Convergent evolution

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Evolutionary biology
Diagrammatic representation of the divergence of modern taxonomic groups from their common ancestor
Example: Two succulent plant genera, Euphorbia and Astrophytum, are only distantly related, but the species within each have independently converged on a similar body form.

Convergent evolution is the independent evolution of similar features in species of different lineages. Convergent evolution creates analogous structures that have similar form or function but were not present in the last common ancestor of those groups.[1] The cladistic term for the same phenomenon is homoplasy.[2] The recurrent evolution of flight is a classic example, as flying insects, birds, and bats have independently evolved the useful capacity of flight.

Functionally similar features that arose through convergent evolution are analogous, whereas homologous structures or traits have a common origin but can have dissimilar functions.[1] The British anatomist Richard Owen was the first scientist to recognise the fundamental difference between analogies and homologies.[3] 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. On a molecular level, that can happen from random mutation unrelated to adaptive changes; see long branch attraction. Convergent evolution is similar to but different from parallel evolution. Parallel evolution occurs when two independent but similar 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.

Many instances of convergent evolution are known in plants, including the repeated development of C4 photosynthesis and of seed dispersal by fleshy fruits adapted to be eaten by animals.

Causes[edit]

Mammals and insects are part of different homologous and analogous evolutionary groups. In the horizontal direction, the structures are homologous in their morphology, or anatomy, but different in their function due to differences in habitat. In the vertical direction, the structures are analogous in function due to similar lifestyles of organisms but anatomically different since they are part of different groups.

In morphology, analogous traits will often arise where different species live in similar ways and/or similar environment, and so face the same environmental factors. When occupying similar ecological niches (that is, a distinctive way of life) similar problems lead to similar solutions.[4]

In biochemistry, physical and chemical constraints on mechanisms have caused some active site arrangements such as the catalytic triad to evolve independently in separate enzyme superfamilies.[5]

Significance[edit]

In his 1989 book Wonderful Life, Stephen Jay Gould argued that if the tape of life were re-wound and played back, life would have taken 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]

Distinctions[edit]

Cladistic definitions of homoplasy, synapomorphy, autapomorphy, apomorphy and plesiomorphy

Convergent evolution is defined and named differently in different fields of biology.

Cladistic definition[edit]

In cladistics, a homoplasy is a trait shared by two or more taxa because of convergence, parallelism or reversal.[8] Homoplastic character states require extra steps to explain their distribution on a most parsimonious cladogram. Homoplasy is only recognizable when other characters imply an alternative hypothesis of grouping, because in the absence of such evidence, shared features are always interpreted as similarity due to common descent. Homoplasious traits or changes (derived trait values acquired in unrelated organisms in parallel) can be compared with synapomorphy (a derived[a] trait present in all members of a monophyletic clade), autapomorphy (derived trait present in only one member of a clade), or apomorphies, derived traits acquired in all members of a monophyletic clade following divergence where the most recent common ancestor had the ancestral trait (the ancestral trait manifesting in paraphyletic species as a plesiomorphy).[9]

Atavism[edit]

Main article: 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. 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.[10]

Parallel vs. convergent evolution[edit]

Evolution at an amino acid position. In each case, the left-hand species changes from incorporating alanine (A) at a specific position within a protein in a hypothetical common ancestor deduced from comparison of sequences of several species, and now incorporates serine (S) in its present-day form. The right-hand species may undergo divergent, parallel, or convergent evolution at this amino acid position relative to that of the first species.

