SYMBIOSIS

Some of the best examples of adaptations arising through coevolution come from two different species living in continuing, intimate associations, called symbiosis (Gr. sym, together bio, life). Such interspecific interactions influence the species involved in dramatically different ways. In some instances, one member of the association benefits, and the other is harmed. In other cases, life without the partner would be impossible for both.
Parasitism is a common form of symbiosis in which one organism lives in or on a second organism, called a host. The host usually survives at least long enough for the parasite to complete one or more life cycles. The relationships between a parasite and its host(s) are often complex. Some parasites have life histories involving multiple hosts. The definitive or final host is the host that harbors the sexual stages of the parasite. A fertile female in a definitive host may produce and release hundreds of thousands of eggs in its lifetime. Each egg gives rise to an immature stage that may be a parasite of a second host. This second host is called an intermediate host, and asexual reproduction may occur in this host. Some life cycles may have more than one intermediate host and more than one immature stage. For the life cycle to be completed, the final immature stage must have access to a definitive host. Many examples of coevolutionary interactions between host and parasite are cited in Part Two of this text.

Commensalism is a symbiotic relationship in which one member of the relationship benefits, and the second is neither helped nor harmed. The distinction between parasitism and commensalism is somewhat difficult to apply in natural situations. Whether or not the host is harmed often depends on such factors as the host’s nutritional state. Thus, symbiotic relationships may be commensalistic in some situations and parasitic in others.

Mutualism is a symbiotic relationship that benefits both members. Examples of mutualism abound in the animal kingdom, and many examples are described elsewhere in this text.

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