Difference between revisions of "Symbiosis" - New World Encyclopedia

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[[image:Common_clownfish.jpg|thumb|250px|right|Common [[Clownfish]] (Amphiprion ocellaris) in their magnificent sea anemone (Heteractis magnifica) home.]]
 
[[image:Common_clownfish.jpg|thumb|250px|right|Common [[Clownfish]] (Amphiprion ocellaris) in their magnificent sea anemone (Heteractis magnifica) home.]]
'''Symbiosis''' (pl. symbioses) (from the [[Greek language|Greek]] words ''syn'' = con/plus and ''biono'' = living) is an [[Biological interaction|interaction]] between two organisms living together in more or less intimate association or even the merging of two dissimilar organisms.
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'''Symbiosis''' (pl. symbioses) is the intimate and interactive association (living together) of two or more species.  The term symbiosis  had been coined by German de Barry in 1879 from Greek words: ''sym'' = sum or plus and ''bios'' = life = living together. In the narrowest sense of the term, symbiosis has been defined as interactive association of two species in a mutually beneficial relationship. However, according to the original concept of de Barry, symbiosis is the living together of two different species and it can range from neutralism to parasitism.
The term [[Host (biology)|host]] is usually used for the larger (macro) of the two members of a symbiosis. The smaller (micro) member is called the '''symbiont''' (plural: '''symbionts'''), or alternately, '''symbiote''' (plural: '''symbiotes''').  When a microscopic symbiont lives inside the cells of a host, it is referred to as an [[endosymbiont]].
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In a symbiosis, the larger partner (macro-species) is known as 'host' whereas the smaller partner (micro-species) is known as the '''symbiont''' (plural: '''symbionts'''), or alternately, '''symbiote''' (plural: '''symbiotes''').  . For example, in the case of cattle and cattle egret, cattle is host and the bird cattle egret is the symbiont as it feeds on the ticks and other ectoparasites of the former. When a microscopic symbiont lives inside the cells of a host, it is referred to as an [[endosymbiont]].
  
 
In nature, we don't find any organisms having nothing to do with other organisms and physical factors of their environment. They are not made to live alone in isolation. It is not only that they interact for their own survival, but also to contribute to others. Thus, the existence of every organism has significant value for the wholesome of the ecosystem it is living in. One of such inter-relationships is expressed in the form of a food web, i.e., the relationship of eating and being eaten. In this relationship also, we find the dual purposes of each being: purpose of oneself (eating) and the purpose of wholesome (being eaten). To observe such inter-relationships among the organisms of different categories more closely, we can explore the phenomenon of symbiosis.
 
In nature, we don't find any organisms having nothing to do with other organisms and physical factors of their environment. They are not made to live alone in isolation. It is not only that they interact for their own survival, but also to contribute to others. Thus, the existence of every organism has significant value for the wholesome of the ecosystem it is living in. One of such inter-relationships is expressed in the form of a food web, i.e., the relationship of eating and being eaten. In this relationship also, we find the dual purposes of each being: purpose of oneself (eating) and the purpose of wholesome (being eaten). To observe such inter-relationships among the organisms of different categories more closely, we can explore the phenomenon of symbiosis.
The term symbiosis (pleural: SYMBIOSES) had been coined by German de Barry in 1879 from Greek words: ''sun'' = syn; ''bios'' = life or ''sumbios'' = living together. In the narrowest sense, it has been defined as interactive association of two species (unit of taxonomy) for mutual benefits. However, according to the original concept of de Barry, it can be defined as the intimate and interactive association (living together) of two or more species ranging from neutralism to parasitism.
 
In a symbiosis, the bigger partner (macro-species) is known as 'host' whereas the smaller partner (micro-species) is known as 'symbiont' or 'symbiote'. For example, in the case of cattle and cattle egret, cattle is host and the bird cattle egret is the symbiont as it feeds on the ticks and other ectoparasites of the former.
 
 
  
  

Revision as of 21:31, 26 March 2006

Common Clownfish (Amphiprion ocellaris) in their magnificent sea anemone (Heteractis magnifica) home.

Symbiosis (pl. symbioses) is the intimate and interactive association (living together) of two or more species. The term symbiosis had been coined by German de Barry in 1879 from Greek words: sym = sum or plus and bios = life = living together. In the narrowest sense of the term, symbiosis has been defined as interactive association of two species in a mutually beneficial relationship. However, according to the original concept of de Barry, symbiosis is the living together of two different species and it can range from neutralism to parasitism.

