Coevolution

From Wikipedia, the free encyclopedia

The pollinating wasp Dasyscolia ciliata in pseudocopulation with a flower of Ophrys speculum

In biology, coevolution occurs when two or more species reciprocally affect each other's evolution.

Charles Darwin mentioned evolutionary interactions between flowering plants and insects in On the Origin of Species (1859). The term coevolution was coined by Paul R. Ehrlich and Peter H. Raven in 1964. The theoretical underpinnings of coevolution are now well-developed, 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 influences the structure and function of ecological communities as well as 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 guild or diffuse coevolution, several species may evolve a trait in reciprocity with a trait in another species, as has happened between the flowering plants and pollinating insects such as bees, flies, and beetles.

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 entomophilous flowers

Honey bee taking a reward of nectar and collecting pollen in its pollen baskets from white melilot 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. The term coevolution was coined by Paul R. Ehrlich and Peter H. Raven in 1964, to describe the evolutionary interactions of plants and butterflies.

Several highly successful insect groups—especially the Hymenoptera (wasps, bees and ants) and Lepidoptera (butterflies) 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 ornithophilous flowers

Purple-throated carib feeding from and pollinating a flower

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 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 allows the plant to place pollen on a certain part of the bird's body, permitting a variety of morphological co-adaptations.

A fig exposing its many tiny matured, seed-bearing gynoecia. These are pollinated by the fig wasp, Blastophaga psenes. In the cultivated fig, there are also asexual varieties.

Ornithophilous flowers need to be conspicuous to birds. 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. Each of the two subfamilies of hummingbirds, the Phaethornithinae (hermits) and the Trochilinae, 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.

Fig reproduction and fig wasps

The genus Ficus is composed of 800 species of vines, shrubs, and trees, including the cultivated fig, defined by their syconiums, the fruit-like vessels that either hold female flowers or pollen on the inside. Each fig species has its own fig wasp which (in most cases) pollinates the fig, so a tight mutual dependence has evolved and persisted throughout the genus.

Acacia ants and acacias

Pseudomyrmex ant on bull thorn acacia (Vachellia cornigera) with Beltian bodies that provide the ants with protein

The acacia ant (Pseudomyrmex ferruginea) is an obligate plant ant that protects at least five species of "Acacia" (Vachellia) from preying insects and from other plants competing for sunlight, and the tree provides nourishment and shelter for the ant and its larvae. Such mutualism is not automatic: other ant species exploit trees without reciprocating, following different evolutionary strategies. These cheater ants impose important host costs via damage to tree reproductive organs, though their net effect on host fitness is not necessarily negative and, thus, becomes difficult to forecast.

Hosts and parasites

Parasites and sexually reproducing hosts

Host–parasite coevolution is the coevolution of a host and a parasite. A general characterization of many viruses, obligate parasites, is that they coevolved alongside their respective hosts. Correlated mutations between the two species enter them into an evolution arms race. 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. The Red Queen hypothesis predicts that sexual reproduction allows a host to stay just ahead of its parasite, similar to the Red Queen's race in Through the Looking-Glass: "it takes all the running you can do, to keep in the same place". The host reproduces sexually, producing some offspring with immunity over its parasite, which then evolves in response.

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 variability for fitness not seen in the asexual reproduction, which produces another generation of the organism susceptible to infection by the same parasite. Coevolution between host and parasite may accordingly be responsible for much of the genetic diversity seen in normal populations, including blood-plasma polymorphism, protein polymorphism, and histocompatibility systems.

Brood parasite: Eurasian reed warbler raising a common cuckoo

Brood parasites

Brood parasitism demonstrates close coevolution of host and parasite, for example in cuckoos. These birds do not make their own nests, but lay their eggs in nests of other species, ejecting or killing the eggs and young of the host and thus having a strong negative impact on the host's reproductive fitness. Their eggs are camouflaged as eggs of their hosts, implying that hosts can distinguish their own eggs from those of intruders and are in an evolutionary arms race with the cuckoo between camouflage and recognition. Cuckoos are counter-adapted to host defences with features such as thickened eggshells, shorter incubation (so their young hatch first), and flat backs adapted to lift eggs out of the nest.

Antagonistic coevolution

Antagonistic coevolution is seen in the harvester ant species Pogonomyrmex barbatus and Pogonomyrmex rugosus, in a relationship both parasitic and mutualistic. The queens are unable to produce worker ants by mating with their own species. Only by crossbreeding can they produce workers. The winged females act as parasites for the males of the other species as their sperm will only produce sterile hybrids. But because the colonies are fully dependent on these hybrids to survive, it is also mutualistic. While there is no genetic exchange between the species, they are unable to evolve in a direction where they become too genetically different as this would make crossbreeding impossible.

Predators and prey

Predator and prey: a leopard killing a bushbuck

Predators and prey interact and coevolve, the predator to catch the prey more effectively, the prey to escape. The coevolution of the two mutually imposes selective pressures. These often lead to an evolutionary arms race between prey and predator, resulting in antipredator adaptations.

