Convergent evolution is the independentevolution of similar features in species of different periods or epochs in time. Convergent evolution createsanalogous structures that have similar form or function but were not present in thelast common ancestor of those groups. Thecladistic term for the same phenomenon ishomoplasy. Therecurrent evolution of flight is a classic example, as flyinginsects,birds,pterosaurs, andbats have independently evolved the useful capacity of flight. Functionally similar features that have arisen through convergent evolution areanalogous, whereashomologous structures or traits have a common origin but can have dissimilar functions. Bird, bat, and pterosaurwings are analogous structures, but their forelimbs are homologous, sharing an ancestral state despite serving different functions.
The opposite of convergence isdivergent evolution, where related species evolve different traits. Convergent evolution is similar toparallel evolution, which occurs when two independent species evolve in the same direction and thus independently acquire similar characteristics; for instance,gliding frogs have evolved in parallel from multiple types oftree frog.
Homology and analogy in mammals and insects: on the horizontal axis, the structures are homologous in morphology, but different in function due to differences in habitat. On the vertical axis, the structures are analogous in function due to similar lifestyles but anatomically different with differentphylogeny.[a]
In morphology, analogous traits arise when different species live in similar ways and/or a similar environment, and so face the same environmental factors. When occupying similarecological niches (that is, a distinctive way of life) similar problems can lead to similar solutions.[1][2][3] The British anatomistRichard Owen was the first to identify the fundamental difference between analogies andhomologies.[4]
In his 1989 bookWonderful Life,Stephen Jay Gould argued that if one could "rewind the tape of life [and] the same conditions were encountered again, evolution could take a very different course."[6]Simon Conway Morris disputes this conclusion, arguing that convergence is a dominant force in evolution, and given that the same environmental and physical constraints are at work, life will inevitably evolve toward an "optimum" body plan, and at some point, evolution is bound to stumble uponintelligence, a trait presently identified with at leastprimates,corvids, andcetaceans.[7]
In cladistics, a homoplasy is a trait shared by two or moretaxa for any reason other than that they share a common ancestry. Taxa which do share ancestry are part of the sameclade; cladistics seeks to arrange them according to their degree of relatedness to describe theirphylogeny. Homoplastic traits caused by convergence are therefore, from the point of view of cladistics, confounding factors which could lead to an incorrect analysis.[8][9][10][11]
In some cases, it is difficult to tell whether a trait has been lost and then re-evolved convergently, or whether a gene has simply been switched off and then re-enabled later. Such a re-emerged trait is called anatavism. From a mathematical standpoint, an unused gene (selectively neutral) has a steadily decreasingprobability of retaining potential functionality over time. The time scale of this process varies greatly in different phylogenies; in mammals and birds, there is a reasonable probability of remaining in the genome in a potentially functional state for around 6 million years.[12]
Evolution at anamino acid position. In each case, the left-hand species changes from having alanine (A) at a specific position in a protein in a hypothetical ancestor, and now has serine (S) there. The right-hand species may undergodivergent, parallel, or convergent evolution at this amino acid position relative to the first species.
When two species are similar in a particular character, evolution is defined as parallel if the ancestors were also similar, and convergent if they were not.[b] Some scientists have argued that there is a continuum between parallel and convergent evolution,[13][14][15][16] while others maintain that despite some overlap, there are still important distinctions between the two.[17][18]
When the ancestral forms are unspecified or unknown, or the range of traits considered is not clearly specified, the distinction between parallel and convergent evolution becomes more subjective. For instance, the striking example of similar placental and marsupial forms is described byRichard Dawkins inThe Blind Watchmaker as a case of convergent evolution, because mammals on each continent had a long evolutionary history prior to the extinction of the dinosaurs under which to accumulate relevant differences.[19]
Many proteins share analogousstructural elements that arose independently across different genomes. There are several examples of convergent protein motifs sharing similar arrangements of structural elements.[20] Whole protein structures too have arisen through convergent evolution.[21]