In a symbiosis, the larger partner (macro-species) is known as 'host' whereas the smaller partner (micro-species) is known as the symbiont (plural: symbionts), or alternately, symbiote (plural: symbiotes). . For example, in the case of cattle and cattle egret, cattle is host and the bird cattle egret is the symbiont as it feeds on the ticks and other ectoparasites of the former. When a microscopic symbiont lives inside the cells of a host, it is referred to as an endosymbiont.

In nature, we don't find any organisms having nothing to do with other organisms and physical factors of their environment. They are not made to live alone in isolation. It is not only that they interact for their own survival, but also to contribute to others. Thus, the existence of every organism has significant value for the wholesome of the ecosystem it is living in. One of such inter-relationships is expressed in the form of a food web, i.e., the relationship of eating and being eaten. In this relationship also, we find the dual purposes of each being: purpose of oneself (eating) and the purpose of wholesome (being eaten). To observe such inter-relationships among the organisms of different categories more closely, we can explore the phenomenon of symbiosis.


The various forms of symbiosis include: -

  • parasitism, in which the association is disadvantageous or destructive to one of the organisms and beneficial to the other (+ -)
  • mutualism, in which the association is advantageous to both (+ +)
  • commensalism, in which one member of the association benefits while the other is not affected (+ 0)
  • amensalism, in which the association is disadvantageous to one member while the other is not affected (- 0)

In some cases, the term symbiosis is used only if the association is obligatory and benefits both organisms. Symbiosis as defined in this article does not restrict the term to only the mutually beneficial interactions.

Types of Symbioses

A. Based on the occurrence of symbionts on the host

Two types of symbioses have been recognized:

1. Ectosymbiosis

In this case, the symbionts occur on the body surface of the host. The body surface can be external surface like - skin, or internal lining surfaces of digestive canal, respiratory tract, etc. Leech on the skin of vertebrate and Escherichia coli on the mucosal layer of intestine of man are some suitable examples.

2. Endosymbiosis

It is the case, where symbionts occur inside the cell (intracellular space) or in the tissue (intercellular space of the host. The classical example of endosymbiosis is Rhizobium sps. in the cells of root nodules of leguminous plants

B. Based on what the symbiotic partners get

Symbioses can be categorized into following types:
Table showing different types of symbiotic interactions depending on what the partners get - benefit, harm or none.
Benefit None Harm
+ 0 -
Benefit + Mutualism Commensalism Parasitism
None 0 Commensalism Neutralism
Harm - Parasitism
From the table, we can derived four categories of symbiotic relationships among organisms. They are:
  1. 0 0 Type (Neutralism)
  2. 0 + Type (Phoresis, Commensalism)
  3. + + Type (Mutualism)
  4. - + Type (Parasitism, Predation, Amensalism, Competition, Allelopathic interference)
Before discussing each of the categories in detail, it is necessary to point out the conclusion that we can draw from the table above. There is no symbioses known where there is harm to both partners or harm to one but without any benefits to another. This proves that the interactions among species are intentionally aimed at fulfilling dual purposes as mentioned above.

1. 0 0 Type: 'Neutralism'

The special term used for this 0 0 type category is neutralism. This term has been used in ecology to signify the biological interaction in which the population density of one species has absolutely no effect whatsoever on the other. Examples given for this symbiosis are pelicans and cormorants feeding in the ocean, spider and mantis preying insects of the same bush, numerous songs birds feeding and nesting together in a woodland, large number of microorganisms that normally act as residents of different body organs of human beings such as skin, nose, mouth, etc.
When you critically evaluate each of the examples of neutralism on the basis of ecological principles, then you will come to the conclusion that neutralism is unlikely to exist and is impossible to prove. Because, living together always involves at least some degree of positive or negative impact on the partner. Thus, living together is a purposeful interaction of different degree of intimacy.

