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 opposite of convergence is
divergent evolution, where related species evolve different traits. Convergent evolution is similar to
parallel 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 of
tree frog.
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 niches (that is, a distinctive way of life) similar problems can lead to similar solutions.[1][2][3] The British anatomist
Richard Owen was the first to identify the fundamental difference between analogies and
homologies.[4]
In his 1989 book Wonderful 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 upon intelligence, a trait presently identified with at least
primates,
corvids, and
cetaceans.[7]
In cladistics, a homoplasy is a trait shared by two or more
taxa for any reason other than that they share a common ancestry. Taxa which do share ancestry are part of the same
clade; cladistics seeks to arrange them according to their degree of relatedness to describe their
phylogeny. 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 an
atavism. From a mathematical standpoint, an unused gene (
selectively neutral) has a steadily decreasing
probability 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]
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.[b] Some scientists have argued that there is a continuum between parallel and convergent evolution,[13][14] while others maintain that despite some overlap, there are still important distinctions between the two.[15][16]
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 in The 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.[17]
The
enzymology of
proteases 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][18]
Serine and cysteine proteases use different amino acid functional groups (alcohol or thiol) as a
nucleophile. In order to activate that nucleophile, they orient an acidic and a basic residue in a
catalytic triad. The chemical and physical constraints on
enzyme catalysis have caused identical triad arrangements to evolve independently more than 20 times in different
enzyme superfamilies.[5]
Threonine proteases use the amino acid threonine as their catalytic
nucleophile. Unlike cysteine and serine, threonine is a
secondary 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 such
steric clashes.
Several evolutionarily independent
enzyme superfamilies with different
protein folds use the N-terminal residue as a nucleophile. This commonality of
active site but difference of protein fold indicates that the active site evolved convergently in those families.[5][19]
Cone snail and fish insulin
Conus geographus produces a distinct form of
insulin that is more similar to fish insulin protein sequences than to insulin from more closely related molluscs, suggesting convergent evolution,[20] though with the possibility of
horizontal gene transfer.[21]
Ferrous iron uptake via protein transporters in land plants and chlorophytes
Distant homologues of the metal ion transporters
ZIP in
land plants and
chlorophytes have converged in structure, likely to take up Fe2+ efficiently. The IRT1 proteins from Arabidopsis thaliana and
rice have extremely different amino acid sequences from Chlamydomonas's IRT1, but their three-dimensional structures are similar, suggesting convergent evolution.[22]
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 six insect orders.[23][24][25] 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.[25] 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.[23][25]
Swimming animals including
fish such as
herrings,
marine mammals such as
dolphins, and
ichthyosaurs (
of the Mesozoic) all converged on the same streamlined shape.[32][33] A similar shape and swimming adaptations are even present in molluscs, such as Phylliroe.[34] 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 environment.[35] Similar body shapes are found in the
earless seals and the
eared seals: they still have four legs, but these are strongly modified for swimming.[36]
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 the
thylacine (Tasmanian tiger or Tasmanian wolf) converged with those of
Canidae such as the red fox, Vulpes vulpes.[37]
As a sensory adaptation,
echolocation has evolved separately in
cetaceans (dolphins and whales) and bats, but from the same genetic mutations.[38]
Electric fishes
The
Gymnotiformes of South America and the
Mormyridae of Africa independently evolved
passive electroreception (around 119 and 110 million years ago, respectively). Around 20 million years after acquiring that ability, both groups evolved active
electrogenesis, producing weak electric fields to help them detect prey.[39]
One of the best-known examples of convergent evolution is the camera eye of
cephalopods (such as squid and octopus),
vertebrates (including mammals) and
cnidaria (such as jellyfish).[41] Their last common ancestor had at most a simple photoreceptive spot, but a range of processes led to the
progressive 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, cephalopods lack a
blind spot.[7]
Birds and
bats have
homologous limbs because they are both ultimately derived from terrestrial
tetrapods, 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 of
feathers, strongly attached to the forearm (the ulna) and the highly fused bones of the wrist and hand (the
carpometacarpus), 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][42] Birds and bats also share a high concentration of
cerebrosides in the skin of their wings. This improves skin flexibility, a trait useful for flying animals; other mammals have a far lower concentration.[43] The extinct
pterosaurs independently evolved wings from their fore- and hindlimbs, while
insects have
wings that evolved separately from different organs.[44]
Flying squirrels and
sugar gliders are much alike in their body plans, with gliding wings stretched between their limbs, but flying squirrels are placental mammals while sugar gliders are marsupials, widely separated within the mammal lineage from the placentals.[45]
Insect mouthparts show many examples of convergent evolution. The mouthparts of different insect groups consist of a set of
homologous 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, the
proboscis of flower-visiting insects such as
bees and
flower beetles,[47][48][49] or the biting-sucking mouthparts of blood-sucking insects such as
fleas and
mosquitos.
