Convergent evolution is the independent
evolution
Evolution is change in the heritable
Heredity, also called inheritance or biological inheritance, is the passing on of Phenotypic trait, traits from parents to their offspring; either through asexual reproduction or sexual reproduction, ...

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
Cladistics (; ) is an approach to Taxonomy (biology), biological classification in which organisms are categorized in groups ("clades") based on hypotheses of most recent common ancestry. The evidence for hypothesized relationships is typically ...
term for the same phenomenon is
homoplasy
Homoplasy, in biology and phylogenetics, is when a Phenotypic trait, trait has been gained or lost independently in separate lineages over the course of evolution. This is different from Homology (biology), homology, which is the similarity of trait ...
. The
recurrent evolution
Recurrent evolution is the repeated evolution of a particular trait, character, or mutation (biology), mutation. Most evolution is the result of Genetic drift, drift, often interpreted as the random chance of some alleles being passed down to the ...
of flight is a classic example, as flying
insect
Insects (from Latin
Latin (, or , ) is a classical language
A classical language is a language
A language is a structured system of communication
Communication (from Latin ''communicare'', meaning "to share" or "to be in ...

s,
bird
Birds are a group of warm-blooded vertebrate
Vertebrates () comprise all species of animal
Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With ...

s,
pterosaurs
Pterosaurs (; from Greek ''pteron'' and ''sauros'', meaning "wing lizard") were flying reptile
Reptiles are tetrapod
Tetrapods (; from Greek 'four' and 'foot') are four-limbed animals constituting the superclass Tetrapoda . It includes ...

, and
bat
Bats are mammal
Mammals (from Latin
Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the po ...

s 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
A wing is a type of fin that produces lift while moving through air or some other fluid. Accordingly, wings have streamlined cross-sections that are subject to aerodynamic forces and act as airfoil
An airfoil (American English
Am ...

are analogous structures, but their forelimbs are homologous, sharing an ancestral state despite serving different functions.
The opposite of convergence is
divergent evolution
Divergence is a function that associates a scalar with every point of a vector field.
Divergence or Divergent may also refer to:
Mathematics
* Divergence (computer science), a computation which does not terminate (or terminates in an exception ...
, where related species evolve different traits. Convergent evolution is similar to
parallel evolution
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.Zhang, J. and Kumar, S. 1997Detection of convergent and parall ...
, 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 of
tree frog
A treefrog, or tree frog, is any species of frog
A frog is any member of a diverse and largely Carnivore, carnivorous group of short-bodied, tailless amphibians composing the order (biology), order Anura (literally ''without tail'' in Ancient ...

.
Many instances of convergent evolution are known in
plant
Plants are predominantly photosynthetic
Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, through cellular respiration, can later be released to fuel ...

s, including the repeated development of
,
seed dispersal
Seed dispersal is the movement, spread or transport of seed
A seed is an embryonic
''Embryonic'' is the twelfth studio album by experimental rock band the Flaming Lips released on October 13, 2009, on Warner Bros. Records, Warner Bros. The ban ...
by fleshy
fruit
In botany
Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist who specialises in this field. The term "botany" comes from the ...

s adapted to be eaten by animals, and
carnivory
A carnivore , meaning "meat
Meat is animal flesh that is eaten as food. Humans have hunted and killed animals for meat since prehistoric times. The advent of civilization allowed the domestication of animals such as chickens, sheep, rabbi ...

.
Overview

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 similar
ecological niche
In ecology
Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers organisms
...

s (that is, a distinctive way of life) similar problems can lead to similar solutions.
[ The British anatomist ]Richard Owen
Sir Richard Owen (20 July 1804 – 18 December 1892) was an English biologist, comparative anatomist and paleontologist. Despite being a controversial figure, Owen is generally considered to have been an outstanding naturalist with a remark ...

was the first to identify the fundamental difference between analogies and homologies.
In biochemistry, physical and chemical constraints on have caused some arrangements such as the catalytic triad
A catalytic triad is a set of three coordinated amino acids that can be found in the active site of some enzymes. Catalytic triads are most commonly found in hydrolase and transferase enzymes (e.g. proteases, amidases, esterases, acylases, lipas ...

to evolve independently in separate enzyme superfamiliesA protein superfamily is the largest grouping (clade
A clade (; from grc, , ''klados'', "branch"), also known as a monophyletic group or natural group, is a group of organisms that are monophyly, monophyletic—that is, composed of a common anc ...
.
In his 1989 book '' Wonderful Life'', Stephen Jay Gould
Stephen Jay Gould (; September 10, 1941 – May 20, 2002) was an American Paleontology, paleontologist, Evolutionary biology, evolutionary biologist, and History of science, historian of science. He was one of the most influential and widely read ...
argued that if one could "rewind the tape of life the same conditions were encountered again, evolution could take a very different course." Simon Conway Morris
Simon Conway Morris (born 1951) is an English palaeontologist
Paleontology, also spelled palaeontology or palæontology (), is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene
The ...
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 upon intelligence, a trait presently identified with at least primates
A primate ( ) (from Latin , from 'prime, first rank') is a eutherian mammal constituting the Taxonomy (biology), taxonomic order (biology), order Primates (). Primates arose 85–55 million years ago first from small Terrestrial animal, ...
, corvids
Corvidae is a Cosmopolitan distribution, cosmopolitan family (biology), family of oscine passerine birds that contains the crows, ravens, rook (bird), rooks, jackdaws, jays, magpies, treepies, choughs, and nutcracker (bird), nutcrackers. In Engli ...
, and cetaceans
Cetaceans (from la, Cetus (mythology), cetus, lit=whale, from grc, κῆτος, translit=Cetus (mythology), kētos, lit = huge fish, sea monster) are aquatic mammals constituting the infraorder Cetacea (). Key characteristics are their fully aq ...