The same applies to herbivores, animals that eat plants, and the plants that they eat. In the Rocky Mountains, red squirrels and crossbills (seed-eating birds) compete for seeds of the lodgepole pine.  The squirrels get at pine seeds by gnawing through the cone scales, whereas the crossbills get at the seeds by extracting them with their unusual crossed mandibles. In areas where there are squirrels, the lodgepole's cones are heavier, and have fewer seeds and thinner scales, making it more difficult for squirrels to get at the seeds. Conversely, where there are crossbills but no squirrels, the cones are lighter in construction, but have thicker scales, making it more difficult for crossbills to get at the seeds. The lodgepole's cones are in an evolutionary arms race with the two kinds of herbivore.

Sexual conflict has been studied in Drosophila melanogaster (shown mating, male on right).

Competition

Both intraspecific competition, with features such as sexual conflict and sexual selection, and interspecific competition, such as between predators, may be able to drive coevolution.

Guild or diffuse coevolution

Long-tongued bees and long-tubed flowers coevolved, whether pairwise or "diffusely" in groups known as guilds.

The types of coevolution listed so far have been described as if they operated pairwise (also called specific coevolution), in which traits of one species have evolved in direct response to traits of a second species, and vice versa. This is not always the case. Another evolutionary mode arises where evolution is still reciprocal, but is among a group of species rather than exactly two. This is called guild or diffuse coevolution. For instance, a trait in several species of flowering plant, such as offering its nectar at the end of a long tube, can coevolve with a trait in one or several species of pollinating insects, such as a long proboscis. More generally, flowering plants are pollinated by insects from different families including bees, flies, and beetles, all of which form a broad guild of pollinators which respond to the nectar or pollen produced by flowers.

Outside biology

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

In algorithms

Coevolutionary algorithms are used for generating artificial life as well as for optimization, game learning and machine learning. Daniel Hillis added "co-evolving parasites" to prevent an optimization procedure from becoming stuck at local maxima. Karl Sims coevolved virtual creatures.

In 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 an exhibition project named 'Co-evolution', 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." 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."

In technology

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, where a system is understood to consist of 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. Such systems work best when the technical and social systems are deliberately developed together.

In sociology

Models of coevolution have been proposed for sociology and international political economy. Richard Norgaard's 2006 book Development Betrayed proposes a "Co-Evolutionary Revisioning of the Future" of social and economic life.

Mutualism (biology)

From Wikipedia, the free encyclopedia

Hummingbird hawkmoth drinking from Dianthus. Pollination is a classic example of mutualism.

Mutualism or interspecific cooperation is the way two organisms of different species exist in a relationship in which each individual fitness benefits from the activity of the other. Similar interactions within a species are known as co-operation. Mutualism can be contrasted with interspecific competition, in which each species experiences reduced fitness, and exploitation, or parasitism, in which one species benefits at the "expense" of the other. Symbiosis involves two species living in close proximity and may be mutualistic, parasitic, or commensal, so symbiotic relationships are not always mutualistic.

A well-known mutualism is the relationship between ungulates (such as bovines) and bacteria within their intestines. The ungulates benefit from the cellulase produced by the bacteria, which facilitates digestion; the bacteria benefit from having a stable supply of nutrients in the host environment. This can also be found in many different symbiotic relationships.

Mutualism plays a key part in ecology. For example, mutualistic interactions are vital for terrestrial ecosystem function as more than 48% of land plants rely on mycorrhizal relationships with fungi to provide them with inorganic compounds and trace elements. In addition, mutualism is thought to have driven the evolution of much of the biological diversity we see, such as flower forms (important for pollination mutualisms) and co-evolution between groups of species. However mutualism has historically received less attention than other interactions such as predation and parasitism.

Measuring the exact fitness benefit to the individuals in a mutualistic relationship is not always straightforward, particularly when the individuals can receive benefits from a variety of species, for example most plant-pollinator mutualisms. It is therefore common to categorise mutualisms according to the closeness of the association, using terms such as obligate and facultative. Defining "closeness", however, is also problematic. It can refer to mutual dependency (the species cannot live without one another) or the biological intimacy of the relationship in relation to physical closeness (e.g., one species living within the tissues of the other species).

The term mutualism was introduced by Pierre-Joseph van Beneden in his 1876 book Animal Parasites and Messmates.

Types

Mutualistic relationships can be thought of as a form of "biological barter" in mycorrhizal associations between plant roots and fungi, with the plant providing carbohydrates to the fungus in return for primarily phosphate but also nitrogenous compounds. Other examples include rhizobia bacteria that fix nitrogen for leguminous plants (family Fabaceae) in return for energy-containing carbohydrates.

Service-resource relationships

The red-billed oxpecker eats ticks on the impala's coat, in a cleaning symbiosis.

Service-resource relationships are common. Three important types are pollination, cleaning symbiosis, and zoochory.

In pollination, a plant trades food resources in the form of nectar or pollen for the service of pollen dispersal.

Phagophiles feed (resource) on ectoparasites, thereby providing anti-pest service, as in cleaning symbiosis. Elacatinus and Gobiosoma, genera of gobies, also feed on ectoparasites of their clients while cleaning them.