2. 0 + Type

In this symbiosis, the host is said to experience neither benefit nor harm, whereas the symbiont gets benefit ranging from a simple transport to protection, food and shelter.
Phoresis (Phoresy)
It is the simplest form of 0 + type of symbiosis occurring mainly among the species sharing the same ecological niche. This relationship is not species specific. By chance, the two involved organisms come in contact, establish fairly loose mechanical association and the smaller one (specially known as phoront) is carried by the transport host (mechanical vector) indifferently. Food and water borne disease organisms (virues, bacteria, protozoans and multicellular parasites) are transported from their point of sources to the destination by flies and other insects because of direct contacts. Fungi and free living nematodes are carried by dung beetles as they share the same common niche. Remosa (sucker fish) is equipped with a sucker disk on the dorsal side of its head. With the help of the disc, the fish attaches itself to the belly of larger fish so as to get benefit of faster movement of the latter. Because it can also get some food scraps of the larger fish, this association can also be considered as commensalism.
Commensalism
This term was coined to mean "eating at the same table" type of living together for food. It is, therefore, a loose, usually facultative (neither dies if the relationship ends) and interspecific association between a larger 'host' and a smaller 'commensal' without any metabolic dependency. Here, only commensal derives benefit of sharing space, substrate, defense, shelter, transport and/or food without helping or harming the host. In some cases, the interaction is obligatory for the commensal. Escherichia coli, and a variety other of bacteria are harbored
File:Oncidium-longipes.jpg
An orchid plant perched on a tree
comfortably in the large intestine of man to feed on anyway out going food rest. There is no obvious harm or benefit to the host. Entamoeba gingivalis is also a commensal in human mouth. Examples of metazoan commensals are barnacles on whale, epiphytes on tree, etc. Sedentary adult barnacles by attaching itself on whale or any other moving objects get renewed sources of food and oxygen supply without giving any trouble to the carrying host. Similarly, epiphytic plants like orchids and bromeliads grow perched on sturdier trees for better exposer to sunlight and for absorbing moisture and nutrients from air.
Even though enumerable examples of commensalism have been recognized, however, when you inspect them more closely, then you may conclude that pure commensal relationship tends to be rather rare. Pin worm in the large intestine of human being has been considered as a commensal, but at the time of its reproduction, it intentionally causes itching at the anal region. Shrimp is a commensal on sea cucumber for passive transport. From time to time, it leaves the host for food. But it has been pointed out that the commensal may feed on the parasites of the host body surface and also provide protection to the host because of its warning coloration. Thus, commensalism seems to be, in reality, either mutualism or parasitism.

3. + + Type: 'Mutualism'