.[
]
Distinctions
Cladistics
In cladistics, a homoplasy is a trait shared by two or more taxa
In biology
Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechani ...
for any reason other than that they share a common ancestry. Taxa which do share ancestry are part of the same clade
A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyly, monophyletic – that is, composed of a common ancestor and all its lineage (evolution), lineal descendants - on a phylogenetic tree. R ...

; cladistics seeks to arrange them according to their degree of relatedness to describe their phylogeny
A phylogenetic tree (also phylogeny or evolutionary tree Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA.) is a branching diagram or a tree (graph theory), tree showing the evolutionary relationships among va ...

. Homoplastic traits caused by convergence are therefore, from the point of view of cladistics, confounding factors which could lead to an incorrect analysis.
Atavism
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 an atavism
In biology, an atavism is a modification of a biological structure whereby an ancestral genetic Trait (biological), trait reappears after having been lost through evolutionary change in previous generations. Atavisms can occur in several ways; one ...
. From a mathematical standpoint, an unused gene () has a steadily decreasing probability
Probability is the branch of mathematics
Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained ...

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.
Parallel vs. convergent evolution
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. Some scientists have argued that there is a continuum between parallel and convergent evolution, while others maintain that despite some overlap, there are still important distinctions between the two.
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 by Richard Dawkins
Richard Dawkins (born 26 March 1941) is a British evolutionary biology, evolutionary biologist and author. He is an Oxford fellow, emeritus fellow of New College, Oxford and was Simonyi Professor for the Public Understanding of Science, Prof ...

in ''The Blind Watchmaker
''The Blind Watchmaker: Why the Evidence of Evolution Reveals a Universe without Design'' is a 1986 book by , in which the author presents an explanation of, and argument for, the theory of by means of . He also presents arguments to refute ...
'' 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.
At molecular level
Proteins
Protease active sites
The enzymology
Enzymes () are protein
Proteins are large biomolecule
, showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958 ...
of proteases
A protease (also called a peptidase or proteinase) is an enzyme that catalysis, catalyzes (increases reaction rate or "speeds up") proteolysis, breaking down proteins into smaller polypeptides or single amino acids, and spurring the formation of n ...
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.
Serine and cysteine proteases use different amino acid functional groups (alcohol or thiol) as a nucleophile
In chemistry
Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo during a ...

. In order to activate that nucleophile, they orient an acidic and a basic residue in a catalytic triad
A catalytic triad is a set of three coordinated amino acids that can be found in the active site of some enzymes. Catalytic triads are most commonly found in hydrolase and transferase enzymes (e.g. proteases, amidases, esterases, acylases, lipas ...

. The chemical and physical constraints on enzyme catalysis
Enzyme catalysis is the increase in the rate of a process
A process is a series or set of activities that interact to produce a result; it may occur once-only or be recurrent or periodic.
Things called a process include:
Business and managem ...

have caused identical triad arrangements to evolve independently more than 20 times in different enzyme superfamiliesA protein superfamily is the largest grouping (clade
A clade (; from grc, , ''klados'', "branch"), also known as a monophyletic group or natural group, is a group of organisms that are monophyly, monophyletic—that is, composed of a common anc ...
.
Threonine protease
Threonine proteases are a family of proteolytic
Proteolysis is the breakdown of proteins into smaller polypeptides or amino acids. Uncatalysed, the hydrolysis of peptide bonds is extremely slow, taking hundreds of years. Proteolysis is typically ...
s use the amino acid threonine as their catalytic nucleophile
In chemistry
Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo during a ...

. Unlike cysteine and serine, threonine is a secondary alcohol
upright=0.8, The bond angle between a hydroxyl group (-OH) and a chain of carbon atoms (R)
In chemistry
Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, ...