Zoochory is the dispersal of the seeds of plants by animals. This is similar to pollination in that the plant produces food resources (for example, fleshy fruit, overabundance of seeds) for animals that disperse the seeds (service).

Another type is ant protection of aphids, where the aphids trade sugar-rich honeydew (a by-product of their mode of feeding on plant sap) in return for defense against predators such as ladybugs.

Service-service relationships

Ocellaris clownfish and Ritter's sea anemones is a mutual service-service symbiosis, the fish driving off butterflyfish and the anemone's tentacles protecting the fish from predators.

Strict service-service interactions are very rare, for reasons that are far from clear. One example is the relationship between sea anemones and anemone fish in the family Pomacentridae: the anemones provide the fish with protection from predators (which cannot tolerate the stings of the anemone's tentacles) and the fish defend the anemones against butterflyfish (family Chaetodontidae), which eat anemones. However, in common with many mutualisms, there is more than one aspect to it: in the anemonefish-anemone mutualism, waste ammonia from the fish feed the symbiotic algae that are found in the anemone's tentacles. Therefore, what appears to be a service-service mutualism in fact has a service-resource component. A second example is that of the relationship between some ants in the genus Pseudomyrmex and trees in the genus Acacia, such as the whistling thorn and bullhorn acacia. The ants nest inside the plant's thorns. In exchange for shelter, the ants protect acacias from attack by herbivores (which they frequently eat, introducing a resource component to this service-service relationship) and competition from other plants by trimming back vegetation that would shade the acacia. In addition, another service-resource component is present, as the ants regularly feed on lipid-rich food-bodies called Beltian bodies that are on the Acacia plant.

In the neotropics, the ant, Myrmelachista schumanni makes its nest in special cavities in Duroia hirsute. Plants in the vicinity that belong to other species are killed with formic acid. This selective gardening can be so aggressive that small areas of the rainforest are dominated by Duroia hirsute. These peculiar patches are known by local people as "devil's gardens".

In some of these relationships, the cost of the ant’s protection can be quite expensive. Cordia sp. trees in the Amazonian rainforest have a kind of partnership with Allomerus sp. ants, which make their nests in modified leaves. To increase the amount of living space available, the ants will destroy the tree’s flower buds. The flowers die and leaves develop instead, providing the ants with more dwellings. Another type of Allomerus sp. ant lives with the Hirtella sp. tree in the same forests, but in this relationship the tree has turned the tables on the ants. When the tree is ready to produce flowers, the ant abodes on certain branches begin to wither and shrink, forcing the occupants to flee, leaving the tree’s flowers to develop free from ant attack.

The term "species group" can be used to describe the manner in which individual organisms group together. In this non-taxonomic context one can refer to "same-species groups" and "mixed-species groups." While same-species groups are the norm, examples of mixed-species groups abound. For example, zebra (Equus burchelli) and wildebeest (Connochaetes taurinus) can remain in association during periods of long distance migration across the Serengeti as a strategy for thwarting predators. Cercopithecus mitis and Cercopithecus ascanius, species of monkey in the Kakamega Forest of Kenya, can stay in close proximity and travel along exactly the same routes through the forest for periods of up to 12 hours. These mixed-species groups cannot be explained by the coincidence of sharing the same habitat. Rather, they are created by the active behavioural choice of at least one of the species in question.

Mathematical modeling

Mathematical treatments of mutualisms, like the study of mutualisms in general, has lagged behind those of predation, or predator-prey, consumer-resource, interactions. In models of mutualisms, the terms "type I" and "type II" functional responses refer to the linear and saturating relationships, respectively, between benefit provided to an individual of species 1 (y-axis) on the density of species 2 (x-axis).

Type I functional response

One of the simplest frameworks for modeling species interactions is the Lotka–Volterra equations. In this model, the change in population density of the two mutualists is quantified as:

${\displaystyle {\begin{aligned}{\frac {dN}{dt}}&=r_{1}N\left(1-{\cfrac {N}{K_{1}}}+\beta _{12}{\cfrac {M}{K_{1}}}\right)\\[8pt]{\frac {dM}{dt}}&=r_{2}M\left(1-{\cfrac {M}{K_{2}}}+\beta _{21}{\cfrac {N}{K_{2}}}\right)\end{aligned}}}$

where

• N and M=the population densities.
• r=intrinsic growth rate of the population.
• K=carrying capacity of its local environmental setting.
• β=coefficient converting encounters with one species to new units of the other.

Mutualism is in essence the logistic growth equation + mutualistic interaction. The mutualistic interaction term represents the increase in population growth of species one as a result of the presence of greater numbers of species two, and vice versa. As the mutualistic term is always positive, it may lead to unrealistic unbounded growth as it happens with the simple model. So, it is important to include a saturation mechanism to avoid the problem.

The type I functional response is visualized as the graph of ${\displaystyle {\cfrac {\beta _{12}}{K_{1}}}M}$ vs. M.

