Abstract
Two facets of symbiosis theory are discussed: the role of symbiosis in evolution and the evolution and ecology of symbiosis. Research on symbiosis developed in virtual conflict with the aims and doctrines of the major biological disciplines for most of the 20th century. To gain a better understanding of this aspect of biology, I briefly sketch some of the phenomena and issues that have shaped discourse over its scope and significance. Diverse explanations for the various relations exhibited by symbiosis have been proposed from the 19th century to the present. The usefulness of the word itself has been debated, and its anthropomorphisms have been discussed. Research on mycorrhizas has been paradigmatic for revealing the dynamic nature of symbiosis and the inherent complexities of cost–benefit accounting across the parasitism–mutualism continuum.Key words: ecology, evolution, parasitism, mutualism, mycorrhizas, symbiosis.