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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]
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]
A number of examples of parallel evolution are provided by the two main branches of themammals, theplacentals andmarsupials, which have followed independent evolutionary pathways following the break-up of land-masses such asGondwanaland roughly 100 million years ago. InSouth America, marsupials and placentals shared the ecosystem (before theGreat American Interchange); inAustralia, marsupials prevailed; and in theOld World andNorth America the placentals won out. However, in all these localities mammals were small and filled only limited places in the ecosystem until themass 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); theTasmanian wolf and the Europeanwolf; likewise marsupial and placentalmoles,flying squirrels, and (arguably)mice.[citation needed]
Hummingbirds andsunbirds, twonectarivorous bird lineages in the New and Old Worlds have parallelly evolved a suite of specializedbehavioral andanatomical traits. These traits (bill shape, digestive enzymes, andflight) allow the birds to optimally fit the flower-feeding-and-pollinationecological niche they occupy, which is shaped by the birds' suites of parallel traits. Thus, a parallel coevolved behavioral syndrome within the birds creates anemergentguild 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]
Thebill shape of nectarivores, being long and needle-like, allows them to reach down a flower'spistil/stamen and get at thenectar within. Nectarivores may also use their specialized bills to engage innectar 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'scorolla 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 andornithophilous 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 digestsucrose is far greater than that of other aviantaxa. This difference is due to an analogous high concentration ofsucrase-isomaltase, anenzyme thathydrolyzes 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]
