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In biology, symbiosis is the generalized term for any intimate relationship between two or more organisms. Symbiosis is often used erroneously to refer to a mutually beneficial relationship, but this is a misuse of the term.
Types of symbiosis
Symbiotic relationships can be broken into three major types. However, it is important to note that these are artificial distinctions. In reality, symbiotic relationships exist on a continuum, and one type of relationship might transition to another type if conditions change.
Mutualistic relationships are those in which both partners benefit from the interaction. In some cases, this symbiosis is obligative (necessary for survival), while in others it may be facultative. Depending on the history of the relationship, the two may become so close that they are indistinguishable from each other (see endosymbiosis).
Perhaps the most famous example of a mutualistic relationship is that between clownfish (subfamily Amphiprioninae) and sea anemones (order Actiniaria). However, this mutualism is controversial within marine biology, because the gains for the anemone are less than clear. Better examples include the interaction between corals and zooxanthellae (photosynthetic algae), between cleaner shrimp or cleaner fish and larger fish congregating at cleaning stations, or the relationship between plants and some fungi within their roots (mycorrhizal relationships).
A commensal relationship is one in which one partner benefits but the other is not affected. There are many examples of this, and the clownfish/anemone example above might arguably be commensalism (because the anemone does not always gain from the relationship). Many epiphytes (plants which attach themselves to other organism's surface) are commensal. Commensalism may be considered the middle of the symbiotic continuum.
In a parasitic relationship, one partner benefits at the cost of the other partner. Examples of this abound in nature, and include malaria, all viral life, and many other parasites. Parasitic relationships often vary in the amount of harm done to their host (remember, it's a continuum), with some parasites going as far as killing their host outright. Some apparent parasites may actually have non-obvious benefits for the host. For example, exposure to helminths (parasitic worms) has been shown to suppress asthma and allergies in humans.
The type of relationship can change between these three categories. For instance, E. coli would be considered a mutualistic partner with humans, but it can become parasitic if the hosts immune system is weakened. Richard Dawkins suggests that the relationship's place on the continuum is largely related to whether or not the partners' genes share a common reproductive pathway.
Symbiosis and evolution
Creationists and their ilk commonly complain that symbiosis (though they mean mutualism) cannot be explained by evolution, because the two organisms would not be able to survive without each other. In reality, symbiotic relationships are actually very strong evidence for evolution. In fact, most symbiotic relationships display high degrees of co-speciation, as would be expected from evolutionary theory's predictions (in other words, the two organisms did evolve together).
Another problem with using symbiosis as an argument against evolution is that it assumes that the partners in such relationships cannot survive without each other. In almost every case, this is simply not true; otherwise there would be no non-symbiotic life. Additionally, examples of obligative mutualism are ancient and display high degrees of co-speciation, indicating that the necessary nature of the relationship is a derived characteristic, not a pre-existing one.