Coevolution and Symbiosis
The plants, animals, protist, fungi, and bacteria that live together in communities have changed and adjusted to one another continually over a period of millions of years. for example, many feature of flowering plants have evolved in relation to the dispersal of the plant's gametes by animals. these animals, in turn have evovlved a number of special traits that enable them to obtain food or other resources effeciently from the plants they visit, often from their flowers. while doing so, the animals pick up pollen, which they may deposit on the next plant they visit, or seeds, which may be left elsewhere in the environment, sometimes a great distance from the parental plant.
Such inretactions which involve the long-term, mutual evolutionary adjusment of the characteristic of the members of biological communities, are example of coevolutio, a phenomenon we have already seen in predator-prey interactions.
Symbiosis is Widespread
Another type of coevolution involves symbiotic relationships in which two or more kinds of organism live together in often elaborate and more-or-less permanent relationships. all symbiotic relationships carry the potential for coevolution between the organism involved and in many instances the result of this coevolution are fascinating. examples of symbiosis include lichens which are associations of certain fungi with green algae or cyanobacteria. another important example are mycorrhizae, the association between fungi and the roots of most kinds of plants. the fungi expedite the plant's absorption of certain nutrients, and the plants in turn provide the fungi with carbohydrates. similarly, root nodules that occur in legumes and certain other kinds of plants contaian bacteria that fix atmospheric nitrogen and make it available to their host plants
In the tropics, leafcutter ants are often so abundant that they can remove a quarter or more of the total leaf surface of the plants in a given area. they do not eat these leaves directly; rather they take them to underground nests, where they chew them up and inoculate them with the spores of particular fungi. these fungi are cultivated by the ants and brought from one specially prepared bed to another, where they grow and reproduce. in turn, the fungi constitute the primary food of the ants and their larvae. the relationship between leaf-cutter ants and these fungi is an excellent example of symbiosis.
The major kinds of symbiotic relationship include (1) commensalism, in which one species benefits while the other neither benefits nor is harmed; (2) mutualism, in which both participating species benefits; (3) parasitism, in which one species benefits but the other is harmed. parasitism can also be viewed as a form of predation, although the organism that is preyed upon does not necessarily die.
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