It is the species specific interaction of two organisms for mutual support and help. Sometimes, we come across the instances where the term symbiosis has been used specifically for this + + type of association synonymously to mutualism. However, considering the overall interspecific associations, Mutualism is the most appropriate term for this type of symbiosis. It is a loose to intimate and facultative to obligatory interaction, in which both the involved partners are called 'mutuals'. When the relationship is intimate and obligatory, each mutual is physiologically (metabolically) dependent to another for the survival and reproduction, it means that one derives benefit of another with respect to developmental stimuli, nutrition, digestive enzyme and maturation factors, so neither can survive and reproduce successfully in absence of another partner. Examples of mutualism can be categorized as follow:
Nutritional Symbioses
Both mutuals contribute to each other either simple organic nutrients, inorganic minerals or digestive enzymes or digestion process. Some important nutritional symbioses are -
In nitrogen fixation, Rhizobium sps. of root and/or shoot nodules of legumes fix atmospheric nitrogen to make it available for plant, whereas the plant provides glucose and other organic nutrients as well as biological environment to the bacteria.
Mycorrhiza (fungus-root) has been defined as an apparent underground structure developed as a result of mutualistic association between fungi (mainly basidiomycetes) and plant root of almost all plant species. There are different degrees of complexity in association. Plants benefit of having induced root development and increased absorption of water and minerals. Fungus partner in return gets organic food supply from the plant. So mycorrhiza can be used to grow plants in efficient way, even in poor soil condition
Syntrophy is the obligatory association of two microorganisms for mutual production of biochemical substances what another partner need for its physiological processes. For example, sulfur reducing bacterium, Desulfuromonas acetoxidans need sulfur for oxidizing acetic acid so as to produce carbon dioxide, hydrogen sulfide and energy. Its partner, green photosynthetic bacterium (Chlorobium), in the same culture constantly use the provided hydrogen sulfide for its photosynthesis to make the first one continuous supply of sulfur. Based on the principle of syntrophy, effective microorganism (EM) preparation is in use for enhancing all sort of microbial processes.
About 50 percent of the food stuff of ruminants is composed of cellulose, for the digestion of which, they have to take assistant from protozoans (ciliates like Diplodinium sp. and Entodinium sp.) and obligate anaerobic bacteria (Ruminococcus albus, R. flavefaciens, etc.) lodged in rumen (a special compartment) of their stomach. Because of anaerobic milieu, the bacterial fermentation produces different fatty acids, carbon dioxide and hydrogen gas, which are in turn used by methanogenic bacteria (Methanobacterium ruminantium)present there to produce methane. Thus, ruminants are the important source of methane in the atmosphere. Like the ruminants, termites and others harbor protozoan (flagellates) and bacterial flora in gastrointestinal (GI) tract for digestion of cellulose.
Lichen is the very common example of nutritional symbiosis and is the obligate
File:Lichen02.jpg
Lichen over the rock
and intimate but ectosymbiotic association of a fungus (especially ascomycetes and basidiomycetes) and an alga (green or blue green algae). The thallus (body proper) is mainly made of close network of fungal mycelium (mycobiont) and is responsible for absorption of water, minerals and gases and to form protective tissue structure for algal partner. Algal cells (phycobiont) constitute about 10 percent of the thallus, are embedded in the compact mass of mycelium and are responsible for photosynthesis, nitrogen fixation and production of food for both partners.
Cleaning Symbioses
These are the facultative and loose associations of mutuals based on the principle that one's parasites are someone's food. Here, therefore, the larger mutual cooperates with and allows smaller mutual to remove its ectoparasites like leech, ticks, etc. even from gills, inside of mouth and nostrils. Some of the cleaning partners are crocodile and crocodile bird (Egyptian Plover), larger fishes and crustaceans or fishes, shark and remosa fish, buffalo and oxpecker, rhino and tick bird, cattle and cattle egret.
Transport Symbioses
In the transport symbiosis, one mutual takes advantage of fast and secure movement of another mutual, whereas the latter get mainly the cleaning services. Remosa and shark, and shrimp and sea cucumber have already been described above. Typical example of this association is torsalo fly (Dermatobia hominis) of New World Tropics making use of small insect for transport of its eggs to the vertebrate host, including man. The female parasitizes small flies and mosquitoes and glues its eggs underneath their bodies and let them free to reach to the hosts. Upon contact with the warm skin of host, larva comes out of egg, penetrates the skin and develops underneath the skin of the host. This association can also be considered as a type of parasitism.
Pollination symbioses
Rufous humming bird on a flower
Pollination symbiosis refer to the mutual benefit exchanged between flower visiting small birds, insects like butterfly, bee, etc. and flower bearing plants, as the former while getting nectar cause pollination in the latter.
Defense Symbioses
An example of defense symbiosis is the relationship
clownfish & sea anemone
between clownfish of the genus Amphiprion (family: Pomacentridae) that dwell among the tentacles of tropical sea anemones. The territorial fish protects the anemone from anemone-eating fish, and in turn the stinging tentacles of the anemone protect the anemone fish from its predators (a special mucus on the anemone fish protects it from the stinging tentacles). Some goby fish species live in symbiosis with a shrimp. The
shrimp and goby fish
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 of imminent danger. When that happens both the shrimp and goby fish quickly retract into the burrow.
From the survey of biological world, one can be convinced that there are enumerable cases of viable mutualistic associations enabling organisms to explore diverse ecological niches. Hence, the mutualism has enable organisms even to colonize stressful habitats and survive successfully even at self-insufficiency, thus promoting the efficient and optimal use of biological features as well as natural resources together.