Opposable thumbs
Opposable thumbs allowing the grasping of objects are most often associated with
primates, like humans, monkeys, apes, and lemurs. Opposable thumbs also evolved in
giant 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.[50]
Despite the similar lightening of
skin colour after moving
out 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.[51] When humans migrated
out of Africa, they moved to more northern latitudes with less intense sunlight.[51] It was beneficial to them to reduce their
skin pigmentation.[51] It appears certain that there was some lightening of skin colour before European and East Asian lineages diverged, as there are some skin-lightening genetic differences that are common to both groups.[51] However, after the lineages diverged and became genetically isolated, the skin of both groups lightened more, and that additional lightening was due to different genetic changes.[51]
Humans
Lemurs
Despite the similarity of appearance, the genetic basis of blue eyes is different in humans and
lemurs.
Lemurs and
humans 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 one
gene locus is responsible, say with brown dominant to blue
eye 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.[52]
In plants
The annual life-cycle
While most plant species are
perennial, about 6% follow an
annual life cycle, living for only one growing season.[53] The annual life cycle independently emerged in over 120 plant families of angiosperms.[54][55] The prevalence of annual species increases under hot-dry summer conditions in the four species-rich families of annuals (
Asteraceae,
Brassicaceae,
Fabaceae, and
Poaceae), indicating that the annual life cycle is adaptive.[53][56]
Fruits with a wide variety of structural origins have converged to become edible.
Apples are
pomes with five
carpels; their accessory tissues form the apple's core, surrounded by structures from outside the botanical fruit, the
receptacle or
hypanthium. Other edible fruits include other plant tissues;[63] the fleshy part of a
tomato is the walls of the
pericarp.[64] This implies convergent evolution under selective pressure, in this case the competition for
seed dispersal by animals through consumption of fleshy fruits.[65]
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.[66]
Carnivory
Carnivory has evolved multiple times independently in plants in widely separated groups. In three species studied, Cephalotus follicularis, Nepenthes alata and Sarracenia purpurea, there has been convergence at the molecular level. Carnivorous plants secrete
enzymes into the digestive fluid they produce. By studying
phosphatase,
glycoside hydrolase,
glucanase,
RNAse and
chitinaseenzymes as well as a
pathogenesis-related protein and a
thaumatin-related protein, the authors found many convergent
amino 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 that
homologous genes in the non-carnivorous plant Arabidopsis 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.[67]
Methods of inference
Phylogenetic reconstruction and
ancestral 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 of
natural selection.[68]
Pattern-based measures
Earlier methods for measuring convergence incorporate ratios of phenotypic and
phylogenetic distance by simulating evolution with a
Brownian motion model of trait evolution along a phylogeny.[69][70] More recent methods also quantify the strength of convergence.[71] 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.[68]
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.[68]
Process-based measures
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 the
Ornstein–Uhlenbeck process to test different scenarios of selection. Other methods rely on an a priori specification of where shifts in selection have occurred.[72]
See also
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. Retrieved 23 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.
^Arendt, J; Reznick, D (January 2008). "Convergence and parallelism reconsidered: what have we learned about the genetics of adaptation?". Trends in Ecology & Evolution. 23 (1): 26–32.
doi:
10.1016/j.tree.2007.09.011.
PMID18022278.
^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.
^Wilhelmi, Andreas P.; Krenn, Harald W. (2012). "Elongated mouthparts of nectar-feeding Meloidae (Coleoptera)". Zoomorphology. 131 (4): 325–37.
doi:
10.1007/s00435-012-0162-3.
S2CID9194699.
^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.
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: 43–55.
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.