Coevolution

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Coevolution

Species are said to "coevolve" when their respective levels of fitness depend not only on their own genetic structure and adaptations but also the development of another species as well. The gene pool of one species creates selection pressure for another species. Although the changes are generally reciprocal, they may also be unilateral and still be considered coevolutionary.

The process of coevolution arises from interactions that establish structure in communities. A variety of different types of interactions can occur–symbiotic, where neither member suffers, or parasitic, predatory, and competitive relationships, where one member of a species pair suffers.

Coevolution can result from mutually positive selection pressure. For example, certain plants have in an evolutionary sense created positive situations for insects by providing valuable food sources for them. In return the insects provide a means to distribute pollen that is more efficient than the distribution of pollen by wind. Unfortunately, the plant and the insect species could evolve into a position of total dependency through increased specialization, thus enhancing the risk of extinction if either species declines.

Coevolution can also arise from negative pressures. Prey species will continually adapt defensive or evasive systems to avoid predation. Predators respond by developing mechanisms to surmount these defenses. However, these species pairs are "familiar" with one another, and neither of the strategies is perfect. Some prey are always more vulnerable, and some predators are less efficient due to the nature of variability in natural populations. Therefore the likelihood of extinction from this association is limited.

Several factors influence the likelihood and strength of coevolved relationships. Coevolution is more likely to take place in pairs of species where high levels of co-occurrence are present. It is also common in cases where selective pressure is strong, influencing important functions such as reproduction or mortality . The type of relationship—be it mutualistic, predator-prey, or competitor—also influences coevolution. Species that have intimate relationships, such as that of a specialist predator or a host-specific parasite, interact actively and thus are more likely to influence each other's selection. Species that do not directly encounter each other but interact through competition for resources are less likely candidates to coevolve, but the strength of the competition may influence the situation.

The result of coevolved relationships is structure in communities. Coevolution and symbiosis create fairly distinct relationships in communities, relationships that are mirrored in distribution, community energetics, and resistance to disturbances in species. Coevolution allows for recurring groupings of plant and animal communities.

[David A. Duffus ]