In commensal interactions, one species benefits and the other is unaffected. The commensal organism may depend on its host for food, shelter, support, transport, or a combination of these.

One example of commensalism involves a small crab that lives inside an oyster’s shell. The crab enters the shell as a larva and receives shelter while it grows. Once fully grown, however, it is unable to exit through the narrow opening of the two valves, and so it remains within the shell, snatching particles of food from the oyster but not harming its unwitting benefactor. Another form of commensalism occurs between small plants called epiphytes and the large tree branches on which they grow. Epiphytes depend on their hosts for structural support but do not derive nourishment from them or harm them in any way.

Many other kinds of interaction, however, range from antagonism to commensalism to mutualism, depending on the ecological circumstances. For example, plant-feeding insects may have large detrimental effects on plant survival or reproduction if they attack small or nonvigorous plants but may have little or no effect on large or vigorous plants of the same species. Some human diseases may cause only temporary discomfort or be life-threatening, depending on the age and physical condition of the person.

No interaction between species fits neatly into the categories of antagonism, commensalism, or mutualism. The interaction depends on the genetic makeup of both species and the age, size, and physical condition of the individuals. Interactions may even depend on the composition of the community in which the interaction takes place. For example, the moth Greya politella pollinates the flowers of a small herb called the prairie star (Lithophragma parviflorum). The female moth pollinates while she lays eggs (oviposits) in the corolla of the flower. As she pushes her abdomen down into a flower, pollen adheres to her. She flies on to the next flower to lay more eggs, where some of the pollen rubs off onto the stigma of the flower, causing pollination to occur. Although this unusual pollination mechanism is very effective in some local populations, in other communities different pollinators such as bee flies and bees are so common that their visits to the flowers swamp the pollination efforts of the moths. As a result, pollination by the moths makes up a very tiny proportion of all the pollinator visits that occur within that community and probably has little effect on plant reproduction or natural selection. This moth, therefore, is a commensal in some populations and a mutualist in others, depending on the local assemblage of pollinator species.

The study of coevolution

To understand how coevolution shapes interactions within communities, researchers must distinguish between traits that have coevolved and those that were already present in ancestors before the interspecific interaction began. For example, hummingbirds use their wings and bills to reach the nectar within flowers. A hummingbird with a long bill may have evolved its bill as a result of coevolution with a particular species of flower; however, its wings are not the result of coevolution. Wings were already present in birds before hummingbirds evolved. Therefore, both the evolutionary ecology and the history (phylogeny) of the interacting species must be studied. The phylogeny indicates when each species arose within a lineage and when each new trait made its first appearance. The ecological studies can then show how each of those traits has been shaped by and used under different ecological conditions.

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The study of the coevolution between moths of the family Prodoxidae and their host plants illustrates the interplay of phylogeny and ecology. Prodoxid moths include some species that have become major pollinators of plants. These pollinators include yucca moths (of the genera Tegeticula and Parategeticula) and Greya moths (see above Commensalism and other types of interaction). Greya moths inadvertently, or passively, pollinate the flowers they visit, but their close relatives the yucca moths purposely, or actively, perform this function. Female yucca moths collect and carry pollen on specialized appendages attached to their mouthparts. They visit yucca flowers to lay their eggs in the floral ovary, and their offspring feed on the developing seeds. While visiting each flower, a female moth takes some of the pollen she is carrying and places it directly on the stigma of the flower. Her offspring are therefore guaranteed developing seeds on which to feed. The yuccas have evolved to depend solely on these moths for pollination. Unlike many other plant species, they do not produce nectar or any other reward for pollinators and so do not waste energy to attract pollinators. They lose some of their seeds to the yucca moth larvae, but this is the cost of coevolution with this highly efficient pollinator.

Phylogenetic studies have shown that the loss of nectar production in yuccas and the evolution of active pollination in yucca moths are novel traits that have arisen through coevolution, as the relatives of yuccas produce nectar, and the relatives of yucca moths, the Greya moths, do not actively pollinate their host plants. Some other aspects of the interaction make use of traits that did not coevolve between yuccas and yucca moths. Instead, the traits were present in ancestors. Laying eggs in flowers and local specialization to one plant species are two traits that are common to all the close relatives of yucca moths, regardless of the plants on which they feed.

Therefore, by combining ecological and phylogenetic studies, researchers can piece together the history of coevolution between these species. The coevolved mutualism between yuccas and yucca moths began when their ancestors inadvertently became more successful at survival and reproduction as a result of their interactions. Yuccas that did not waste energy on nectar production to attract other pollinators achieved an advantage over those plants that did; yucca moths that ensured the availability of developing seeds for their offspring by actively pollinating the flowers in which they laid their eggs also gained an advantage over populations that did not do so. The process undoubtedly involved many other twists and turns along the way, but the combination of evolutionary ecological and phylogenetic studies allows at least part of the coevolutionary process to be reconstructed.

All coevolved interactions are similar to those between yuccas and yucca moths in that natural selection operates on traits that are already present within species, molding them in new ways by favouring new mutations that fine-tune the relationship. Tinkering rather than engineering is how the biologist François Jacob described the process of evolution, and his analogy certainly extends to the coevolutionary process. Coevolved interactions are not designed from scratch for maximum efficiency. Instead, evolution fiddles with existing structures and behaviours and adapts them to perform new functions—in effect, jury-rigging them. Consequently, the organization of biological communities reflects this makeshift nature of adaptation and coevolution.

The coevolutionary “arms race” versus reduced antagonism

Nothing is absolutely predictable about the direction of coevolution. How an interaction coevolves depends not only on the current genetic makeup of the species involved but also on new mutations that arise, the population characteristics of each species, and the community context in which the interaction takes places. Under some ecological conditions, an antagonistic interaction between two species can coevolve to enhance the antagonism; the species “build up” methods of defense and attack, much like an evolutionary arms race. Under other ecological conditions, however, the antagonism may be lessened, resulting in reduced antagonism.