Symbiosis refers to intimate relations between life forms that involve strong interdependence. Some well-known examples are the associations between flowers and bees, fungi and tree roots, the lichen symbiosis between fungi and algae, and even the mitochondria that make chemical energy for eukaryotic cells. In this list, the examples go from relatively loose associations to ones that are ever more tightly coupled. While the flower and the bee are interdependent products of co-evolution, they are still distinct and distinguishable organisms. For some biologists, including Lynn Margulis and Dorion Sagan (2002, 18), they do not count as examples of symbiosis because they do not live in close or “obligate” contact. The wasp can find other sources of food; the orchid can be pollinated by other species. But mycorrhizae, lichens, and mitochondria are much more integrated. Lichens, the example for which biologists first used the term symbiosis in the late 19th century...

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