Biological symbiosis

The term symbiosis (from the Greek: συμ, sym, "with"; and βίοσίς, biosis, "living") can be used to describe various degrees of close relationship between organisms of different species. The term was first used in 1879 by the German mycologist, Heinrich Anton de Bary, who defined it as: "the living together of unlike organisms".

There is no single universally agreed upon definition of symbiosis. Some define symbiosis in the sense that De Bary intended, describing a close relationship between organisms in which the outcome for each is highly dependent upon the other. The relationship may be categorized as mutualism, parasitism, commensalism, or any biological interaction in which at least one organism benefits. Others define it more narrowly, as only those relationships from which both organisms benefit, in which case it would be synonymous with mutualism.

Symbiotic relationships may involve an organism living on another (ectosymbiosis), inside another (endosymbiosis), or organisms related by mutual stereotypic behaviours. Further, symbiotic relationships, may be either obligate, which is to say necessary to the survival of at least one organism, or facultative, where the relationship is useful but not vital.

Physical interaction
Endosymbiosis, is any sybiotic relationship in which the symbiont lives within the tissues of the host; either in the intracellular space or extracellularly. Examples are nitrogen-fixing bacteria (called rhizobia) which live in root nodules on legume roots, single-celled algae inside reef-building corals, and bacterial endosymbionts that provide essential nutrients to about 10%–15% of insects.

Ectosymbiosis, also referred to as exosymbiosis, is any symbiotic relationship in which the symbiont lives on the body surface of the host, including the inner surface of the digestive tract or the ducts of exocrine glands. Examples of this include ectoparasites such as lice, commensal ectosymbionts, such as the barnacles that attach themselves to the jaw of baleen whales, and mutualist ectosymbionts such as cleaner fish.

Mutualism
The term Mutualism, describes any relationship between individuals of different species where both individuals derive a fitness benefit. Generally only lifelong interactions involving close physical and biochemical contact, can properly be considered symbiotic. Mutualistic relationships, may be either obligate for both species, obligate for one but facultative for the other, or facultative for both. Many biologist restrict the definition of symbiosis to close mutualist relationships.

A large percentage of herbivores have mutualistic gut fauna that help them digest plant matter, which is more difficult to digest than animal prey. Coral reefs are the result of mutualisms between coral organisms and various types of algae that live inside them. Most land plants and land ecosystems rely on mutualisms between the plants which fix carbon from the air, and Mycorrhyzal fungi which help in extracting minerals from the ground.

Another example is the goby fish, which sometimes lives together with a shrimp. The shrimp digs and cleans up a burrow in the sand in which both the shrimp and the goby fish live. The shrimp is almost blind leaving it vulnerable to predators when above ground. In case of danger the goby fish touches the shrimp with its tail to warn it. When that happens both the shrimp and goby fish quickly retract into the burrow.

One of the most spectacular examples of obligate mutualism is between the siboglinid tube worms and symbiotic bacteria that live at hydrothermal vents and cold seeps. The worm has no digestive tract and is solely reliant on their internal symbionts for nutrition. The bacteria oxidize either hydrogen sulfide or methane which the host supplies to them. These worms were discovered in the late 1980s at the hydrothermal vents near the Galapagos Islands and have since been found at deep-sea hydrothermal vents and cold seeps in all of the world's oceans.

Commensalism
Commensalism describes a relationship between two living organisms where one benefits and the other is not significantly harmed or helped. It is derived from the English word commensal, meaning the sharing of food, and used of human social interaction. The word derives from the Latin com mensa, meaning sharing a table.

Commensal relationships may involve an organism using another for transportation (phoresy), for housing (inquilinism), or it may also involve an organism using something another created, after the death of the first (metabiosis). An example is the hermit crabs that use gastropod shells to protect their bodies.

Parasitism
A parasitic relationship is one in which one member of the association benefits while the other is harmed. Parasitic symbioses take many forms, from endoparasites, that live within the hosts body, to ectoparasites that live on its surface. In addition, parasites may be necrotrophic, which is to say they kill their host, or biotrophic, meaning they rely on their host surviving. Biotrophic parasitism is an extremely successful mode of life. Depending on the definition used, as many as half of all animals have at least one parasitic phase in their life cycles, and it is also frequent in plants and fungi. Moreover, almost all free-living animals are host to one or more parasite taxa.

Symbiosis and evolution
While historically, symbiosis has received less attention than other interactions such as predation or competition, it is increasingly recognised as an important selective force behind evolution, with many species having a long history of interdependent co-evolution. In fact the evolution of all eukaryotes (plants, animals, fungi, protists) is believed to have resulted from a symbiosis between various sorts of bacteria.

Symbiogenesis
The biologist Lynn Margulis, famous for the work on endosymbiosis, contends that symbiosis is a major driving force behind evolution. She considers Darwin's notion of evolution, driven by competition, as incomplete, and claims evolution is strongly based on co-operation, interaction, and mutual dependence among organisms. According to Margulis and Dorion Sagan, "Life did not take over the globe by combat, but by networking."

Co-evolution
Symbiosis played a major role in the co-evolution of flowering plants and the animals that pollinate them. Many plants that are pollinated by insects, bats or birds, have very specialized flowers modified to promote pollination by a specific pollinator that is also correspondingly adapted. The first flowering plants in the fossil record had relatively simple flowers. Adaptive speciation quickly gave rise to many diverse groups of plants, and at the same time, corresponding speciation occurred in certain insects groups. Some groups of plants developed nectar and large sticky pollen while insects evolved more specialized morphologies to access and collect these rich food sources. In some taxa of plants and insects the relationship has become dependent, were the plant species can only be pollinated by one species of insect.

Objections
Creationists have long claimed that obligate symbioses are evidence against evolution, arguing that since neither organism can survive without the other, they must have come into existence at exactly the same time. This simplistic point of view ignores the extreme variety of symbiotic relationships as well the mutability of species over time. Obligate mutualisms could easily evolve from facultative relationships in which neither species is fully committed. These arguments persist despite many examples of facultative symbioses and multiple theoretical and computational models describing how such a relationship would evolve.