Coevolution

Bumblebees and the flowers they pollinate have coevolved so that both have become dependent on each other for survival.

In biology, coevolution is "the change of a biological object triggered by the change of a related object". In other words, when changes in at least two species' genetic compositions reciprocally affect each other’s evolution, coevolution has occurred.

There is evidence for coevolution at the level of populations and species. Charles Darwin briefly described the concept of coevolution in On the Origin of Species (1859) and developed it in detail in Fertilisation of Orchids (1862). It is likely that viruses and their hosts coevolve in various scenarios.

However, there is little evidence of coevolution driving large-scale changes in Earth's history, since abiotic factors such as mass extinction and expansion into ecospaces seem to guide the shifts in the abundance of major groups. One proposed specific example was the evolution of high-crowned teeth in grazers when grasslands spread through North America - long held up as an example of coevolution. We now know that these events happened independently.

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), and examples of mutualism evolving through time. Evolution in response to abiotic factors, such as climate change, is not biological coevolution (since climate is not alive and does not undergo biological evolution).

The general conclusion is that coevolution may be responsible for much of the genetic diversity seen in normal populations including: blood-plasma polymorphism, protein polymorphism, histocompatibility systems, etc.

The parasite/host relationship probably drove the prevalence of sexual reproduction over the more efficient asexual reproduction. It seems that when a parasite infects a host, sexual reproduction affords a better chance of developing resistance (through variation in the next generation), giving sexual reproduction viability for fitness not seen in the asexual reproduction, which produces another generation of the organism susceptible to infection by the same parasite.

Coevolution is primarily a biological concept, but researchers have applied it 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". This hypothesis predicts that sexual reproduction allows a host to stay just ahead of its parasite by a generation, similar to the Red Queen in "Through the Looking Glass". …always running ….. Just ahead. The essence is that the host reproduces sexually giving it immunity over its parasite, which then evolves in response. This requires the next generation to repeat the sequence. 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. A model based on adaptive dynamics and experimental data of floral and proboscis lengths, as well as nectar consumed and pollen deposited during the pollination of the long-tubed iris (Lapeirousia anceps) by the long-proboscid fly (Moegistorhynchus longirostris) has generated diverse coevolutionary dynamics, including two types of Red Queen dynamics, evolutionary branching (backed by observations of coexisting irises of short and long tubes in a single population) and trap.

Coevolution types

Coevolution can occur between pairs of entities (often referred to as pairwise coevolution) exists, with examples including predator and prey, host and symbiont or host and parasite. However, some instances of coevolution are less clearcut: a species may evolve in response to more than one species. If this coevolution is occurring in a non-additive way, then this type of coevolution is known as diffuse coevolution.

A general characterization that can be made of many viruses, widely known to be obligate parasites, is that they coevolved alongside their respective hosts. This is suggested by the similar genetic arrangement between virus and host. Correlated mutations between the two species enter them into an evolution arms race: the host must develop a defense mechanism to overcome the parasite, and the parasite must overcome the new defense mechanism in order to persist and reproduce. Whichever organism, host or parasite, that cannot keep up with the other will be eliminated from their habitat, as the species with the higher average population fitness survives. This race is known as the Red Queen hypothesis.

Escape and radiate coevolution occurs when an organism under selectional constraints by predation develops a defense mechanism allowing it to "escape", and then "radiate" into another species entirely. The predator then evolves accordingly, and the process continues.

Coevolution in the fossil record

Many examples of coevolution have been observed among living species but coevolution has not been conclusively shown in the fossil record. This may be because the fossil record does not tend to preserve high resolution data (e.g. on the level of species) and because coevolution does not drive large-scale changes in Earth's history. Even classic examples of coevolution in the fossil record such as the interaction between bees and the flowers they pollinate or the development of grazers in spreading grasslands are little supported by evidence. In the case of bees and flowers, these two organisms evolved at different times - Archaefructus is a flowering plant from 125 million years ago but bees are known from nests 220 million years ago. Similarly in the case of grazing ungulates and the spread of grasslands - the timing in evolution of the two organisms is very different - this is called the Grit, not grass hypothesis.