Theenzymology ofproteases provides some of the clearest examples of convergent evolution. These examples reflect the intrinsic chemical constraints on enzymes, leading evolution to converge on equivalent solutions independently and repeatedly.[5][22]
Serine and cysteine proteases use different amino acid functional groups (alcohol or thiol) as anucleophile. To activate that nucleophile, they orient an acidic and a basic residue in acatalytic triad. The chemical and physical constraints onenzyme catalysis have caused identical triad arrangements to evolve independently more than 20 times in differentenzyme superfamilies.[5]
Threonine proteases use the amino acid threonine as their catalyticnucleophile. Unlike cysteine and serine, threonine is asecondary alcohol (i.e. has a methyl group). The methyl group of threonine greatly restricts the possible orientations of triad and substrate, as the methyl clashes with either the enzyme backbone or the histidine base. Consequently, most threonine proteases use an N-terminal threonine in order to avoid suchsteric clashes.Several evolutionarily independentenzyme superfamilies with differentprotein folds use the N-terminal residue as a nucleophile. This commonality ofactive site but difference of protein fold indicates that the active site evolved convergently in those families.[5][23]
Conus geographus produces a distinct form ofinsulin that is more similar to fish insulin protein sequences than to insulin from more closely related molluscs, suggesting convergent evolution,[24] though with the possibility ofhorizontal gene transfer.[25]
Ferrous iron uptake via protein transporters in land plants and chlorophytes
Distant homologues of the metal ion transportersZIP inland plants andchlorophytes have converged in structure, likely to take up Fe2+ efficiently. The IRT1 proteins fromArabidopsis thaliana andrice have extremely different amino acid sequences fromChlamydomonas's IRT1, but their three-dimensional structures are similar, suggesting convergent evolution.[26]
Na+,K+-ATPase and Insect resistance to cardiotonic steroids
Many examples of convergent evolution exist in insects in terms of developing resistance at a molecular level to toxins. One well-characterized example is the evolution of resistance to cardiotonic steroids (CTSs) via amino acid substitutions at well-defined positions of the α-subunit ofNa+,K+-ATPase (ATPalpha). Variation in ATPalpha has been surveyed in various CTS-adapted species spanning six insect orders.[27][28][29] Among 21 CTS-adapted species, 58 (76%) of 76 amino acid substitutions at sites implicated in CTS resistance occur in parallel in at least two lineages.[29] 30 of these substitutions (40%) occur at just two sites in the protein (positions 111 and 122). CTS-adapted species have also recurrently evolvedneo-functionalized duplications of ATPalpha, with convergent tissue-specific expression patterns.[27][29]
Swimming animals includingfish such asherrings,marine mammals such asdolphins, andichthyosaurs (of the Mesozoic) all converged on the same streamlined shape.[36][37] A similar shape and swimming adaptations are even present in molluscs, such asPhylliroe.[38] The fusiform bodyshape (a tube tapered at both ends) adopted by many aquatic animals is an adaptation to enable them totravel at high speed in a highdrag environment.[39] Similar body shapes are found in theearless seals and theeared seals: they still have four legs, but these are strongly modified for swimming.[40]
The marsupial fauna of Australia and the placental mammals of the Old World have several strikingly similar forms, developed in two clades, isolated from each other.[7] The body, and especially the skull shape, of thethylacine (Tasmanian tiger or Tasmanian wolf) converged with those ofCanidae such as the red fox,Vulpes vulpes.[41]
TheGymnotiformes of South America and theMormyridae of Africa independently evolvedpassive electroreception (around 119 and 110 million years ago, respectively). Around 20 million years after acquiring that ability, both groups evolved activeelectrogenesis, producing weak electric fields to help them detect prey.[43]
The camera eyes ofvertebrates (left) andcephalopods (right) developed independently and are wired differently; for instance,optic nerve(3) fibres(2) reach the vertebrateretina(1) from the front, creating ablind spot(4).[44]
One of the best-known examples of convergent evolution is the camera eye ofcephalopods (such as squid and octopus),vertebrates (including mammals) andcnidarians (such as jellyfish).[45] Their last common ancestor had at most a simple photoreceptive spot, but a range of processes led to theprogressive refinement of camera eyes—with one sharp difference: the cephalopod eye is "wired" in the opposite direction, with blood and nerve vessels entering from the back of the retina, rather than the front as in vertebrates. As a result, vertebrates have ablind spot.[7]
Vertebrate wings are partlyhomologous (from forelimbs), but analogous as organs of flight in (1)pterosaurs, (2)bats, (3)birds, evolved separately.
