Mutualism

Mutualism is an obligatory and interdependent interaction that benefits both of the participating species. Lichens are a classic example: a lichen represents an intimate mutualistic relationship between a fungus and a photosynthetic partner, either an alga or a cyanobacterium, visible in a vertical transverse section (Fig. 13.7) as fungal tissue enclosing a distinct layer formed by its algal or cyanobacterial partner. The most spectacular and evolutionarily interesting examples of mutualism, however, are found in plant-animal relationships. Plants require the help of animals to pollinate their flowers and to disperse their seeds, and in return the animals are rewarded -- with pollen and nectar for pollinators, and with juicy, nutritious fruit for seed dispersers. Such relationships very often show co-evolution, meaning that the flower and its specific pollinator species evolve their structures in tandem, tightly linked to one another over time -- the long, curved bill of a hummingbird matching the shape of the tubular flower it feeds from (Fig. 13.8) is a good illustration. A m …