Type II functional response

In 1989, David Hamilton Wright modified the Lotka–Volterra equations by adding a new term, βM/K, to represent a mutualistic relationship. Wright also considered the concept of saturation, which means that with higher densities, there are decreasing benefits of further increases of the mutualist population. Without saturation, species' densities would increase indefinitely. Because that isn't possible due to environmental constraints and carrying capacity, a model that includes saturation would be more accurate. Wright's mathematical theory is based on the premise of a simple two-species mutualism model in which the benefits of mutualism become saturated due to limits posed by handling time. Wright defines handling time as the time needed to process a food item, from the initial interaction to the start of a search for new food items and assumes that processing of food and searching for food are mutually exclusive. Mutualists that display foraging behavior are exposed to the restrictions on handling time. Mutualism can be associated with symbiosis.

Handling time interactions In 1959, C. S. Holling performed his classic disc experiment that assumed the following: that (1), the number of food items captured is proportional to the allotted searching time; and (2), that there is a variable of handling time that exists separately from the notion of search time. He then developed an equation for the Type II functional response, which showed that the feeding rate is equivalent to

${\displaystyle {\cfrac {ax}{1+axT_{H}}}}$

where,

• a=the instantaneous discovery rate
• x=food item density
• T_H=handling time

The equation that incorporates Type II functional response and mutualism is:

${\displaystyle {\frac {dN}{dt}}=N\left[r(1-cN)+{\cfrac {baM}{1+aT_{H}M}}\right]}$

where

• N and M=densities of the two mutualists
• r=intrinsic rate of increase of N
• c=coefficient measuring negative intraspecific interaction. This is equivalent to inverse of the carrying capacity, 1/K, of N, in the logistic equation.
• a=instantaneous discovery rate
• b=coefficient converting encounters with M to new units of N

or, equivalently,

${\displaystyle {\frac {dN}{dt}}=N[r(1-cN)+\beta M/(X+M)]}$

where

• X=1/a T_H
• β=b/T_H

This model is most effectively applied to free-living species that encounter a number of individuals of the mutualist part in the course of their existences. Wright notes that models of biological mutualism tend to be similar qualitatively, in that the featured isoclines generally have a positive decreasing slope, and by and large similar isocline diagrams. Mutualistic interactions are best visualized as positively sloped isoclines, which can be explained by the fact that the saturation of benefits accorded to mutualism or restrictions posed by outside factors contribute to a decreasing slope.

The type II functional response is visualized as the graph of ${\displaystyle {\cfrac {baM}{1+aT_{H}M}}}$ vs. M.

Structure of networks

Mutualistic networks made up out of the interaction between plants and pollinators were found to have a similar structure in very different ecosystems on different continents, consisting of entirely different species. The structure of these mutualistic networks may have large consequences for the way in which pollinator communities respond to increasingly harsh conditions and on the community carrying capacity.

Mathematical models that examine the consequences of this network structure for the stability of pollinator communities suggest that the specific way in which plant-pollinator networks are organized minimizes competition between pollinators, reduce the spread of indirect effects and thus enhance ecosystem stability and may even lead to strong indirect facilitation between pollinators when conditions are harsh. This means that pollinator species together can survive under harsh conditions. But it also means that pollinator species collapse simultaneously when conditions pass a critical point. This simultaneous collapse occurs, because pollinator species depend on each other when surviving under difficult conditions.

Such a community-wide collapse, involving many pollinator species, can occur suddenly when increasingly harsh conditions pass a critical point and recovery from such a collapse might not be easy. The improvement in conditions needed for pollinators to recover, could be substantially larger than the improvement needed to return to conditions at which the pollinator community collapsed.

Humans

Dogs and sheep were among the first animals to be domesticated.

Humans are involved in mutualisms with other species: their gut flora is essential for efficient digestion. Infestations of head lice might have been beneficial for humans by fostering an immune response that helps to reduce the threat of body louse borne lethal diseases.

Some relationships between humans and domesticated animals and plants are to different degrees mutualistic. For example, agricultural varieties of maize provide food for humans and are unable to reproduce without human intervention because the leafy sheath does not fall open, and the seedhead (the "corn on the cob") does not shatter to scatter the seeds naturally.

In traditional agriculture, some plants have mutualist as companion plants, providing each other with shelter, soil fertility and/or natural pest control. For example, beans may grow up cornstalks as a trellis, while fixing nitrogen in the soil for the corn, a phenomenon that is used in Three Sisters farming.

One researcher has proposed that the key advantage Homo sapiens had over Neanderthals in competing over similar habitats was the former's mutualism with dogs.

Cheating (biology)

From Wikipedia, the free encyclopedia

Cheating is a term used in behavioral ecology and ethology to describe behavior whereby organisms receive a benefit at the cost of other organisms. Cheating is common in many mutualistic and altruistic relationships. A cheater is an individual who does not cooperate (or cooperates less than their fair share) but can potentially gain the benefit from others cooperating. Cheaters are also those who selfishly use common resources to maximize their individual fitness at the expense of a group. Natural selection favors cheating, but there are mechanisms to regulate it.

