convergence

In evolutionary biology, convergent evolution is the process whereby organisms not closely related (not monophyletic), independently evolve similar traits as a result of having to adapt to similar environments or ecological niches^[1]. It is the opposite of divergent evolution, where related species evolve different traits. On a molecular level, this can happen due to random mutation unrelated to adaptive changes; see long branch attraction.

In cultural evolution, convergent evolution is the development of similar cultural adaptations to similar environmental conditions by different peoples with different ancestral cultures.

An example of convergent evolution is the similar nature of the flight/wings of insects, birds, pterosaurs, and bats. All four serve the same function and are similar in structure, but each evolved independently. Some aspects of the lens of eyes also evolved independently in various animals.

Convergent evolution is similar to, but distinguishable from, the phenomena of evolutionary relay and parallel evolution. Evolutionary relay refers to independent species acquiring similar characteristics through their evolution in similar ecosystems, but not at the same time (e.g. dorsal fins of extinct ichthyosaurs and sharks). Parallel evolution occurs when two independent species evolve together at the same time in the same ecospace and acquire similar characteristics (extinct browsing-horses and extinct paleotheres).

Structures that are the result of convergent evolution are called analogous structures or homoplasies; they should be contrasted with homologous structures, which have a common origin.

Animal examples

Mammals

• The marsupial Thylacine and placental Canids.

Thylacine, a canine-like marsupial

Golden Jackal, a true canine

• Several mammal groups have independently evolved prickly protrusions of the skin, called spines - echidnas (monotremes), hedgehogs (insectivores), Old World porcupines (rodents) and New World porcupines (a separate group of rodents). In this case, because the two groups of porcupines are relatively closely related, they would be considered to be an example of parallel evolution; neither echidnas nor hedgehogs, however, are closely related to rodents at all. In fact, the last common ancestor of all four groups was a contemporary of the dinosaurs.
• Cat-like, sabre-toothed predators evolved in three distinct lineages of mammals — sabre-toothed cats, Nimravids (false sabre-tooths), and the marsupial thylacosmilids. Gorgonopsids and creodonts also developed long canines, but that is the only physical similarity.
• A number of mammals have developed claws and long, sticky tongues that allow them to open the homes of social insects (e.g. ants and termites) and eat them. These include the four species of anteater, about 20 species of armadillo, eight species of pangolin, the African aardvark, four species of echidna, and the Australian numbat.
• Koalas of Australasia have evolved fingerprints, very similar to those of humans. The Australian honey possum has developed a long tongue for taking nectar from flowers, the same sort of structure that butterflies possess to accomplish the same task.

Birds

• The Little Auk of the north Atlantic (Charadriiformes) and the diving petrels of the southern oceans (Procellariiformes) are remarkably similar in appearance and habits.
• The similar evolution of auks in the Northern Hemisphere and penguins in the Southern Hemisphere.
• Vultures come in two varieties as a result of convergent evolution: both Old World vultures and New World vultures eat carrion, but Old World vultures are in the eagle and hawk family and use eyesight for food discovery; the New World vultures are related to storks and use the sense of smell (as well as sight) to find carrion. In both cases they search for food by soaring, circle over carrion, and group in trees, and both have featherless necks.

Nubian vulture, an Old World vulture

Turkey vulture, a New World vulture

Hummingbird, a New World bird, with Sunbird, an old world bird

• The Flightless Cormorant of the Galapagos Islands, unlike other cormorants, now has wings developed for swimming rather than flight, equal in proportion to penguins.

Other

• The similarities in diet and activity patterns between the thorny devil (Moloch horridus) and the Texas horned lizard (Phrynosoma cornutum) both in different clades.
• Modern Crocodilians, and prehistoric phytosaurs, champsosaurs, and certain labyrinthodont amphibians. The resemblance between the crocodilians and phytosaurs in particular is quite striking.
• The Neotropical poison dart frog and the Mantella of Madagascar have independently developed similar mechanisms for obtaining alkaloids from a diet of ants and storing the toxic chemicals in skin glands. They have also independently evolved similar bright skin colors that warn predators of their toxicity–(by the opposite of crypsis, namely aposematism).
• Assassin spiders are a group comprising two lineages that evolved independently. They have very long necks and fangs proportionately larger than those of any other spider, and hunt other spiders by snagging them from a distance.
• The smelling organs of the terrestrial coconut crab are similar to those of insects.
• The body shape of the prehistoric fish-like reptile Ophthalmosaurus and other ichthyosaurians, dolphins (aquatic mammals), and tuna (scombrid fish).
• The brachiopods and bivalve molluscs, which both have very similar shells.

Plant examples

• Prickles, thorns and spines are all modified plant tissues that have evolved to prevent or limit herbivory, these structures have evolved independently a number of times.
• The aerial rootlets found in ivy (Hedera) are similar to those of the Climbing Hydrangea (Hydrangea petiolaris) and some other vines. These rootlets are not derived from a common ancestor but have the same function of clinging to whatever support is available.
• Many Euphorbia and Cactaceae species occur in hot, dry environments and have similar modifications (see picture below).

Euphorbia obesa

Astrophytum asterias

Examples for convergent evolution of enzymes and biochemical pathways

• The existence of distinct families of carbonic anhydrase is believed to illustrate convergent evolution.
• The use of (Z)-7-dodecen-1-yl acetate as a sex pheromone by the Asian elephant (Elephas maximus) and by more than 100 species of Lepidoptera.
• The independent development of the catalytic triad in serine proteases independently with subtilisin in prokaryotes and the chymotrypsin clan in eukaryotes.
• The repeated independent evolution of nylonase in two different strains of Flavobacterium and one strain of Pseudomonas.
• The biosynthesis of plant hormones such as gibberellin and abscisic acid by different biochemical pathways in plants and fungi. ^[1][2]

References

1. ^ Tudzynski B. (2005). "Gibberellin biosynthesis in fungi: genes, enzymes, evolution, and impact on biotechnology". Appl Microbiol Biotechnol. 66: 597-611. PMID 15578178. 
2. ^ Siewers V, Smedsgaard J, Tudzynski P. (2004). "The P450 monooxygenase BcABA1 is essential for abscisic acid biosynthesis in Botrytis cinerea.". Appl Environ. Microbiol. 70: 3868-3876. PMID 15240257. 

• Rasmussen, L.E.L., Lee, T.D., Roelofs, W.L., Zhang, A., Doyle Davies Jr, G. (1996). Insect pheromone in elephants. Nature. 379: 684
• Convergent Evolution Examples- Ecological Equivalents, Department of Biology, Bellarmine University

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