4. - + Type

This association involves obvious harm in one partner for the benefit of another. This type of relationship occurs in many forms.
Parasitism
It may be defined as an interspecific association in which one species, the 'parasite', lives on or in a second species, the 'host', for a significant period of its life and exploits the host to obtain nourishment, shelter and/or protection. This relationship can be loose or mostly intimate, and for the parasite, it is usually obligatory. As the parasite has to break through the different layers of defense mechanism of the host for localizing on the host, establishing in the host as well as for growth and reproduction in the host, the parasite must be equipped with specific mechanism too. Therefore, the relationship is strongly species specific, i.e., particular parasite can infect only particular host, not any other. The parasite causes harm to the host in many ways, like by depriving of digested food, erecting mechanical blockade of food, blood, lymph, bile passages, causing tissue damages by rupture, enzymatic digestion, induction of autolysis or secretion of toxin.
Types of Parasites:- Macroparasites are multicellular parasites visible to naked eyes, e.g., helminth parasites. Mesoparasites are the ones which penetrate the external openings and infect in like buccal cavity, cloaca, external ear. Microparasites are unicellular and invisible to naked eyes, e.g., protozoan parasites. Ectoparasites, like leech, ticks, lice are macroparasites occurring on the body surface of the host for shorter or longer period of time. Endoparasites, on the other hand, enter the body of the host via different means and remain in specific site permanently or for longer period of time, e.g., intestinal parasites, blood parasites. Temporary parasites (leech, bed bug) visit its host only for a short period of time. Permanent parasites spent whole or a part of their life cycle in the host. Facultative parasites need association of host, but can also survive without parasitic mode of life. So, they are opportunistic. For example, parasitic nematode, Strongyloides stercoralis, can also be free living. Majority of parasites are obligatory parasites and are totally dependent on the host for food, shelter and/or protection, so they cannot survive without host. Accidental parasites are those which happen to infect unusual host, other than normal definite host. Wandering or aberrant parasite instead of arriving at the site of infection in the definitive host reach unusual place as dead end and becoming unable to complete the life cycle. For example, tape worm (Taenia solium) reach to brain and remains there unless operated out. When a parasite is being parasitized by another organism, then the latter one is known as hyperpasasite or secondary parasite. It can be used for biological control of the pest and parasites. Monogenic parasites complete the whole life cycle in one host, for example, Entamoeba histolytica, but the digenetic parasite needs, in addition to primary host, also secondary host to complete the whole life cycle. Plasmodium vivax (malaria parasite) complete its asexual part of life cycle in man and the sexual part in female Anopheles mosquito.
Types of Hosts:- Definitive host is usually the main host. For digenetic parasites, it is the host for adult stage and for the completion of sexual part of life cycle. Intermediate or secondary host occurs only in the case of digenetic parasite for the completion of larval stage, asexual reproduction and for transmission to the definitive host. Accidental host is the one which can function as normal host but is infected only occasionally because of some reasons for example due to the lack of exposer or means of transmission. Permissive host is either definitive, intermediate or accidental host which allow parasite to complete its life cycle part or the whole. Non-permissive host, on the other hand, is host organism other than true definitive host but receiving the parasite in the position of definitive host so that the parasite find itself in a dead end. Paratenic host or transport host refer to host organism other than true intermediate host but receiving the parasite in the position of intermediate host so that the parasite are helped to go to the definitive host. For example Echinococcus granulosus normally passes to the dog through intermediate host like goat, sheep, etc. But the parasite instead of passing to intermediate host, may come to infect human being and remain in causing hydatiditis, as dog has no chance to get it from man. Reservoir host are permissive host alternative to definitive host so that infective stage can be passed from the host to the population of definitive host. Vector is usually the intermediate host playing active role in the transmission of the parasite.
In parasitism, there is always exploitation and harm. However, such an association has been continuing and there has been no record of species extinction because of parasitism. There are also examples where the parasitism help the host in fulfilling its purpose of life: existence and species continuation. Further, some parasitism result in something which has higher value than the both host and parasite together. Cordyceps sinesis is a fungus and infects in the head a type of underground caterpillar, in such a way that it finally projects itself out of the head giving the appearance of half fungi and half caterpillar. The whole body is now known as yarsagumba (gold rush) and is very much valuable in medicine as a vitalizing agent. Further, when we carefully analyze, we find that all the human parasitic diseases are the result of failure of human responsibility.
Predation
It is a relationship between two species but not an association and represents the obvious example of eating and being eaten process. For example, a tiger which hunts is called 'predator' and a deer which is being hunted is known as 'prey'. Even though, it is included in - + type of relationship, this relationship contribute for the well being of both predator and prey species. The predators get food and the preys are protected from mass collapse. This is a good example of individual sacrifice for species' existence and development.
Amensalism
In this interspecific interaction, one species of microorganism like Penicillium, by the secretion of antibiotics or antimicrobial metabolites inhibits the growth and development of other microorganisms, like bacteria, without being self affected. Because the first species avoids competitors of its resources in this way, this interaction is a - + type.
Competition
Interspecific competition is the relationship between the populations of two species resulting from utilization of same resources of the same place and at the same time. This interaction may not need the direct confrontation, but still seems that each is hampering the another. Because of this, some authors have catagorized competition in - - type. However, this is an interaction to set demarcation between species for coexistence or mutual survival, as every time at least one of the involved sides gets benefit in terms of species development in the dynamic environment.
Allelopathic Interference
This is similar to amensalism and competition. For example, desert plant, because of scarcity of nutrients and water, secrete certain chemicals and inhibits the seed germination of other plants in the vicinity. The interference can be both intraspecific as well as interspecific. Pine tree exhibits allelopathic interference by making the surrounding soil acidic.