Theoretical models

Organisms communicate and cooperate to perform a wide range of behaviors. Mutualism, or mutually beneficial interactions between species, is common in ecological systems. These interactions can be thought of "biological markets" in which species offer partners goods that are relatively inexpensive for them to produce and receive goods that are more expensive or even impossible for them to produce. However, these systems provide opportunities for exploitation by individuals that can obtain resources while providing nothing in return. Exploiters can take on several forms: individuals outside a mutualistic relationship who obtain a commodity in a way that confers no benefit to either mutualist, individuals who receive benefits from a partner but have lost the ability to give any in return, or individuals who have the option of behaving mutualistically towards their partners but chose not to do so.

Cheaters, who do not cooperate but benefit from others who do cooperate, gain a competitive edge. In an evolutionary context, this competitive edge refers to a greater ability to survive or to reproduce. If individuals who cheat are able to gain survivorship and reproductive benefits while incurring no costs, natural selection should favor cheaters. What then prevents cheaters from undermining mutualistic systems? One main factor is that the advantages of cheating are often frequency-dependent. Frequency-dependent selection occurs when the fitness of a phenotype depends on its frequency relative to other phenotypes in a population. Cheater phenotypes often display negative frequency-dependent selection, where fitness increases as a phenotype becomes less common and vice versa. In other words, cheaters do best (in terms of evolutionary benefits such as increased survival and reproduction) when there are relatively few of them, but as cheaters become more abundant, they do worse.

For example, in Escherichia coli colonies, there are antibiotic-sensitive "cheaters" that persist at low numbers on antibiotic-laced mediums when in a cooperative colony. These cheaters enjoy the benefit of others producing antibiotic-resistant agents while producing none themselves. However, as numbers increase, if they persist in not producing the antibiotic agent themselves, they are more likely to be negatively impacted by the antibiotic substrate because there is less antibiotic agent to protect everyone. Thus, cheaters can persist in a population because their exploitative behavior gives them an advantage when they exist at low frequencies but these benefits are diminished when they are greater in number.

Others have proposed that cheating (exploitive behavior) can stabilize cooperation in mutualistic systems. In many mutualistic systems, there will be feedback benefits to those that cooperate. For instance, the fitness of both partners may be improved. If there is a high reward or many benefits for the individual that initiated the cooperative behavior, mutualism should be selected for. When researchers investigated the co-evolution of cooperation and choice in a choosy host and its symbiont (an organism that lives in a relationship that benefits all parties involved), their model indicated that although choice and cooperation may be initially selected for, this would often be unstable. In other words, one cooperative partner will choose another cooperative partner if given a choice. However, if this choice is made over and over, variation is removed and this selection can no longer be maintained. This situation is similar to the lek paradox in female choice. For example, in lek paradox, if females consistently choose for a particular male trait, genetic variance for that trait should eventually be eliminated, removing the benefits of choice. However, that choice somehow still persists.

What maintains genetic variability in the face of selection for mutualism (cooperative behavior)? One theory is that cheating maintains this genetic variation. One study shows that a small influx of immigrants with a tendency to cooperate less can generate enough genetic variability to stabilize selection for mutualism. This suggests that the presence of exploitive individuals, otherwise known as cheaters, contribute enough genetic variation to maintain mutualism itself. Both this theory and the negative frequency-dependent theory suggest that that cheating exists as part of a stable mixed evolutionary strategy with mutualism. In other words, cheating is a stable strategy used by individuals in a population where many other individuals cooperate. Another study supports that cheating can exist as a mixed strategy with mutualism using a mathematical game model. Thus, cheating can arise and be maintained in mutualistic populations.

Examples

Stalked slime mould fruiting bodies

Studies of cheating and dishonest communication in populations presupposes an organismal system that cooperates. Without a collective population that has signaling and interactions among individuals, behaviors such as cheating do not manifest. In other words, in order to study cheating behavior, a model system that engages in cooperation is needed. Models that provide insight on cheating include the social amoeba Dictyostelium discoideum; eusocial insects, such as ants, bees, and wasps; and inter-specific interactions found in cleaning mutualisms. Common examples of cleaning mutualisms include cleaner fish such as wrasses and gobies, and some cleaner shrimp.

In Dictyostelium discoideum

Dictyostelium discoideum is a widely used model for cooperation and the development of multicellularity. This species of amoeba are most commonly found in a haploid, single-celled state that feed independently and undergo asexual reproduction. However, when the scarcity of food sources cause individual cells to starve, roughly 10⁴ to 10⁵ cells aggregate to form a mobile, multicellular structure dubbed a "slug". In the wild, aggregates generally contain multiple genotypes, resulting in chimeric mixtures. Unlike clonal (genetically identical) aggregates typically found in multicellular organisms, the potential for competition exists in chimeric aggregates. For example, because individuals in the aggregate contain different genomes, differences in fitness can result in conflict of interest among cells in the aggregate, where different genotypes could potentially compete against each other for resources and reproduction. In Dictyostelium discoideum, roughly 20% of the cells in the aggregate become dead to make the stalk of a fruiting body. The remaining 80% of cells become spores in the sorus of the fruiting body, which can germinate again once conditions are more favorable. In this case, 20% of the cells must give up reproduction so that fruiting body forms successfully. This makes chimeric aggregates of Dictyostelium discoideum susceptible to cheating individuals that take advantage of the reproductive behavior without paying the fair price. In other words, if certain individuals tend to become a part of the sorus more frequently, they can gain increased benefit from the fruiting body system without sacrificing their own opportunities to reproduce. Cheating behavior in D. discoideum is well established, and many studies have attempted to elucidate the evolutionary and genetic mechanisms underlying the behavior. Having a 34Mb genome that is completely sequenced and well annotated makes D. discoideum a useful model in studying the genetic bases and molecular mechanisms of cheating, and in a broader sense, social evolution.