(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 such steric clashes.
Several evolutionarily independent enzyme superfamiliesA protein superfamily is the largest grouping (clade
A clade (; from grc, , ''klados'', "branch"), also known as a monophyletic group or natural group, is a group of organisms that are monophyly, monophyletic—that is, composed of a common anc ...
with different protein foldA protein superfamily is the largest grouping (clade
A clade (; from grc, , ''klados'', "branch"), also known as a monophyletic group or natural group, is a group of organisms that are monophyly, monophyletic—that is, composed of a common anc ...

s use the N-terminal residue as a nucleophile. This commonality of but difference of protein fold indicates that the active site evolved convergently in those families.
Cone snail and fish insulin
'' Conus geographus'' produces a distinct form of insulin
Insulin (, from Latin ''insula'', 'island') is a peptide hormone produced by beta cells of the pancreatic islets; it is considered to be the main Anabolism, anabolic hormone of the body. It regulates the metabolism of carbohydrates, fats and p ...

that is more similar to fish insulin protein sequences than to insulin from more closely related molluscs, suggesting convergent evolution.
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 of Na+,K+-ATPase (ATPalpha). Variation in ATPalpha has been surveyed in various CTS-adapted species spanning 6 insect orders. 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. 30 of these substitutions (40%) occur at just two sites in the protein (positions 111 and 122). CTS-adapted species have also recurrently evolved neo-functionalized duplications of ATPalpha, with convergent tissue-specific expression patterns.
Nucleic acids
Convergence occurs at the level of DNA
Deoxyribonucleic acid (; DNA) is a molecule
File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings.
A molecule is an electrically neutral gro ...

and the amino acid
Amino acids are organic compound
In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compounds are known. The study of the properties, reactions, a ...

sequences produced by structural genes into protein
Proteins are large biomolecule
, showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a No ...

s. Studies have found convergence in amino acid sequences in echolocating bats and the dolphin; among marine mammals; between giant and red pandas; and between the thylacine and canids. Convergence has also been detected in a type of non-coding DNA
Non-coding DNA sequences are components of an organism's DNA
Deoxyribonucleic acid (; DNA) is a molecule
File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of ...
, cis-regulatory element
''Cis''-regulatory elements (CREs) or ''Cis''-regulatory modules (CRMs) are regions of non-coding DNA
Non-coding DNA sequences are components of an organism's DNA
Deoxyribonucleic acid (; DNA) is a molecule
File:Pentacene on Ni(111) ...
s, such as in their rates of evolution; this could indicate either positive selection
upThree types of selection
In population genetics, directional selection, or positive selection is a mode of natural selection
Natural selection is the differential survival and reproduction of individuals due to differences in pheno ...
or relaxed purifying selection
In natural selection, negative selection or purifying selection is the selective removal of alleles that are Mutation#Harmful mutations, deleterious. This can result in stabilizing selection through the purging of deleterious genetic polymorphisms ...
.
In animal morphology
Bodyplans
Swimming animals including fish
Fish are aquatic
Aquatic means relating to water
Water (chemical formula H2O) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the ...

such as herring
Herring are forage fish
Forage fish, also called prey fish or bait fish, are small pelagic fish which are preyed on by larger predators for food. Predators include other larger fish, seabirds and marine mammals. Typical ocean forage fish feed ...

s, marine mammals
Marine mammals are aquatic mammals that rely on the ocean and other marine ecosystems for their existence. They include animals such as seals, whales, manatees, sea otters and polar bears. They are an informal group, unified only by their ...
such as dolphins
Dolphin is a common name of aquatic mammals within the infraorder Cetacea
Cetaceans (from la, Cetus (mythology), cetus, lit=whale, from grc, κῆτος, translit=Cetus (mythology), kētos, lit = huge fish, sea monster) are aquatic mamma ...

, and ichthyosaur
Ichthyosaurs (Ancient Greek
Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Myce ...
s ( of the Mesozoic) all converged on the same streamlined shape. A similar shape and swimming adaptations are even present in molluscs, such as '' Phylliroe''. The fusiform bodyshape (a tube tapered at both ends) adopted by many aquatic animals is an adaptation to enable them to travel at high speed in a high drag
Drag or The Drag may refer to:
Places
* Drag, Norway, a village in Tysfjord municipality, Nordland, Norway
* ''Drág'', the Hungarian name for Dragu Commune in Sălaj County, Romania
* Drag (Austin, Texas), the portion of Guadalupe Street adja ...
environment. Similar body shapes are found in the earless seal
The earless seals, phocids or true seals are one of the three main groups of mammal
Mammals (from Latin
Latin (, or , ) is a classical language
A classical language is a language
A language is a structured system of comm ...
s and the eared seals: they still have four legs, but these are strongly modified for swimming.
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. The body and especially the skull shape of the thylacine
The thylacine ( , or , also ) (''Thylacinus cynocephalus'') is an extinct carnivorous marsupial that was native to the Mainland Australia, Australian mainland and the islands of Tasmania and New Guinea. The last known live animal was capture ...

(Tasmanian tiger or Tasmanian wolf) converged with those of Canidae
Canidae (; from Latin, ''canis'', "dog") is a biological family
Family ( la, familia, plural ') is one of the eight major hierarchical taxonomic ranks in Linnaean taxonomy; it is classified between order (biology), order and genus. A famil ...
such as the red fox, ''Vulpes vulpes
The red fox (''Vulpes vulpes'') is the largest of the true foxes and one of the most widely distributed members of the order
Order or ORDER or Orders may refer to:
* Orderliness
Orderliness is associated with other qualities such as cleanli ...

''.
marsupial
Marsupials are any members of the mammal
Mammals (from Latin
Latin (, or , ) is a classical language
A classical language is a language
A language is a structured system of communication
Communication (from Latin ' ...
and