Conclusion

There is no such categorization of symbioses in nature as described above. In fact, it is a continuum of different degree of intimacy and cooperation between the involved species. Therefore, demarcation line between the categories are artificial and many examples surpass the boundary, e.g., remosa and shark.

As it has been explained in the introduction, different species interact not only for their own benefit, but also to contribute to others. This is obvious in phoresis, commensalism and in mutualism. Mutual sharing reduces interspecific competition, avoids competitive exclusion as suggested Gause principle, promotes efficient use of resources as well as contributes to the dynamicity and diversity of the natural ecosystem, as more niches are created and new interactions are generated.

Even the - + type of associations or relationships should be considered as contribution to the existence of others even at one's own expense or species development at the expense of its individual instead of simply saying exploitation. If the exploitation of one by another leads to something like yarsagumba, then we should consider it as dictated by the directive force of nature to maintain dynamicity and diversity and ultimately to serve the purpose of human being.

In case of human parasites, we can see that it is the result of coupling of human beings failure to fulfill their responsibility and organisms inherent tendency to colonize new niches. Person practicing good hygienic life style does not get intestinal parasites. People with sexual discipline need not worry about HIV infection.

Further references

General Information on Parasitology With Particular Reference to Parasitic Helminths

Symbiosis

Symbiotic Relationships


Symbiosis may be divided into two distinct categories: ectosymbiosis and endosymbiosis. In ectosymbiosis, the symbiont lives on the body surface of the host, including the inner surface of the digestive tract or the ducts of exocrine glands. In endosymbiosis, the symbiont lives either in the intracellular space of the host or extracellularly.

An example of mutual symbiosis is the relationship between clownfish of the genus Amphiprion (family, Pomacentridae) that dwell among the tentacles of tropical sea anemones. The territorial fish protects the anemone from anemone-eating fish, and in turn the stinging tentacles of the anemone protect the anemone fish from its predators (a special mucus on the anemone fish protects it from the stinging tentacles).

Some goby fish species live in symbiosis with a shrimp.

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 of imminent danger. When that happens both the shrimp and goby fish quickly retract into the burrow.

A famous land version of symbiosis is the relationship of the Egyptian Plover bird and the crocodile. In this relationship, the bird is well known for preying on parasites that feed on crocodiles which are potentially harmful for the animal. To that end, the crocodile openly invites the bird to hunt on his body, even going so far as to open the jaws to allow the bird enter the mouth safely to hunt. For the bird's part, this relationship not only is a ready source of food, but a safe one considering that few predator species would dare strike at the bird at such close proximity to its host.

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 Sagan (1986), "Life did not take over the globe by combat, but by networking". As in humans, organisms that cooperate with others of their own or different species often outcompete those that don't.

However, mutualism, parasitism, and commensalism are often not discrete categories of interactions and should rather be perceived as a continuum of interaction ranging from parasitism to mutualism. In fact, the direction of a symbiotic interaction can change during the lifetime of the symbionts due to developmental changes as well as changes in the biotic/abiotic environment in which the interaction occurs.

See also

  • List of symbiotic relationships


References
ISBN links support NWE through referral fees

  • Lynn Margulis and Dorion Sagan, Microcosmos: Four Billion Years of Evolution from Our Microbial Ancestors. Summit Books, New York, 1986. ISBN 0520210646
  • Jan Sapp Evolution by Association, Oxford University Press, 1994. ISBN 0195088212


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