Parallel evolution

• Colouration that serves as a warning to predators and for mating displays has evolved in many different species.
• In the plant kingdom, the most familiar examples of parallel evolution are the forms of leaves, where very similar patterns have appeared again and again in separate genera and families.
• In Arabidopsis thaliana it has been suggested that populations adapt to local climate through parallel evolution [5]
• In butterflies, many close similarities are found in the patterns of wing colouration, both within and among families.
• Old and New World porcupines shared a common ancestor, both evolved strikingly similar quill structures; this is also an example of convergent evolution as similar structures evolved in hedgehogs, echidnas and tenrecs.
• Some extinct archosaurs evolved an upright posture and likely were warm-blooded. These two characteristics are also found in most mammals. Modern crocodiles have a four chambered heart and a crurotarsal, the latter being also a characteristic of therian mammals.
• The extinct pterosaurs and the birds both evolved wings as well as a distinct beak, but not from a recent common ancestor.
• Internal fertilization has evolved independently in sharks, some amphibians and amniotes.
• The patagium is a fleshy membrane that is found in gliding mammals such as flying lemurs, flying squirrels, sugar gliders and the extinct Volaticotherium. These mammals all acquired the patagium independently.
• Pyrotherians evolved a body plan similar to proboscideans.
• The extinct South American litoptern ungulate Thoatherium had legs that are difficult to distinguish from those of horses.
• The eye of the octopus has the same complicated structure as the human eye. As a result, it is often substituted in studies of the eye when using a human eye would be inappropriate. As the two species diverged at the time animals evolved into deuterostomes and protostomes this is extraordinary.
• Certain arboreal frog species, 'flying' frogs, in both Old World families and New World families have developed the ability of gliding flight. They have "enlarged hands and feet, full webbing between all fingers and toes, lateral skin flaps on the arms and legs, and reduced weight per snout-vent length".[6]
• The tree plant habit has evolved separately in unrelated classes of plants.

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.

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.[7]

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.[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 than other taxa and in a quicker manner because the enzymes hydrolyzing sucrose are more abundant in their digestive tract.[9] Moreover, the Adaptive Modulation Hypothesis[10] 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 sharing this high enzyme concentration, even when the similarities of their diets are considered. In other words, specialized nectarivory is not necessarily associated with a high capacity to digest sucrose, meaning that the specialized sunbirds' and hummingbirds' sucrose digestive capacity is parallel because it is not required, yet both lineages possess it.

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.[11] 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.

Moreover, the digestive enzyme activity in nectarivores matching the nectar composition in their respective flowers appears to be an example of parallel coevolution 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 that evolved via the same means.[11][9]