Coevolution

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

In a broad sense, biological coevolution is "the change of a biological object triggered by the change of a related object".^[1] Coevolution can occur at multiple levels of biology: 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 others' evolution. Species-level coevolution includes the evolution of a host species and its parasites, 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). Evolution in a one-on-one interaction, such as that between predator and prey, host-symbiont or host-parasitic pair, is coevolution. 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". And, certainly, for many organisms, the biotic (living) environment is the most prominent selective pressure, resulting in evolutionary change.

The concept of coevolution was briefly described by Charles Darwin in On the Origin of Species, and developed in detail in Fertilisation of Orchids.^[2][3][4]

One model of the coevolution processes was Leigh Van Valen's Red Queen's hypothesis. Emphasizing the importance of the sexual conflict, Thierry Lodé privileged the role of antagonist interactions (notably sexual) in evolution leading to an antagonist coevolution.^[5]

Coevolution does not imply mutual dependence. The host of a parasite, or prey of a predator, does not depend on its enemy for survival.

The existence of mitochondria within eukaryote cells is an example of coevolution as the mitochondria has a different DNA sequence than that of the nucleus in the host cell. This concept is described further by the endosymbiotic theory.

Coevolutionary algorithms are also a class of algorithms used for generating artificial life as well as for optimization, game learning and machine learning. Pioneering results in the use of coevolutionary methods were by Daniel Hillis (who coevolved sorting networks) and Karl Sims (who coevolved virtual creatures).

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.^[6]

Models

Coevolution branching strategies for asexual population dynamics in limited resource environments have been modeled using the generalized Lotka–Volterra equations^[7]

Specific examples

Hummingbirds and ornithophilous flowers

Hummingbirds and ornithophilous flowers have evolved to form a mutualistic relationship. It is prevalent in the bird’s biology as well as in the flower’s. Hummingbird flowers have nectar chemistry associated with the bird’s diet. Their color and morphology also coincide with the bird’s vision and morphology. The blooming times of these ornithophilous flowers have also been found to coincide with hummingbirds' breeding seasons.

Flowers have converged to take advantage of similar birds.^[8] Flowers compete for pollinators and adaptations reduce deleterious effects of this competition.^[8] Bird-pollinated flowers usually show higher nectar volumes and sugar production.^[9] This reflects high energy requirements of the birds.^[9] Energetic criteria are the most important determinants of flower choice by birds.^[9] Following their respective breeding seasons, several species of hummingbirds co-occur in North America, and several hummingbird flowers bloom simultaneously in these habitats. These flowers seem to have converged to a common morphology and color.^[9] Different lengths and curvatures of the corolla tubes can affect the efficiency of extraction in hummingbird species in relation to differences in bill morphology.^[9] 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.^[9] This opens the door for a variety of morphological co-adaptations.

An important requisite for attraction is conspicuousness to birds, which reflects the properties of avian vision and habitat features.^[9] Birds have their greatest spectral sensitivity and finest hue discrimination at the long wavelength end of the visual spectrum.^[9] This is why red is so conspicuous to birds. Hummingbirds may also be able to see ultraviolet “colors” (Stiles 1981). The prevalence of ultraviolet patterns and nectar guides in nectar-poor entomophilous flowers allows the bird to avoid these flowers on sight.^[9] Two subfamilies in the family Trochilidae are Phaethorninae and Trochlinae. Each of these groups has evolved in conjunction with a particular set of flowers. Most Phaethorninae species are associated with large monocotyledonous herbs, and members of the subfamily Trochilinae are associated with dicotyledonous plant species.^[9]

Angracoid orchids and African moths

Another example of coevolution is pollination of Angraecoid orchids by African moths. These species 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. The evolutionary process 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 which lives on Fringed Rue (Ruta chalepensis) plant. Rue plant produces etheric oils which repel plant-eating insects. The Old World Swallowtail caterpillar developed resistance to these poisonous substances, thus reducing competition of other plant-eating insects.

Old World Swallowtail (Papilio machaon) caterpillar on Fringed Rue (Ruta chalepensis) plant

Garter snake and Rough-skinned newt

Coevolution can occur between predator and prey species as in the case of the Rough-skinned Newt (Taricha granulosa) and the common garter snake (Thamnophis sirtalis). In this case, the newts produce a potent nerve toxin that concentrates in their skin. Garter snakes have evolved resistance to this toxin through a set 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.

California buckeye and pollinators

When bee-hives are kept whose bee species that have not coevolved with the California Buckeye, toxicity to aesculin, a neurotoxin present in the flower nectar of the Aesculus californica tree may be noted; this toxicity is only thought to be present in the case of honeybees and other insecta, which species did not coevolve with A. californica.^[10]

Acacia ant and Swollen thorn acacia tree

The ant provides protection for the tree against preying insects and other plants competing for sunlight, and the tree provides nourishment and shelter for the ant and the ants' larvae.^[11][12]

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.

Bibliography

• 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
• Dawkins, R. Unweaving the Rainbow and other books.
• Geffeney, Shana L., et al. “Evolutionary diversification of TTX-resistant sodium channels in a predator-prey interaction”. Nature 434 (2005): 759–763.

See also

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

References

• Darwin, Charles (1859), On the Origin of Species (1st ed.), London: John Murray, http://darwin-online.org.uk/content/frameset?itemID=F373&viewtype=text&pageseq=1, retrieved 2009-02-07 
• Darwin, Charles (1862), On the various contrivances by which British and foreign orchids are fertilised by insects, and on the good effects of intercrossing, London: John Murray, http://darwin-online.org.uk/content/frameset?viewtype=text&itemID=F800&pageseq=1, retrieved 2009-02-07 
• Darwin, Charles (1877), The various contrivances by which orchids are fertilised by insects, London: John Murray, http://darwin-online.org.uk/content/frameset?itemID=F801&viewtype=text&pageseq=1, retrieved 2009-07-27 
• Thompson, John L. (1994), The coevolutionary process, Chicago: University of Chicago Press, ISBN 0-226-79760-0, http://books.google.co.uk/books?id=AyXPQzEwqPIC&pg=PA27&lpg=PA27&dq=Wallace+%22creation+by+law%22+Angr%C3%A6cum&source=bl&ots=rvzjyAL80w&sig=1Kjl6J3lm-4I0xoDDb_umU9anf8&hl=en&ei=qXppSsCoE-WrjAezxPSyCw&sa=X&oi=book_result&ct=result&resnum=9, retrieved 2009-07-27 

Further reading

• Futuyma, D. J. and M. Slatkin (editors) (1983). Coevolution. Sunderland, Massachusetts: Sinauer Associates. pp. 555 pp. ISBN 0878932283. 
• Thompson, J. N. (1994). The Coevolutionary Process. Chicago: University of Chicago Press. pp. 376 pp. ISBN 0226797597.