In eusocial insects

Eusocial insects also serve as valuable tools in studying cheating. Eusocial insects behave cooperatively, where members of the community forgo reproduction to assist a few individuals to reproduce. Such model systems have potential for conflict of interest to arise among individuals, and thus also have potential for cheating to occur. Eusocial insects in the order Hymenoptera, which includes bees and wasps, exhibit good examples of conflicts of interest present in insect societies. In these systems, queen bees and wasps can mate and lay fertilized eggs that hatch into females. On the other hand, workers of most species in Hymenoptera can produce eggs, but cannot produce fertilized eggs due to loss of mating ability. Workers that lay eggs represent a cost for the colony because workers that lay eggs often do significantly less work, and thus negatively impact the health of the colony (for example: decreased amount of collected food, or less attention given to tending the queen's eggs). In this case, a conflict of interest arises between the workers and the colony. The workers should lay eggs in order to pass on their genes; however, as a colony, having only the queen reproduce leads to better productivity. If workers sought to pass their own genes by laying eggs, foraging activities would diminish, leading to decreased resources for the entire colony. This, in turn, can cause a tragedy of the commons, where selfish behavior lead to the depletion of resources, with long-term negative consequences for the group. However, in natural bee and wasp societies, only 0.01–0.1% and 1%, respectively, of the workers lay eggs, suggesting that strategies exist to combat cheating to prevent tragedy of the commons. These insect systems have given scientists opportunities to study strategies that keep cheating in check. Such strategies are commonly referred to as "policing" strategies, generally where additional costs are imposed on cheaters to discourage or eliminate cheating behaviors. For example, honeybees and wasps may eat eggs produced by workers. In some ant species and yellowjackets, policing may occur via aggression towards or killing egg-laying individuals to minimize cheating.

Fish cleaned by smaller cleaner wrasses on Hawaiian reefs

In cleaning symbiosis

Cleaning symbiosis that develop between small and larger marine organisms often represent models useful for studying the evolution of stable social interactions and cheating. In the cleaning fish Labroides dimidiatus (Bluestreak cleaner wrasse), as in many cleaner species, client fish seeks to have ectoparasites removed by the cleaners. In these situations, instead of picking off the parasites on the surface of the client fish, the cleaner can cheat by feeding on the client's tissue (mucus layer, scales, etc.), thereby gaining additional benefit from the symbiotic system. It has been well documented that cleaners will feed on mucus when their clients are unable to control the cleaner's behavior; however, in natural settings, client fish often jolt, chase after cheating cleaners, or terminate interactions of swimming way, effectively controlling the cheating behavior. Studies on cleaning mutualisms generally suggest that cheating behavior is often adjusted depending on the species of the client. In cleaning shrimp, cheating is predicted to occur less often because shrimps bear a higher cost if the clients use aggression to control the cleaner's behavior. Studies have found that cleaner species can strategically adjust cheating behavior according to the potential associated risk. For example, predatory clients, which present a significantly high cost for cheating, experience less cheating behavior. On the other hand, nonpredatory clients present a lower cost for cheating, and thus experience more cheating behaviors from the cleaners. Some evidence suggest that physiological processes can mediate the cleaners' decision to switch from cooperating to cheating in mutualistic interactions. For example, in the bluestreak cleaner wrasse, changes in cortisol levels are associated with behavior changes. For smaller clients, increase cortisol levels in the water lead to more cooperative behavior, while for larger clients, the same treatment lead to more dishonest behavior. It has been suggested that "good behavior" toward smaller clients often allow wrasses to attract larger clients that are often cheated.

Other

Other models of cheating include the European tree frog, "Hyla arborea". In many sexually reproducing species such as this, some males can access mates by exploiting resources of more competitive males. Many species have dynamic reproductive strategies that can change in response to changes in the environment. In these instances, several factors contribute to the decision to switch between mating strategies. For example, in the European tree frog, a sexually competitive (as in, perceived to be attractive by females) male tend to call to attract mates. This is often referred to as the "bourgeois" tactic. On the other hand, a smaller male that would likely fail to attract mates using the bourgeois tactic will tend to hide near attractive males and attempt to access females. In this instance, the males can gain access to females without having to defend territories or acquiring additional resources (which often serve as the basis for attractiveness). This is referred to as the "parasitic" tactic, where the smaller male effectively cheats its way to accessing females, by reaping the benefit of sexual reproduction without contributing resources that normally attract females. Models such as this provide valuable tools for research aimed at energetic constraints and environmental cues involved in cheating. Studies find that mating strategies are highly adaptable and depend on a variety of factors, such as competitiveness, energetic costs involved in defending territory or acquiring resources.