Birds andbats havehomologous limbs because they are both ultimately derived from terrestrialtetrapods, but their flight mechanisms are only analogous, so their wings are examples of functional convergence. The two groups have independently evolved their own means of powered flight. Their wings differ substantially in construction. The bat wing is a membrane stretched across four extremely elongated fingers and the legs. The airfoil of the bird wing is made offeathers, strongly attached to the forearm (the ulna) and the highly fused bones of the wrist and hand (thecarpometacarpus), with only tiny remnants of two fingers remaining, each anchoring a single feather. So, while the wings of bats and birds are functionally convergent, they are not anatomically convergent.[3][47] Birds and bats also share a high concentration ofcerebrosides in the skin of their wings. This improves skin flexibility, a trait useful for flying animals; other mammals have a far lower concentration.[48] The extinctpterosaurs independently evolved wings from their fore- and hindlimbs, whileinsects havewings that evolved separately from different organs.[49]
Flying squirrels andsugar gliders are much alike in their mammalian body plans, with gliding wings stretched between their limbs, but flying squirrels are placentals while sugar gliders are marsupials, widely separated within the mammal lineage from the placentals.[50]
Insect mouthparts show many examples of convergent evolution. The mouthparts of different insect groups consist of a set ofhomologous organs, specialised for the dietary intake of that insect group. Convergent evolution of many groups of insects led from original biting-chewing mouthparts to different, more specialised, derived function types. These include, for example, theproboscis of flower-visiting insects such asbees andflower beetles,[52][53][54] or the biting-sucking mouthparts of blood-sucking insects such asfleas andmosquitos.
Opposable thumbs allowing the grasping of objects are most often associated withprimates, like humans and other apes, monkeys, and lemurs. Opposable thumbs also evolved ingiant pandas, but these are completely different in structure, having six fingers including the thumb, which develops from a wrist bone entirely separately from other fingers.[55]
Despite the similar lightening ofskin colour after movingout of Africa, different genes were involved in European (left) and East Asian (right) lineages.
Convergent evolution in humans includes blue eye colour and light skin colour.[56] When humans migratedout of Africa, they moved to more northern latitudes with less intense sunlight.[56] It was beneficial to them to have reducedskin pigmentation.[56] It appears certain that there was some lightening of skin colourbefore European and East Asian lineages diverged, as there are some skin-lightening genetic differences that are common to both groups.[56] However, after the lineages diverged and became genetically isolated, the skin of both groups lightened more, and that additional lightening was due todifferent genetic changes.[56]
Humans
Lemurs
Despite the similarity of appearance, the genetic basis of blue eyes is different in humans andlemurs.
Lemurs andhumans are both primates. Ancestral primates had brown eyes, as most primates do today. The genetic basis of blue eyes in humans has been studied in detail and much is known about it. It is not the case that onegene locus is responsible, say with brown dominant to blueeye colour. However, a single locus is responsible for about 80% of the variation. In lemurs, the differences between blue and brown eyes are not completely known, but the same gene locus is not involved.[57]
While most plant species areperennial, about 6% follow anannual life cycle, living for only one growing season.[58] The annual life cycle independently emerged in over 120 plant families of angiosperms.[59][60] The prevalence of annual species increases under hot-dry summer conditions in the four species-rich families of annuals (Asteraceae,Brassicaceae,Fabaceae, andPoaceae), indicating that the annual life cycle is adaptive.[58][61]
Fruits with a wide variety of structural origins have converged to become edible.Apples arepomes with fivecarpels; their accessory tissues form the apple's core, surrounded by structures from outside the botanical fruit, thereceptacle orhypanthium. Other edible fruits include other plant tissues;[68] the fleshy part of atomato is the walls of thepericarp.[69] This implies convergent evolution under selective pressure, in this case the competition forseed dispersal by animals through consumption of fleshy fruits.[70]
Seed dispersal by ants (myrmecochory) has evolved independently more than 100 times, and is present in more than 11,000 plant species. It is one of the most dramatic examples of convergent evolution in biology.[71]
Carnivory has evolved multiple times independently in plants in widely separated groups. In three species studied,Cephalotus follicularis,Nepenthes alata andSarracenia purpurea, there has been convergence at the molecular level. Carnivorous plants secreteenzymes into the digestive fluid they produce. By studyingphosphatase,glycoside hydrolase,glucanase,RNAse andchitinaseenzymes as well as apathogenesis-related protein and athaumatin-related protein, the authors found many convergentamino acid substitutions. These changes were not at the enzymes' catalytic sites, but rather on the exposed surfaces of the proteins, where they might interact with other components of the cell or the digestive fluid. The authors also found thathomologous genes in the non-carnivorous plantArabidopsis thaliana tend to have their expression increased when the plant is stressed, leading the authors to suggest that stress-responsive proteins have often been co-opted[c] in the repeated evolution of carnivory.[72]
Angiosperm phylogeny of orders based on classification by the Angiosperm Phylogeny Group. The figure shows the number of inferred independent origins of C3-C4 photosynthesis andC4 photosynthesis in parentheses.
