Coevolution is a cyclic process in which the adaptations of one organism in an ecosystem drive the adaptations of another organism. These adaptations then cause the first organism to adapt to keep up (Brockhurst & Koskella, 2013). Coevolution is very environmentally specific so you will not see it develop between generalist species (Abrams, 2006; Aslan, et. al, 2013; Becklin, 2008; Chu, 1985) though it may develop communally. Similarly, coevolution does not account for wide-scale adaptions among groups of organisms better attributed to environmental shifts such as habitat destruction and climate change (Chu, 1985).
Vermeij (1987) described the pattern of predator-prey adaptation as the theory of escalation in which a competitive advantage given through natural selection for either predator or prey pushes for adaptation in its counterpart. This shifts the advantage and begins a new round of competitive adaptation. By nature these systems are antagonistic (Brockhurst & Koskella, 2013) since the effectiveness of one species' adaptations negatively impacts the survival of the other and are measured by the effectiveness of the prey's defense against the predator (Mougi & Iwasa, 2010). Vermeij (1994) identifies predators as the most important selective force in history.
Examples of Predator-Prey
One particularly stark example of coevolutionary predator-prey system exists between the African honey badger and the African honey bee. The African honey badger (Mellivora capensis) primarily feeds on rodents, insects and arachnids in its native environment (Kruuk, 1983). However, as one might expect form its name, the honey badger has a particular love of honey. It uses its long claws to dislodge hives form their mounted positions, cutting teeth to tear open the hives, and its thick, loose skin to ward off stings from the bees (Churcher, 1994). In response, the African bee (Apis meliffera scutellata) developed a behavioural adaptation: massive swarming. Honey badgers, relatively immune to the stings of a bee, require a full-out attack by the hive. As the bees die post-sting, they release a pheromone calling to others to continue the attack. They will also pursue their quarry for up to a kilometer (Bourgain et al., 1998).
Examples of Parasite-Host
Parasitism typically refers to organisms which thrive at the expense of another. This is differentiated from simply predator-prey by the host facilitating one phase of the parasite's life stage. While we typically think of parasites as smaller organisms, the relationship between the koel cuckoo and species of crow and shrike in their environments. Koel are brood parasites, laying their eggs in the nests of other birds and letting the host bird hatch the eggs. The Jungle crow, an oft-parasitized species, has developed a highly aggressive behavioural pattern to prevent the koel from setting their eggs in the crow's nest. However, the koel has coevolved its own behavioral modification. The male will lure the aggressive crow out to chase it while the female koel sets the cuckoo eggs in the crow's nest (Rothstein, 1990).
Antagonistic coevolutionary work towards equilibrium, especially among specialists. If one side develops a long-term evolutionary advantage without the other adapting, the specialist predator or prey will perish (Abrams, 2006). This equilibrium is reached most effectively among predator-prey systems with equitable life cycles. When generations match up, predators and prey coevolve simultaneously. However, in parasite-host systems the life cycle of the parasite is significantly shorter. This along for a greater number of generations and time for quick adaptation. For this reason, parasite-host systems are usually out of equilibrium and oscillate (Mougi & Iwasa, 2010).
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