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.

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Darwin's finches by Gould.jpg
Darwin's finches by John Gould
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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

phylogeny.[a]
Further information: List of examples of convergent evolution

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

cetaceans.[7]

Distinctions

Cladistics

Main article: 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

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.[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

Main article: Catalytic triad

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

enzyme superfamilies.[5]

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, fish-like bacterial copper/zinc superoxide dismutase

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] Although convergent evolution is not impossible in this example, the possibility of horizontal gene transfer cannot be ignored, and it provides the only reasonable explanation of the fish-like copper/zinc superoxide dismutase of Photobacterium leiognathi.[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

positive selection or relaxed purifying selection.[30][31]

In animal morphology

Dolphins and 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.

Vulpes vulpes.[37]

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]
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).[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.