Coevolution

In biology, coevolution occurs when two or more species reciprocally affect each other's evolution through the process of natural selection. The term sometimes is used for two traits in the same species affecting each other's evolution, as well as gene-culture coevolution.

Charles Darwin mentioned evolutionary interactions between flowering plants and insects in ''On the Origin of Species'' (1859). Although he did not use the word coevolution, he suggested how plants and insects could evolve through reciprocal evolutionary changes. Naturalists in the late 1800s studied other examples of how interactions among species could result in reciprocal evolutionary change. Beginning in the 1940s, plant pathologists developed breeding programs that were examples of human-induced coevolution. Development of new crop plant varieties that were resistant to some diseases favored rapid evolution in pathogen populations to overcome those plant defenses. That, in turn, required the development of yet new resistant crop plant varieties, producing an ongoing cycle of reciprocal evolution in crop plants and diseases that continues to this day.

Coevolution as a major topic for study in nature expanded rapidly after the middle 1960s, when Daniel H. Janzen showed coevolution between acacias and ants (see below) and Paul R. Ehrlich and Peter H. Raven suggested how coevolution between plants and butterflies may have contributed to the diversification of species in both groups. The theoretical underpinnings of coevolution are now well-developed (e.g., the geographic mosaic theory of coevolution), and demonstrate that coevolution can play an important role in driving major evolutionary transitions such as the evolution of sexual reproduction or shifts in ploidy. More recently, it has also been demonstrated that coevolution can influence the structure and function of ecological communities, the evolution of groups of mutualists such as plants and their pollinators, and the dynamics of infectious disease.

Each party in a coevolutionary relationship exerts selective pressures on the other, thereby affecting each other's evolution. Coevolution includes many forms of mutualism, host-parasite, and predator-prey relationships between species, as well as competition within or between species. In many cases, the selective pressures drive an evolutionary arms race between the species involved. Pairwise or specific coevolution, between exactly two species, is not the only possibility; in multi-species coevolution, which is sometimes called guild or diffuse coevolution, several to many species may evolve a trait or a group of traits in reciprocity with a set of traits in another species, as has happened between the flowering plants and pollinating insects such as bees, flies, and beetles. There are a suite of specific hypotheses on the mechanisms by which groups of species coevolve with each other.

Coevolution is primarily a biological concept, but researchers have applied it by analogy to fields such as computer science, sociology, and astronomy.

Mutualism

Coevolution is the evolution of two or more species which reciprocally affect each other, sometimes creating a mutualistic relationship between the species. Such relationships can be of many different types.

Flowering plants

Flowers appeared and diversified relatively suddenly in the fossil record, creating what Charles Darwin described as the "abominable mystery" of how they had evolved so quickly; he considered whether coevolution could be the explanation. He first mentioned coevolution as a possibility in ''On the Origin of Species'', and developed the concept further in '' Fertilisation of Orchids'' (1862).

Insects and insect-pollinated flowers

Modern insect-pollinated (entomophilous) flowers are conspicuously coadapted with insects to ensure pollination and in return to reward the pollinators with nectar and pollen. The two groups have coevolved for over 100 million years, creating a complex network of interactions. Either they evolved together, or at some later stages they came together, likely with pre-adaptations, and became mutually adapted.

Several highly successful insect groups—especially the Hymenoptera (wasps, bees and ants) and Lepidoptera (butterflies and moths) as well as many types of Diptera (flies) and Coleoptera (beetles)—evolved in conjunction with flowering plants during the Cretaceous (145 to 66 million years ago). The earliest bees, important pollinators today, appeared in the early Cretaceous. A group of wasps sister to the bees evolved at the same time as flowering plants, as did the Lepidoptera. Further, all the major clades of bees first appeared between the middle and late Cretaceous, simultaneously with the adaptive radiation of the eudicots (three quarters of all angiosperms), and at the time when the angiosperms became the world's dominant plants on land.

At least three aspects of flowers appear to have coevolved between flowering plants and insects, because they involve communication between these organisms. Firstly, flowers communicate with their pollinators by scent; insects use this scent to determine how far away a flower is, to approach it, and to identify where to land and finally to feed. Secondly, flowers attract insects with patterns of stripes leading to the rewards of nectar and pollen, and colours such as blue and ultraviolet, to which their eyes are sensitive; in contrast, bird-pollinated flowers tend to be red or orange. Thirdly, flowers such as some orchids mimic females of particular insects, deceiving males into pseudocopulation.

The yucca, ''Yucca whipplei'', is pollinated exclusively by ''Tegeticula maculata'', a yucca moth that depends on the yucca for survival. The moth eats the seeds of the plant, while gathering pollen. The pollen has evolved to become very sticky, and remains on the mouth parts when the moth moves to the next flower. The yucca provides a place for the moth to lay its eggs, deep within the flower away from potential predators.

Birds and bird-pollinated 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. The floral characteristics of ornithophilous plants vary greatly among each other compared to closely related insect-pollinated species. These flowers also tend to be more ornate, complex, and showy than their insect pollinated counterparts. It is generally agreed that plants formed coevolutionary relationships with insects first, and ornithophilous species diverged at a later time. There is not much scientific support for instances of the reverse of this divergence: from ornithophily to insect pollination. The diversity in floral phenotype in ornithophilous species, and the relative consistency observed in bee-pollinated species can be attributed to the direction of the shift in pollinator preference.

Flowers have converged to take advantage of similar birds. Flowers compete for pollinators, and adaptations reduce unfavourable effects of this competition. The fact that birds can fly during inclement weather makes them more efficient pollinators where bees and other insects would be inactive. Ornithophily may have arisen for this reason in isolated environments with poor insect colonization or areas with plants which flower in the winter. 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, the most important determinants of flower choice. In '' Mimulus'', an increase in red pigment in petals and flower nectar volume noticeably reduces the proportion of pollination by bees as opposed to hummingbirds; while greater flower surface area increases bee pollination. Therefore, red pigments in the flowers of ''Mimulus cardinalis'' may function primarily to discourage bee visitation. In '' Penstemon'', flower traits that discourage bee pollination may be more influential on the flowers' evolutionary change than 'pro-bird' adaptations, but adaptation 'towards' birds and 'away' from bees can happen simultaneously. However, some flowers such as ''Heliconia angusta'' appear not to be as specifically ornithophilous as had been supposed: the species is occasionally (151 visits in 120 hours of observation) visited by '' Trigona'' stingless bees. These bees are largely pollen robbers in this case, but may also serve as pollinators.

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 have converged to a common morphology and color because these are effective at attracting the birds. Different lengths and curvatures of the corolla

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