Convergent evolution

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Two succulent plant genera, Euphorbia and Astrophytum, are only distantly related, but the species within each have converged on a similar body form.

Convergent evolution is the independent

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.

Many instances of convergent evolution are known in

C4 photosynthesis, seed dispersal by fleshy fruits adapted to be eaten by animals, and carnivory
.

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 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 biochemistry, physical and chemical constraints on

enzyme superfamilies.[5]

In his 1989 book

Distinctions

Cladistics

In cladistics, a homoplasy is a trait shared by two or more

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]

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.[12]

Parallel vs. convergent evolution

Evolution at an amino 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 undergo divergent, 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] 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]

At molecular level

prolyl oligopeptidase, TEV protease, and papain
.

Proteins

Protease active sites

The

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

steric clashes
. Several evolutionarily independent
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

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]

Nucleic acids

Convergence occurs at the level of

In animal morphology

ichthyosaurs
converged on many adaptations for fast swimming.

Bodyplans

Swimming animals including

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.

Echolocation

As a sensory adaptation,

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]

  • Convergence of
    weakly electric fishes
  • Gymnotiform electrolocation waveform
    Gymnotiform electrolocation waveform
  • A gymnotiform electric fish of South America
    A gymnotiform electric fish of South America
  • A mormyrid electric fish of Africa
    A mormyrid electric fish of Africa
  • Mormyrid electrolocation waveform
    Mormyrid electrolocation waveform

Eyes

The camera eyes of vertebrates (left) and cephalopods (right) developed independently and are wired differently; for instance, optic nerve (3) fibres (2) reach the vertebrate retina (1) from the front, creating a blind spot (4).[40]

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]

Flight

birds
, evolved separately.

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]

Hummingbird hawk-moths and hummingbirds have evolved similar flight and feeding patterns.[46]

Insect mouthparts

Insect mouthparts show many examples of convergent evolution. The mouthparts of different insect groups consist of a set of

flower beetles,[47][48][49] or the biting-sucking mouthparts of blood-sucking insects such as fleas and mosquitos
.

Opposable thumbs

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]

Primates

Veronika Loncká.jpg
Angela Bassett by Gage Skidmoe.jpg
(미쓰와이프) 제작기영상 엄정화 3m3s.jpg
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.

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
A_blue_eye.jpg
Eye_See_You_(2346693372).jpg
Eulemur_mongoz_(male_-_face).jpg
Blue-eyed_black_lemur.jpg
Despite the similarity of appearance, the genetic basis of blue eyes is different in humans and lemurs.

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

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

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]

Carbon fixation

Fruits

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

Molecular convergence in carnivorous plants

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

C4 photosynthesis
in parentheses.

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

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
  • Morphology (biology) – Study of external forms and structures of organisms
  • Iterative evolution
    – The repeated evolution of a specific trait or body plan from the same ancestral lineage at different points in time.
  • Elvis taxon – Misidentification of later taxon superficially resembling earlier extinct taxon
  • Breeding back – A form of selective breeding to recreate the traits of an extinct species, but the genome will differ from the original species.
  • Orthogenesis (contrastable with convergent evolution; involves teleology)

Notes

  1. ^ However, evolutionary developmental biology has identified deep homology between insect and mammal body plans, to the surprise of many biologists.
  2. ^ 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.
  3. pre-adaptation or exaptation
    .

References

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Further reading

External links