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.
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.
Recent evidence suggests that even plants and animals share a convergently evolved developmental pattern whereby embryos of both lineages pass through a phylotypic stage marked by an organizational checkpoint during mid-embryogenesis.
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.
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.
Main article: Cladistics
In cladistics, a homoplasy is a trait shared by two or more taxa for any reason other than that they share a common ancestry.
Homoplastic traits caused by convergence are therefore, from the point of view of cladistics, confounding factors which could lead to an incorrect analysis.
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.
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 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.
At molecular level
Protease active sites
Main article: catalytic triad
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 order to activate that nucleophile, they orient an acidic and a basic residue in a catalytic triad.
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.
This commonality of active site but difference of protein fold indicates that the active site evolved convergently in those families.
Cone snail and fish insulin
Na,K-ATPase and Insect resistance to cardenolides
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 amino acid substitutions at well-defined positions in the structure of the Na,K-ATPase α-subunit spanning 15 genera and 4 orders.
The synergistic relationship between the Q111 and N122 substitutions are highlighted.
Convergent evolution in this case does not depend on the type of selection or time frame in which it can occur, but has more to do with the co-evolutionary relationship causing a sort of soft selection between cardenolide-producing plants and the insects that prey on them.
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, cis-regulatory elements, such as in their rates of evolution; this could indicate either positive selection or relaxed purifying selection.
In animal morphology
A similar shape and swimming adaptations are even present in molluscs, such as Phylliroe.
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.
Main article: Eye evolution
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.
Further information: Flying and gliding animals § Evolution and ecology of aerial locomotion
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 powered flight, evolved independently.
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.
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.
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.
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.
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.
Further information: Human skin color § Genetics of skin color variation
Convergent evolution in humans includes blue eye colour and light skin colour.
When humans migrated out of Africa, they moved to more northern latitudes with less intense sunlight.
It was beneficial to them to reduce their skin pigmentation.
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.
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.
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.
While convergent evolution is often illustrated with animal examples, it has often occurred in plant evolution.
About 7,600 plant species of angiosperms use C4 carbon fixation, with many monocots including 46% of grasses such as maize and sugar cane, and dicots including several species in the Chenopodiaceae and the Amaranthaceae.
This implies convergent evolution under selective pressure, in this case the competition for seed dispersal by animals through consumption of fleshy fruits.
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.
Carnivory has evolved multiple times independently in plants in widely separated groups.
Carnivorous plants secrete enzymes into the digestive fluid they produce.
By studying phosphatase, glycoside hydrolase, glucanase, RNAse and chitinase enzymes 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 in the repeated evolution of carnivory.
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.
More recent methods also quantify the strength of convergence.
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.
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.
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 (OU) process to test different scenarios of selection.
Other methods rely on an a priori specification of where shifts in selection have occurred.
- 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 a non-crab-like crustacean into a crab-like form
Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/Convergent evolution.