Constraints and countermeasures

Environmental conditions and social interactions affecting microbial cheating

Like many other organisms, bacteria rely on iron intake for its biological processes. However, iron is sometimes difficult to access in certain environments, like soil. Some bacteria have evolved siderophores, iron-chelating particles that seek and bring back iron for the bacteria. Siderophores are not necessarily specific to its producer - sometimes another individual could take up the particles instead. Pseudomonas fluorescens is a bacterium commonly found in the soil. Under low-iron conditions, P. fluorescens produces siderophores, specifically pyoverdine, to retrieve the iron necessary for survival. However, when iron is readily available, either from freely diffusing in environment or another bacterium's siderophores, P. fluorescens ceases production, allowing the bacterium to devote its energy towards growth. One study showed that when P. fluorescens grew in association with Streptomyces ambofaciens, another bacterium that produces the siderophore coelichen, no pyoverdine was detected. This result suggested that P. fluorescens ceased siderophore production in favor of taking up iron-bound coelichen, an association also known as siderophore piracy.

More studies, however, suggested that P. fluorescens' cheating behavior could be suppressed. In another study, two strains of P. fluorescens were studied in the soil, their natural environment. One strain, known as the producer, produced a higher level of siderophores, which meant that other strain, known as the non-producer, ceased siderophore production in favor of using the other's siderophores. Although one would expect that the non-producer would outcompete the producer, like the P. fluorescens and S. ambofaciens association, the study demonstrated that the non-producer was unable to do so in soil conditions, suggesting that the two strains could coexist. Further experiments suggested that this cheating prevention may be due to interactions with other microbes in the soil influencing the relationship or the spatial structure of the soil preventing siderophore diffusion and therefore limiting the non-producer's ability to exploit the producer's siderophores.

Selection pressure in bacteria (intraspecies)

By definition, individuals cheat to gain benefits that their non-cheating counterparts do not receive. How then can a cooperative system exist in face of these cheaters? One answer is that the cheaters actually have a reduced fitness compared to the non-cheaters. In a study by Dandekar et al., the researchers examined the survival rates of cheating and non-cheating bacteria populations (Pseudomonas aeruginosa) under varying environmental conditions. These microorganisms, like many species of bacteria, use a cell-cell communication system called quorum sensing that detect their population density and prompt the transcription of various resources when needed. In this case, the resources are publicly shared proteases that break down a food source like casein, and privately used adenosine hydrolase, which breaks down another food source, adenosine. The problem arises when some individuals ("cheaters") do not respond to these quorum sensing signals and therefore do not contribute to the costly protease production yet enjoys the benefits of the broken down resources.

When P. aeruginosa populations are placed into growth conditions where cooperation (and responding to the quorum signal) is costly, the number of cheaters increases, and the public resources are depleted, which can lead to a tragedy of the commons. However, when P. aeruginosa populations are placed into growth conditions with a proportion of adenosine, the cheaters are suppressed because the bacteria that responds to the quorum signal now produces adenosine hydrolase that they privately use for themselves to digest adenosine food source. In wild populations where the presence of adenosine is common, this is an explanation for how individuals that cooperate could have higher fitness than those that cheat, thereby suppressing the cheaters and maintaining cooperation.

Policing/punishment in insects

Cheating is also commonly found in insects. The social and seemingly altruistic communities found in insects such as ants and bees provide ample opportunities for cheaters to take advantage of the system and accrue additional benefits at the expense of the community.

Wasp nest, with some larvae

Sometimes, a colony of insects is called a "superorganism" for its ability to take on properties greater than those of the sum of individuals. A colony of insects in which different individuals are specialized for specific tasks means a greater colony production and greater efficiency. Moreover, based on the kin-selection theory, it is collectively beneficial for all the individuals in the community to have the queen to lay eggs rather the workers lay eggs. This is because if the workers lay eggs, it benefits the egg-laying worker individually, but the rest of the workers are now twice removed from this worker's offspring. Therefore, though it is beneficial for one individual to have its own offspring, it is collectively beneficial to have the queen lay the eggs. Therefore, a system of worker and queen policing exists against worker-laid eggs.

One form of policing occurs by the oophagy of the worker-laid eggs, found in many ant and bee species. This could be done by both or either the queen or the workers. In a series of experiments with honeybees (Apis mellifera), Ratneiks & Visscher found that other workers effectively removed worker-laid eggs in all colonies, whether the eggs were from originated from the same colonies or not. An example of a combination of queen and worker policing is found in ants, in the genus Diacamma, in which worker-laid eggs are taken by other workers and fed to the "queen". In general, these signals that identify the eggs as queen-laid are likely incorruptible, since it must be an honest signal to be maintained and not be used by cheating workers.

