Mutualism is sometimes termed true symbiosis – each interdependent party gains. Mutualism is a product of coevolution: a process of evolving mutual dependency.
Symbioses are pervasive throughout the tree of life. ~ Canadian evolutionary biologist Jeffrey Joy
Most people think symbiosis means there is an evolution toward harmony, a perfect balance of the two partners. ~ Italian evolutionary biologist Vittorio Boscaro
Endosymbiosis allows hosts to acquire new functional traits, such that the combined host and endosymbiont can exploit vacant ecological niches and occupy novel environments; consequently, endosymbiosis affects the structure and function of ecosystems. However, for many endosymbioses, it is unknown whether their evolutionary basis is mutualism or exploitation. ~ English ecologist Christopher Lowe
Eukaryotes arose through coevolution between an archaeal host and a bacterial endosymbiont. The original nature of the association between the two is unclear. It may not have been mutually beneficial.
What we have always thought of as mutualism, where species gain mutual benefit from interacting with each other, might actually be based on exploitation, where one species gains by capturing and then taking resources from another. ~ Christopher Lowe
Euplotes is a single-celled, ciliate protozoan that incorporates a freshwater bacterium as an endosymbiont: Polynucleobacter. These bacteria live within Euplotes for a time, but the relationship does not last.
They’re being replaced, just like you change your clothes. You take advantage of your symbiont until it’s no longer useful, and then you get a new one. The relationship is more like death row than cooperation; sure, the symbiont is kept safe and well-fed in the short term, but ultimately it’s not a good place to be. ~ Canadian microbiologist Patrick Keeling
Microbes are major mutualists. A ubiquitous mutualism is between digestive bacteria and their animal hosts. The mutual relations between plants and nutrient-providing soil microbes are analogous.
Among themselves, numerous plants and animals have mutually beneficial relations. Plant pollination by insects or birds is exemplary.
Mutualistic networks such as plant-pollinator communities are “nested.” Specialist pollinator species visit plant species that are subsets of those visited by more generalist pollinators. Nestedness prevails because it stabilizes mutualistic networks. ~ Indian biologist Samraat Pawar