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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 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 ]
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]
Hummingbirds are birds native to the Americas and comprise the biological family Trochilidae. With approximately 366 species and 113 genera, they occur from Alaska to Tierra del Fuego, but most species are found in Central and South America. As of 2024, 21 hummingbird species are listed as endangered or critically endangered, with numerous species declining in population.
Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last common ancestor of those groups. The cladistic term for the same phenomenon is homoplasy. The recurrent evolution of flight is a classic example, as flying insects, birds, pterosaurs, and bats have independently evolved the useful capacity of flight. Functionally similar features that have arisen through convergent evolution are analogous, whereas homologous structures or traits have a common origin but can have dissimilar functions. Bird, bat, and pterosaur wings are analogous structures, but their forelimbs are homologous, sharing an ancestral state despite serving different functions.
The honey possum or noolbenger, is a tiny species of marsupial that feeds on the nectar and pollen of a diverse range of flowering plants. Found only in southwest Australia, it is an important pollinator for such plants as Banksia attenuata, Banksia coccinea and Adenanthos cuneatus.
In biology, coevolution occurs when two or more species reciprocally affect each other's evolution through the process of natural selection. The term sometimes is used for two traits in the same species affecting each other's evolution, as well as gene-culture coevolution.
The honeyeaters are a large and diverse family, Meliphagidae, of small to medium-sized birds. The family includes the Australian chats, myzomelas, friarbirds, wattlebirds, miners and melidectes. They are most common in Australia and New Guinea, and found also in New Zealand, the Pacific islands as far east as Samoa and Tonga, and the islands to the north and west of New Guinea known as Wallacea. Bali, on the other side of the Wallace Line, has a single species.
Sunbirds and spiderhunters make up the family Nectariniidae of passerine birds. They are small, slender passerines from the Old World, usually with downward-curved bills. Many are brightly coloured, often with iridescent feathers, particularly in the males. Many species also have especially long tail feathers. Their range extends through most of Africa to the Middle East, South Asia, South-east Asia and southern China, to Indonesia, New Guinea and northern Australia. Species diversity is highest in equatorial regions.
Heliconia is a genus of flowering plants in the monotypic family Heliconiaceae. Most of the 194 known species are native to the tropical Americas, but a few are indigenous to certain islands of the western Pacific and Maluku in Indonesia. Many species of Heliconia are found in the tropical forests of these regions. Most species are listed as either vulnerable or data deficient by the IUCN Red List of threatened species. Several species are widely cultivated as ornamentals, and a few are naturalized in Florida, Gambia, and Thailand.
The southern giant hummingbird is one of two species in the genus Patagona and the second largest hummingbird species, after its close relative the northern giant hummingbird.
Babiana ringens, the rat's tail, is a flowering plant endemic to Cape Province of South Africa. The foliage is long and erect with an inflorescence consisting of a sterile main stalk adapted for ornithophily, pollination by birds. The plant bears bright red, tubular flowers on side branches close to the ground. It is a perennial that grows in nutrient-poor sandy soil and flowers during the winter rains.
Zoophily, or zoogamy, is a form of pollination whereby pollen is transferred by animals, usually by invertebrates but in some cases vertebrates, particularly birds and bats, but also by other animals. Zoophilous species frequently have evolved mechanisms to make themselves more appealing to the particular type of pollinator, e.g. brightly colored or scented flowers, nectar, and appealing shapes and patterns. These plant-animal relationships are often mutually beneficial because of the food source provided in exchange for pollination.
Nectar is a viscous, sugar-rich liquid produced by plants in glands called nectaries, either within the flowers with which it attracts pollinating animals, or by extrafloral nectaries, which provide a nutrient source to animal mutualists, which in turn provide herbivore protection. Common nectar-consuming pollinators include mosquitoes, hoverflies, wasps, bees, butterflies and moths, hummingbirds, honeyeaters and bats. Nectar plays a crucial role in the foraging economics and evolution of nectar-eating species; for example, nectar foraging behavior is largely responsible for the divergent evolution of the African honey bee, A. m. scutellata and the western honey bee.
In zoology, a nectarivore is an animal which derives its energy and nutrient requirements from a diet consisting mainly or exclusively of the sugar-rich nectar produced by flowering plants.
Ornithophily or bird pollination is the pollination of flowering plants by birds. This sometimes coevolutionary association is derived from insect pollination (entomophily) and is particularly well developed in some parts of the world, especially in the tropics, Southern Africa, and on some island chains. The association involves several distinctive plant adaptations forming a "pollination syndrome". The plants typically have colourful, often red, flowers with long tubular structures holding ample nectar and orientations of the stamen and stigma that ensure contact with the pollinator. Birds involved in ornithophily tend to be specialist nectarivores with brushy tongues and long bills, that are either capable of hovering flight or light enough to perch on the flower structures.
Pollination syndromes are suites of flower traits that have evolved in response to natural selection imposed by different pollen vectors, which can be abiotic or biotic, such as birds, bees, flies, and so forth through a process called pollinator-mediated selection. These traits include flower shape, size, colour, odour, reward type and amount, nectar composition, timing of flowering, etc. For example, tubular red flowers with copious nectar often attract birds; foul smelling flowers attract carrion flies or beetles, etc.
The malachite sunbird is a small nectarivorous bird found from the highlands of Ethiopia southwards to South Africa. They pollinate many flowering plants, particularly those with long corolla tubes, in the Fynbos.
The copper-throated sunbird is a small passerine within the Nectariniidae family. They sustain themselves by consuming nectar as well as small invertebrates. As a member of the Nectariniidae family, they possess downward curved beaks and have pectoral tufts of yellow feathers. They are found in south-eastern Asia, predominantly in coastal regions.
Nectar robbing is a foraging behavior used by some organisms that feed on floral nectar, carried out by feeding from holes bitten in flowers, rather than by entering through the flowers' natural openings. Nectar robbers usually feed in this way, avoiding contact with the floral reproductive structures, and therefore do not facilitate plant reproduction via pollination. Because many species that act as pollinators also act as nectar robbers, nectar robbing is considered to be a form of exploitation of plant-pollinator mutualism. While there is variation in the dependency on nectar for robber species, most species rob facultatively.
Eurotrochilus is an extinct genus of stem group hummingbirds (Trochilidae) and are the closest known relatives of the crown group Trochilidae. Despite Eurotrochilus being morphologically very similar to modern hummingbirds, they still retained several primitive features and are not closely related to any specific extant hummingbird in the crown group. There are currently two described species of Eurotrochilus: E. inexpectatus and E. noniewiczi.