The other form of policing occurs through aggression towards egg-laying workers. In a species of tree wasp Dolichovespula sylvestris, Wenseleers et al. found that a combination of aggressive behavior and destruction of worker-laid eggs kept the number of worker-laid eggs low. In fact, 91% of the worker-laid eggs were policed within one day. They also found that about 20% of workers laying eggs were prevented from doing so through both the queen's and workers' aggressive behavior. The workers and the queen would grab the egg-laying worker and try to sting her or push her off the cell. This usually results in the worker removing her abdomen and not depositing her eggs.

Policing/punishment in other organisms

A nest of naked mole rats

Aggression and punishment are not just found in insects. For example, in naked mole rats, punishments by the queen are a way she motivates the lazier, less-related workers in their groups. The queen would shove the lazier workers, with the number of shoves increasing when there are fewer active workers. Reeve found that if the queen is removed when colonies are satiated, there is a significant drop in weight of the active workers because the lazier workers are taking advantage of the system.

Punishment is also a method used by cichlid Neolamprologous pulcher in their cooperative breeding systems. It is a pay-to-stay system where helper fish are allowed to stay in certain territories in exchange for their help. Similar to the naked mole rats, the helpers that were prevented from helping, the "idle helpers", receive more aggression than control helpers in the study. Researchers theorize that this system developed because the fish are usually not closely related (so kinship benefits have little impact), and because there is a high level of predation risk when the fish is outside the group (therefore a strong motivator for the helper fish to stay in the group).

Rhesus monkeys also use aggression as a punishment. These animals have five distinct calls that they can "decide" to produce upon finding food. Whether they call or not are related to their gender and number of kins: females call more often and females with more kins call more often. However, sometimes when food is found, the individual ("discoverer") do not call to attract the kins and presumably to share food. If lower ranked individuals find this discoverer to be in the food drop area of the experiment, they recruit coalition support against this individual by screaming. The formed coalition then chases this individual away. If higher ranked individuals find this discoverer, they either chase the discoverer away or became physically aggressive towards the individual. These results show that aggression as punishment is a way to encourage members to work together and share food when it is found.

Interspecific countermeasures

Cheating and constraints of cheating are not limited to intraspecific interactions; it can also occur in a mutualistic relationship between two species. A common example is the mutualistic relationship between cleaner fish Labroides dimidiatus and reef fish. Bshary and Grutter found that cleaner wrasse prefers the client tissue mucus over ectoparasites. This creates a conflict between the cleaner fish and reef fish, because the reef fish only benefit when the cleaner fish eats the ectoparasites. Further studies revealed that in a lab setting, the cleaner fish undergoes behavioral change in face of deterrents against eating their preferential food. In several trials, the plate of their preferential food source was immediately removed when they eat it, to mimic "client fleeing" in natural settings. In other trials, the plate of their preferential food source chased the cleaner fish when they eat it, mimicking "client chasing" in natural setting. After only six learning trials, the cleaners learned to choose against their preference, indicating that punishment is potentially a very effective countermeasure against cheating in mutualistic relationships.

Nitrogen-fixing nodules in legumes

Finally the countermeasures are not limited to organismal relationships. West et al. found a similar countermeasure against cheating in legume-rhizobium mutualism. In this relationship, nitrogen fixing bacteria rhizobium fixes atmospheric N2 from inside the roots of leguminous plants, providing this essential source of nitrogen to these plants while also receiving organic acids for themselves. However, some bacteria are more mutualistic, while others are more parasitic because they consume the plant's resources but fixes little to no N2. Moreover, these plants cannot tell whether the bacteria are more or less parasitic until they are settled in the plant nodules. To prevent cheating, these plants seem to be able to punish the rhizobium bacteria. In a series of experiments, researchers forced non-cooperation between the bacteria and the plants by placing various nodules in nitrogen-free atmosphere. They saw a decrease in the rhizobium reproductive success by 50%. West et al. created a model for legume sanctioning the bacteria and hypothesizes that these behaviors exist to stabilize mutualistic interactions.

Another well-known example of plant-organism interaction occurs between yuccas and yucca moths. The female yucca moths deposit their eggs one at a time to the yucca flower. At the same time, she also deposits a small amount of pollen from yucca flowers as nutrition for the yucca moths. Because most of the pollen is not consumed by the larva, yucca moths are therefore also the active pollinators for the yucca plant. Moreover, sometimes the female moths do not successfully deposit their eggs the first time, and may try again and again. The yucca plant receives scars from the multiple attempts, but they also receive more pollen, since the moths are depositing pollen with every try.

"Cheating" sometimes happens when the yucca moth deposits too many eggs in one plant. In this case, the yucca plant has little to no benefits from this interaction. However, the plant has a unique way of constraining this behavior. While the constraint against cheating often occurs directly to the individual, in this case, the constraint occurs to the individual's offspring. The yucca plant can "abort" the moths by aborting the flowers. Pellmyr and Huth found that there is selective maturation for flowers that have low egg loads and high number of scars (and therefore a high amount of pollen). In this way, there is selection against the "cheaters" who try to use the yucca plant without providing the benefits of pollination.