Phylogenetic reconstruction andancestral state reconstruction proceed by assuming that evolution has occurred without convergence. Convergent patterns may, however, appear at higher levels in a phylogenetic reconstruction, and are sometimes explicitly sought by investigators. The methods applied to infer convergent evolution depend on whether pattern-based or process-based convergence is expected. Pattern-based convergence is the broader term, for when two or more lineages independently evolve patterns of similar traits. Process-based convergence is when the convergence is due to similar forces ofnatural selection.[73]
Earlier methods for measuring convergence incorporate ratios of phenotypic andphylogenetic distance by simulating evolution with aBrownian motion model of trait evolution along a phylogeny.[74][75] More recent methods also quantify the strength of convergence.[76] One drawback to keep in mind is that these methods can confuse long-term stasis with convergence due to phenotypic similarities. Stasis occurs when there is little evolutionary change among taxa.[73]
Distance-based measures assess the degree of similarity between lineages over time. Frequency-based measures assess the number of lineages that have evolved in a particular trait space.[73]
Methods to infer process-based convergence fit models of selection to a phylogeny and continuous trait data to determine whether the same selective forces have acted upon lineages. This uses theOrnstein–Uhlenbeck process to test different scenarios of selection. Other methods rely on ana priori specification of where shifts in selection have occurred.[77]
Incomplete lineage sorting – Characteristic of phylogenetic analysis: the presence of multiple alleles in ancestral populations might lead to the impression that convergent evolution has occurred.
Carcinisation – Evolution of crustaceans into crab-like forms
^However, all organisms share a common ancestor more or less recently, so the question of how far back to look in evolutionary time and how similar the ancestors need to be for one to consider parallel evolution to have taken place is not entirely resolved within evolutionary biology.
^Kirk, John Thomas Osmond (2007).Science & Certainty. Csiro Publishing. p. 79.ISBN978-0-643-09391-1.Archived from the original on 15 February 2017. Retrieved23 January 2017.evolutionary convergence, which, quoting .. Simon Conway Morris .. is the 'recurring tendency of biological organization to arrive at the same "solution" to a particular "need". .. the 'Tasmanian tiger' .. looked and behaved like a wolf and occupied a similar ecological niche, but was in fact a marsupial not a placental mammal.
^Reece, J.; Meyers, N.; Urry, L.; Cain, M.; Wasserman, S.; Minorsky, P.; Jackson, R.; Cooke, B. (5 September 2011).Cambell Biology, 9th Edition. Pearson. p. 586.ISBN978-1-4425-3176-5.
^Dobler, S., Dalla, S., Wagschal, V., & Agrawal, A. A. (2012). Community-wide convergent evolution in insect adaptation to toxic cardenolides by substitutions in the Na,K-ATPase. Proceedings of the National Academy of Sciences, 109(32), 13040–13045.https://doi.org/10.1073/pnas.1202111109
^Werdelin, L. (1986). "Comparison of Skull Shape in Marsupial and Placental Carnivores".Australian Journal of Zoology.34 (2):109–117.doi:10.1071/ZO9860109.
^Sage, Rowan; Russell Monson (1999)."7".C4 Plant Biology. Elsevier. pp. 228–229.ISBN978-0-12-614440-6.
^Kadereit, G.; Borsch, T.; Weising, K.; Freitag, H (2003). "Phylogeny of Amaranthaceae and Chenopodiaceae and the Evolution of C4 Photosynthesis".International Journal of Plant Sciences.164 (6):959–86.Bibcode:2003IJPlS.164..959K.doi:10.1086/378649.S2CID83564261.
^Lengyel, S.; Gove, A. D.; Latimer, A. M.; Majer, J. D.; Dunn, R. R. (2010). "Convergent evolution of seed dispersal by ants, and phylogeny and biogeography in flowering plants: a global survey".Perspectives in Plant Ecology, Evolution and Systematics.12 (1):43–55.Bibcode:2010PPEES..12...43L.doi:10.1016/j.ppees.2009.08.001.
^abcStayton, C. Tristan (2015). "The definition, recognition, and interpretation of convergent evolution, and two new measures for quantifying and assessing the significance of convergence".Evolution.69 (8):2140–2153.doi:10.1111/evo.12729.PMID26177938.S2CID3161530.
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