Coevolution



In biology, coevolution is "the change of a biological object triggered by the change of a related object." Coevolution can occur at many biological levels: it can be as microscopic as correlated mutations between amino acids in a protein, or as macroscopic as covarying traits between different species in an environment. Each party in a coevolutionary relationship exerts selective pressures on the other, thereby affecting each other's evolution. Coevolution of different species includes the evolution of a host species and its parasites (host–parasite coevolution), or predators and their prey and examples of mutualism evolving through time. Evolution in response to abiotic factors, such as climate change, is not coevolution (since climate is not alive and does not undergo biological evolution). Coevolution between pairs of entities (often referred to as pairwise coevolution) exists, such as that between predator and prey, host and symbiont or host and parasite, but many cases are less clearcut: a species may evolve in response to a number of other species, each of which is also evolving in response to a set of species. This situation has been referred to as "diffuse coevolution."

There is little evidence of coevolution driving large-scale changes in Earth's history, since abiotic factors such as mass extinction and expansion into ecospace seem to guide the shifts in the abundance of major groups. However, there is evidence for coevolution at the level of populations and species. For example, the concept of coevolution was briefly described by Charles Darwin in On the Origin of Species, and developed in detail in Fertilisation of Orchids. It is likely that viruses and their hosts may have coevolved in various scenarios.

Coevolution is primarily a biological concept, but has been applied by analogy to fields such as computer science, sociology / International Political Economy and astronomy.

Models
One model of coevolution was Leigh Van Valen's Red Queen's Hypothesis, which states that "for an evolutionary system, continuing development is needed just in order to maintain its fitness relative to the systems it is co-evolving with".

Emphasizing the importance of sexual conflict, Thierry Lodé described the role of antagonist interactions in evolution, giving rise to a concept of antagonist coevolution.

Coevolution branching strategies for asexual population dynamics in limited resource environments have been modeled using the generalized Lotka–Volterra equations.

Hummingbirds and ornithophilous flowers
Hummingbirds and ornithophilous (bird-pollinated) flowers have evolved a mutualistic relationship. The flowers have nectar suited to the birds' diet, their color suits the birds' vision and their shape fits that of the birds' bills. The blooming times of the flowers have also been found to coincide with hummingbirds' breeding seasons.

Flowers have converged to take advantage of similar birds. Flowers compete for pollinators, and adaptations reduce unfavourable effects of this competition. Bird-pollinated flowers usually have higher volumes of nectar and higher sugar production than those pollinated by insects. This meets the birds' high energy requirements, which are the most important determinants of their flower choice. Following their respective breeding seasons, several species of hummingbirds occur at the same locations in North America, and several hummingbird flowers bloom simultaneously in these habitats. These flowers seem to have converged to a common morphology and color. Different lengths and curvatures of the corolla tubes can affect the efficiency of extraction in hummingbird species in relation to differences in bill morphology. Tubular flowers force a bird to orient its bill in a particular way when probing the flower, especially when the bill and corolla are both curved; this also allows the plant to place pollen on a certain part of the bird's body. This opens the door for a variety of morphological co-adaptations.

An important requirement for attraction is conspicuousness to birds, which reflects the properties of avian vision and habitat features. Birds have their greatest spectral sensitivity and finest hue discrimination at the red end of the visual spectrum, so red is particularly conspicuous to them. Hummingbirds may also be able to see ultraviolet "colors". The prevalence of ultraviolet patterns and nectar guides in nectar-poor entomophilous (insect-pollinated) flowers warns the bird to avoid these flowers.

Hummingbirds form the family Trochilidae, whose two subfamilies are the Phaethornithinae (hermits) and the Trochilinae. Each subfamily has evolved in conjunction with a particular set of flowers. Most Phaethornithinae species are associated with large monocotyledonous herbs, while the Trochilinae prefer dicotyledonous plant species.

Angraecoid orchids and African moths
Angraecoid orchids and African moths coevolve because the moths are dependent on the flowers for nectar and the flowers are dependent on the moths to spread pollen so they can reproduce. Coevolution has led to deep flowers and moths with long probosci.

Old world swallowtail and fringed rue
An example of antagonistic coevolution is the old world swallowtail (Papilio machaon) caterpillar living on the fringed rue (Ruta chalepensis) plant. The rue produces etheric oils which repel plant-eating insects. The old world swallowtail caterpillar developed resistance to these poisonous substances, thus reducing competition with other plant-eating insects.

Garter snake and rough-skinned newt
Coevolution of predator and prey species is illustrated by the Rough-skinned newt (Taricha granulosa) and the common garter snake (Thamnophis sirtalis). The newts produce a potent neurotoxin that concentrates in their skin. Garter snakes have evolved resistance to this toxin through a series of genetic mutations, and prey upon the newts. The relationship between these animals has resulted in an evolutionary arms race that has driven toxin levels in the newt to extreme levels. This is an example of coevolution because differential survival caused each organism to change in response to changes in the other.

California buckeye and pollinators
When beehives are populated with bee species that have not coevolved with the California buckeye (Aesculus californica), sensitivity to aesculin, a neurotoxin present in its nectar, may be noticed; this sensitivity is only thought to be present in honeybees and other insects that did not coevolve with A. californica.

Acacia ant and bullhorn acacia tree
The acacia ant (Pseudomyrmex ferruginea) protects the bullhorn acacia (Acacia cornigera) from preying insects and from other plants competing for sunlight, and the tree provides nourishment and shelter for the ant and its larvae. Nevertheless, some ant species can exploit trees without reciprocating, and hence have been given various names such as 'cheaters', 'exploiters', 'robbers' and 'freeloaders'. Although cheater ants do important damage to the reproductive organs of trees, their net effect on host fitness is difficult to forecast and not necessarily negative.

Yucca Moth and the yucca plant
In this mutualistic symbiotic relationship, the yucca plant (Yucca whipplei) is pollinated exclusively by Tegeticula maculata, a species of yucca moth that in turn relies on the yucca for survival. Yucca moths tend to visit the flowers of only one species of yucca plant. In the flowers, the moth eats the seeds of the plant, while at the same time gathering pollen on special mouth parts. The pollen is very sticky, and will easily remain on the mouth parts when the moth moves to the next flower. The yucca plant also provides a place for the moth to lay its eggs, deep within the flower where they are protected from any potential predators. The adaptations that both species exhibit characterize coevolution because the species have evolved to become dependent on each other.

Coevolution outside biology
Coevolution is primarily a biological concept, but has been applied to other fields by analogy.

Technological coevolution
Computer software and hardware can be considered as two separate components but tied intrinsically by coevolution. Similarly, operating systems and computer applications, web browsers and web applications. All of these systems depend upon each other and advance step by step through a kind of evolutionary process. Changes in hardware, an operating system or web browser may introduce new features that are then incorporated into the corresponding applications running alongside.

Algorithms
Coevolutionary algorithms are a class of algorithms used for generating artificial life as well as for optimization, game learning and machine learning. Coevolutionary methods have been applied by Daniel Hillis, who coevolved sorting networks, and Karl Sims, who coevolved virtual creatures.