Specific examples

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. 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 upon 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 can make them more efficient pollinators in cases in which bees and other insects are 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, which are the most important determinants of their flower choice. In experiments with monkeyflowers, an increase in red pigment in petals and flower nectar volume has been shown to noticeably reduce the proportion of pollination by bees as opposed to solely attract hummingbirds; while greater flower surface area was shown to increase the amount of bee pollination. Additional red coloration of flowers significantly decreased bee visitation but seemingly had no effect on the frequency of hummingbird visitation. Thus, hummingbirds may not necessarily have an innate preference for red and the high concentration of these red pigments in the flowers of M. cardinalis could potentially function primarily to discourage bee visitation. In first generation hybrids of these two species, the composition of pollinator visitors (59% bees; 1,744 visits) was exactly in between the two parental species, implying a strong genetic component to pollinator coevolution. In similar experiments with Penstemon flowers, pollen transfer by both bees and hummingbirds was recorded with two closely related species species differing in main pollinator. The results suggest that the flower traits that discourage bee pollination may be even more influential on the flowers' evolutionary change than ‘pro-bird’ adaptations are. However, adaptation 'towards' birds and adaptation 'away' from bees can and do happen simultaneously. 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 also leading to evolutionary growth. Coevolution has led to deep flowers and moths with long proboscides.

Old world swallowtail and fringed rue

Old world swallowtail caterpillar on 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

See also: Acacia § Symbiosis

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

A flowering yucca plant that would be pollinated by a yucca moth

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.

Biological applications

The study of coevolution in natural populations could help in fields such as conservation, human epidemiology, and improved agriculture.

Algorithms

See also: Evolutionary computation

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.

Architecture

The concept of coevolution was introduced in architecture by the Danish architect-urbanist Henrik Valeur as an antithesis to the concept of "star-architecture". As the curator of the Danish Pavilion at the 2006 Venice Biennale of Architecture he conceived and orchestrated the exhibition project CO-EVOLUTION: Danish/Chinese Collaboration on Sustainable Urban Development in China, which was awarded the Golden Lion for Best National Pavilion.

The exhibition included urban planning projects for the cities of Beijing, Chongqing, Shanghai and Xi’an, which had been developed in collaboration between young professional Danish architects and students and professors and students from leading universities in the four Chinese cities. By creating a framework for collaboration between academics and professionals representing two distinct cultures, it was hoped that the exchange of knowledge, ideas and experiences would stimulate "creativity and imagination to set the spark for new visions for sustainable urban development."

Henrik Valeur later argued that: "As we become more and more interconnected and interdependent, human development is no longer a matter of the evolution of individual groups of people but rather a matter of the co-evolution of all people."

Cosmology and astronomy

In his book The Self-organizing Universe, Erich Jantsch attributed the entire evolution of the cosmos to coevolution.

In astronomy, an emerging theory states that black holes and galaxies develop in an interdependent way analogous to biological coevolution.

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. The idea is closely related to the concept of "joint optimization" in sociotechnical systems analysis and design. STS suggests that all human systems include both a "technical system" encompassing the tools and hardware used for production and maintenance, and a "social system" of relationships and procedures through which the technology is tied into the goals of the system and all the other human and organizational relationships within and outside the system. It is possible for the system to optimize on the technology, giving priority to technical solutions and compelling the social system to adapt to it; or to optimize on the social system, giving priority to existing social patterns and procedures and compelling the technology to fill in what gaps remain. In practice, both solutions are generally suboptimal in terms of outcomes. Better outcomes are usually obtained by a reciprocal process of joint optimization, through which both the technical system and the social system change to some degree in response to each other. This may happen naturally, but it is usually best achieved by a systematic process of sociotechnical design in which both systems are led to recognize each other's value and purposes and their mutual adaptation is negotiated between them.

See also

• Bak–Sneppen model
• Character displacement
• Co-adaptation
• Coextinction
• Convergent evolution
• Ecological fitting
• Genomics of domestication
• Lynn Margulis
• Mosaic evolution
• Parallel evolution
• Sexual conflict
• Sympatric speciation, the creation of two or more species from an ancestor species based on something other than spatial divergence (i.e., geography)
• Technological evolution

Further reading

• Dawkins, R. Unweaving the Rainbow.
• Geffeney, Shana L., et al. "Evolutionary diversification of TTX-resistant sodium channels in a predator-prey interaction". Nature 434 (2005): 759–763.
• Michael Pollan The Botany of Desire: A Plant's-eye View of the World. Bloomsbury. ISBN 0-7475-6300-4. Account of the co-evolution of plants and humans
• Abstract

External links

• Coevolution, video of lecture by Stephen C. Stearns (Open Yale Courses)
• [1], Global Optimization of Some Difficult Benchmark Functions by Host-Parasite Coevolutionary Algorithm: An algorithm for Global Optimization.