For a particular trait, proceeding in each of two lineages from a specified ancestor to a later descendant, parallel and convergent evolutionary trends can be strictly defined and clearly distinguished from one another.[11] However the cutoff point for what is considered convergent and what parallel evolution is somewhat arbitrary. 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. However, this definition is somewhat murky. 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. Some scientists have argued that parallel and convergent evolution are more or less indistinguishable,[12] while others maintain that despite some overlap, there are still important distinctions between the two.[13]

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,[14] because mammals on each continent had a long evolutionary history prior to the extinction of the dinosaurs under which to accumulate relevant differences. Stephen Jay Gould describes many of the same examples as parallel evolution starting from the common ancestor of all marsupials and placentals. Many evolved similarities can be described in concept as parallel evolution from a remote ancestor, with the exception of those where quite different structures are co-opted to a similar function. For example, consider Mixotricha paradoxa, a microbe that has assembled a system of rows of apparent cilia and basal bodies closely resembling that of ciliates but that are actually smaller symbiont micro-organisms, or the differently oriented tails of fish and whales. On the converse, any case in which lineages do not evolve together at the same time in the same ecospace might be described as convergent evolution at some point in time.

The definition of a trait is crucial in deciding whether a change is seen as divergent, or as parallel or convergent. In the image above (with a caption that begins "Evolution at an amino acid position"), note that, since serine and threonine possess similar structures with an alcohol side-chain, the example marked "divergent" would be termed "parallel" if the amino acids were grouped by similarity instead of being considered individually. As another example, if genes in two species independently become restricted to the same region of the animals through regulation by a certain transcription factor, this may be described as a case of parallel evolution—but the actual DNA sequence will likely show only divergent changes in individual base-pair positions, since a new transcription factor binding site can be added in a wide range of places within the gene with similar effect.

Similar to convergent evolution, evolutionary relay describes how independent species acquire similar characteristics through their evolution in similar ecosystems, but not at the same time, as in the dorsal fins of sharks and ichthyosaurs.

Examples[edit]

For more details on this topic, see List of examples of convergent evolution.

Enzymes[edit]

Evolutionary convergence of serine and cysteine protease towards the same catalytic triads organisation of acid-base-nucleophile in different protease superfamilies. Shown are the triads of subtilisin, prolyl oligopeptidase, TEV protease, and papain.

Protease active sites[edit]

Main article: catalytic triad

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 independently converge on equivalent solutions repeatedly.[5][15]

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 basic residue in a catalytic triad. The chemical and physical constraints on enzyme catalysis have caused identical triad arrangements to have evolved independently over 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 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][16]

Nucleic acids[edit]

As more sequence data are becoming available, there is growing interest in convergent changes at the level of DNA and amino acids. In 2013 the first genome-wide study of convergence was published. Comparisons of the genomes of echolocating bats and the dolphin identified numerous convergent amino acid substitutions in genes implicated in hearing and vision.[17]

Animals[edit]

Bodyplans[edit]

Features shared by dolphins and ichthyopterygians

The marsupial fauna of Australia and the placental mammals of the Old World have strikingly similar forms, developed in two clades, isolated from each other.[7] Many body plans, for instance sabre-toothed cats and flying squirrels,[18] evolved independently in the two populations.

The same streamlined shape has been converged upon by fish such as herrings, marine mammals such as dolphins, and ichthyosaurs (of the Mesozoic). This bodyplan is an adaptation to being an active predator in a high drag environment. Similar body shapes are found in the earless seals and the eared seals: they still have four legs, but these are much modified for swimming.[19]

Flight[edit]

See also: Evolution of avian flight, Evolution of insect flight and Evolution of aerial locomotion
Vertebrate wings are homologous as forelimbs, being derived from the same organs, but as organs of flight in (1) pterosaurs, (2) bats and (3) birds, they are analogous, resembling each other in some ways and fulfilling similar functions, but having evolved separately.

Birds and bats have homologous limbs as 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 powered flight, evolved independently. Their wings differ substantially in construction. The bat wing is a membrane stretched across four extremely elongated fingers. 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.

Similarly, the extinct pterosaur also shows an independent evolution of vertebrate forelimb to wing. An even more distantly related group with wings is the insects; they not only evolved separately as wings, but from totally different organs, starting from a fundamentally different bodyplan.

