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Parallel evolution is the similar development of a trait in distinct species that are not closely related, but share a similar original trait in response to similar evolutionary pressure.^[1]^[2]

Parallel vs. convergent evolution

Given a trait that occurs in each of two lineages descended from a specified ancestor, it is possible in theory to define parallel and convergent evolutionary trends strictly, and distinguish them clearly from one another.^[2] However, the criteria for defining convergent as opposed to parallel evolution are unclear in practice, so that arbitrary diagnosis is common. When two species share a trait, evolution is defined as parallel if the ancestors are known to have shared that similarity; if not, it is defined as convergent. However, the stated conditions are a matter of degree; all organisms share common ancestors. Scientists differ on whether the distinction is useful.^[3]^[4]

Parallel evolution between marsupials and placentals

A number of examples of parallel evolution are provided by the two main branches of the mammals, the placentals and marsupials, which have followed independent evolutionary pathways following the break-up of land-masses such as Gondwanaland roughly 100 million years ago. In South America, marsupials and placentals shared the ecosystem (before the Great American Interchange); in Australia, marsupials prevailed; and in the Old World and North America the placentals won out. However, in all these localities mammals were small and filled only limited places in the ecosystem until the mass extinction of dinosaurs sixty-five million years ago. At this time, mammals on all three landmasses began to take on a much wider variety of forms and roles. While some forms were unique to each environment, surprisingly similar animals have often emerged in two or three of the separated continents. Examples of these include the placental sabre-toothed cats (Machairodontinae) and the South American marsupial sabre-tooth (Thylacosmilus); the Tasmanian wolf and the European wolf; likewise marsupial and placental moles, flying squirrels, and (arguably) mice.^{[citation needed]}

Parallel coevolution of traits between hummingbirds and sunbirds contributing to ecological guilds

Hummingbirds and sunbirds, two nectarivorous bird lineages in the New and Old Worlds have parallelly evolved a suite of specialized behavioral and anatomical traits. These traits (bill shape, digestive enzymes, and flight) allow the birds to optimally fit the flower-feeding-and-pollination ecological niche they occupy, which is shaped by the birds' suites of parallel traits. Thus, a parallel coevolved behavioral syndrome within the birds creates an emergent guild of highly specialized birds and highly adapted plants, each exploiting the other's involvement in the flowers' pollination in the Old World and New World alike.^[5]

The bill shape of nectarivores, being long and needle-like, allows them to reach down a flower's pistil/stamen and get at the nectar within. Nectarivores may also use their specialized bills to engage in nectar robbing, a practice seen in both hummingbirds and sunbirds in which the bird gets nectar by making a hole in the base of the flower's corolla tube instead of inserting its bill through the tube as is standard, thus "robbing" the flower of nectar since it is not pollinated it in return.^[6]

Nectarivores and ornithophilous flowers often exist in mutualistic guild relationships facilitated by the bird's bill shape, food source, and digestive ability acting in concert with the flower's tube shape and adaptation to pollination by hovering or perching birds. The birds eat nectar using their long, thin bills and, in so doing, collect pollen on their bills; this pollen is then transferred to the next flower they feed on. This mutualism coevolved in parallel between the Old World and New World birds and their respective flowers.^[7] Moreover, the digestive enzyme activity in nectarivores matching the nectar composition in their respective flowers appears to have coevolved in parallel between plants and pollinators across continents, as the nectarivorous lineages independently evolved the ability to digest the nectar specific to their flowers, resulting in distinct guilds.^[7]^[8]

The capacity of nectarivores to digest sucrose is far greater than that of other avian taxa. This difference is due to an analogous high concentration of sucrase-isomaltase, an enzyme that hydrolyzes sucrose. Sucrase activity per unit intestinal surface area appears to be higher in nectarivores than in other birds, meaning these nectarivorous avians can digest more sucrose more rapidly than other taxa.^[8] Moreover, the Adaptive Modulation Hypothesis does not apply for nectarivores and sugar-digesting enzymes, meaning that two lineages of nectarivores should not necessarily both have high sucrase-isomaltase concentrations even though they both eat nectar. Thus, parallel acquisition of analogous sucrose digestive capability is a reasonable conclusion because there is no apparent cause for the two lineages to share this high enzyme concentration.^[9]

References

^ Parallel evolution, an example may be the Pyrotherians evolved a body plan similar to proboscideans: Online Biology Glossary Archived 2007-07-13 at the Wayback Machine
^ ^a ^b Zhang, J. and Kumar, S. 1997. Detection of convergent and parallel evolution at the amino acid sequence level Archived 2016-03-03 at the Wayback Machine. Mol. Biol. Evol. 14, 527-36.
^ Arendt, J.; REZNICK, D. (January 2008). "Convergence and parallelism reconsidered: what have we learned about the genetics of adaptation?". Trends in Ecology & Evolution. 23 (1): 26–32. doi:10.1016/j.tree.2007.09.011. PMID 18022278.
^ Pearce, T. (10 November 2011). "Convergence and Parallelism in Evolution: A Neo-Gouldian Account". The British Journal for the Philosophy of Science. 63 (2): 429–448. doi:10.1093/bjps/axr046.
^ Janeček, Štěpán; Chmel, Kryštof; Uceda Gómez, Guillermo; Janečková, Petra; Chmelová, Eliška; Sejfová, Zuzana; Luma Ewome, Francis (February 2020). "Ecological fitting is a sufficient driver of tight interactions between sunbirds and ornithophilous plants". Ecology and Evolution. 10 (4): 1784–1793. doi:10.1002/ece3.5942. ISSN 2045-7758. PMC 7042734. PMID 32128116.
^ Juan Francisco Ornelas. Serrate Tomia: An Adaptation for Nectar Robbing in Hummingbirds?. The Auk, Volume 111, Issue 3, January 1994, pp. 703-710.
^ ^a ^b Janeček, Štěpán; Bartoš, Michael; Njabo, Kevin Yana (2015-01-22). "Convergent evolution of sunbird pollination systems of Impatiens species in tropical Africa and hummingbird systems of the New World". Biological Journal of the Linnean Society. 115 (1): 127–133. doi:10.1111/bij.12475. ISSN 0024-4066.
^ ^a ^b McWhorter, Todd J.; Rader, Jonathan A.; Schondube, Jorge E.; Nicolson, Susan W.; Pinshow, Berry; Fleming, Patricia A.; Gutiérrez-Guerrero, Yocelyn T.; Martínez del Rio, Carlos (July 2021). "Sucrose digestion capacity in birds shows convergent coevolution with nectar composition across continents". iScience. 24 (7): 102717. doi:10.1016/j.isci.2021.102717. ISSN 2589-0042. PMC 8246590. PMID 34235412.
^ Karasov, W. H. (1992-09-01). "Tests of the adaptive modulation hypothesis for dietary control of intestinal nutrient transport". American Journal of Physiology. Regulatory, Integrative and Comparative Physiology. 263 (3): R496–R502. doi:10.1152/ajpregu.1992.263.3.R496. ISSN 0363-6119. PMID 1415633.

Notes

Dawkins, R. 1986. The Blind Watchmaker. Norton & Company.
Mayr. 1997. What is Biology. Harvard University Press
Schluter, D., E. A. Clifford, M. Nemethy, and J. S. McKinnon. 2004. Parallel evolution and inheritance of quantitative traits. American Naturalist 163: 809–822.
McGhee, G.R. 2011. Convergent Evolution: Limited Forms Most Beautiful. Vienna Series in Theoretical Biology, Massachusetts Institute of Technology Press, Cambridge (MA). 322 pp.