through coevolved symbioses with microbial taxa. Coevolution also seems to play a crucial role in other fundamental processes such as the mainte-* E-mail: jnt@wsu.edu. Am. Nat. 1999. Vol. 153, pp. S1-S14. ᭧ 1999 by The University of Chicago. 0003-0147/99/5305S-0001$03.00. All rights reserved. nance of sexual reproduction and the dynamics of some genetic polymorphisms. Examples of coevolution continue to proliferate (Thompson 1994; articles in this issue), and protocols are being developed for evaluating reciprocal selection within natural communities We have three goals in this volume: to illustrate how ongoing coevolution can be studied across natural communities, to evaluate what we currently know about the dynamics of coevolution across landscapes, and to suggest specific hypotheses on the geographic mosaic of coevolution. We ask, using a variety of approaches, how the geographic structure of coevolution produces broad ecological patterns and how coevolution contributes to the ongoing organization of the earth's biodiversity. If geographic structure is important to the coevolutionary process, it must be possible to show that it creates patterns and dynamics different from what we have observed simply through studies of local communities or the branching patterns of interacting lineages. Moreover, it must be possible to show that the geographic processes and patterns are testable. In this article, I introduce the problem and suggest how the framework of the geographic mosaic of coevolution provides specific hypotheses on the coevolutionary process and its ecological effects that can be tested in natural communities. The subsequent articles show the range of approaches being taken to evaluate the coevolutionary process within a broad geographic framework.