Opposable thumbs[edit]

Opposable thumbs allowing the grasping of objects are most often associated with primates, like humans, monkeys, apes, and lemurs. Opposable thumbs also evolved in pandas, however, and are completely different in structure, having six fingers including the thumb, which develops from a wrist bone entirely separately from other fingers.[20]

Flying squirrels and sugar gliders[edit]

While they look almost the same, flying squirrels and sugar gliders are very different. Flying squirrels are placental mammals and sugar gliders are marsupials, which practically puts them at opposite ends of the mammal lineage. Humans are more closely related to flying squirrels than flying squirrels are to sugar gliders.

Echolocation[edit]

As a sensory adaptation, echolocation has evolved several times, in cetaceans (dolphins and whales), as well as bats. Humans are capable of echolocation with training; this is mostly observed in people who are blind.[21][22]

Eyes[edit]

Vertebrates and cephalopods developed the camera eye independently. In the vertebrate version the nerve fibers pass in front of the retina, and there is a blind spot, 4, where the nerves pass through the retina. In the cephalopod version, the eye is constructed the "right way out", with the nerves attached to the rear of the retina.[23]
Main article: Eye evolution

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).[24] 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. This means that cephalopods do not have a blind spot.[7]

Insect mouthparts[edit]

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 (which can be experimentally quantified). 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,[25][26][27][28][28] or the biting-sucking mouthparts of blood-sucking insects such as fleas and mosquitos.

Plants[edit]

In myrmecochory, seeds such as those of Chelidonium majus have a hard coating and an attached oil body, an elaiosome, for dispersal by ants.

While convergent evolution is often illustrated with animal examples, it has often occurred in plant evolution. For instance, C4 photosynthesis, one of the three major carbon-fixing biochemical processes, has arisen independently up to 40 times.[29][30] About 7,600 plant species of angiosperms use C4 carbon fixation, with many monocots including 46% of grasses such as maize and sugar cane,[31][32] and dicots including a sizeable minority of species in the Chenopodiaceae and the Amaranthaceae.[33][34]

Good examples of convergence in plants include the evolution of edible fruits such as apples. These pomes incorporate (five) carpels and their accessory tissues forming the apple's core, surrounded by structures from outside the botanical fruit, the receptacle or hypanthium. Other edible fruits include other plant tissues;[35] for example, the fleshy part of a tomato is the walls of the pericarp.[36] This implies convergent evolution under selective pressure, in this case the competition for seed dispersal by animals through consumption of fleshy fruits.[37]

The emergence of 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.[38]

See also[edit]

Notes[edit]

  1. ^ Note that in cladistics, "derived" does not mean "as derived unchanged from ancestors" but "changed by evolution from the ancestral condition".

References[edit]

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  29. ^ Williams, B. P.; Johnston, I. G.; Covshoff, S.; Hibberd, J. M. (September 2013). "Phenotypic landscape inference reveals multiple evolutionary paths to C4 photosynthesis". eLife. 2: e00961. doi:10.7554/eLife.00961. PMID 24082995. 
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  34. ^ 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. 
  35. ^ Ireland, Hilary S.; et al. (2013). "Apple SEPALLATA1/2 -like genes control fruit flesh development and ripening". The Plant Journal. 73: 1044–1056. doi:10.1111/tpj.12094. 
  36. ^ Heuvelink, Ep (2005). Tomatoes. CABI. p. 72. ISBN 978-1-84593-149-0. 
  37. ^ Lorts C, Briggeman T, Sang T (2008). "Evolution of fruit types and seed dispersal: A phylogenetic and ecological snapshot" (PDF). Journal of Systematics and Evolution. 46 (3): 396–404. doi:10.3724/SP.J.1002.2008.08039. Archived from the original (PDF) on 2013-07-18. 
  38. ^ Lengyel Sz; Gove AD; Latimer AM; Majer JD; Dunn RR (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. 

Further reading[edit]

External links[edit]

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