Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.
The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings,
birdBirds are winged, bipedal, endothermic , vertebrate animals that lay eggs. There are around 10,000 living species, making them the most numerous tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Birds range in size from the Bee Hummingbird to the ...
s and
batBats are flying mammals in the order Chiroptera . The forelimbs of bats are webbed and developed as wings, making them the only mammals naturally capable of true and sustained flight. By contrast, other mammals said to fly, such as flying squirrels, gliding possums and colugos, glide rather than...
s do, and are capable of powered flight. The wings are similar in construction, due to the physical constraints imposed upon wing shape. Similarity can also be explained by shared ancestry, as evolution can only work with what is already there—thus wings were modified from limbs, as evidenced by their bone structure.
Traits arising through convergent evolution are termed
analogousTwo structures in biology are said to be analogous if they perform the same or similar function by a similar mechanism but evolved separately...
structures, in contrast to
homologousIn evolutionary biology, homology refers to any similarity between characteristics of organisms that is due to their shared ancestry. The word homologous derives from the ancient Greek ομολογειν, 'to agree'. There are examples in different branches of biology...
structures, which have a common origin.
Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.
The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings,
birdBirds are winged, bipedal, endothermic , vertebrate animals that lay eggs. There are around 10,000 living species, making them the most numerous tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Birds range in size from the Bee Hummingbird to the ...
s and
batBats are flying mammals in the order Chiroptera . The forelimbs of bats are webbed and developed as wings, making them the only mammals naturally capable of true and sustained flight. By contrast, other mammals said to fly, such as flying squirrels, gliding possums and colugos, glide rather than...
s do, and are capable of powered flight. The wings are similar in construction, due to the physical constraints imposed upon wing shape. Similarity can also be explained by shared ancestry, as evolution can only work with what is already there—thus wings were modified from limbs, as evidenced by their bone structure.
Traits arising through convergent evolution are termed
analogousTwo structures in biology are said to be analogous if they perform the same or similar function by a similar mechanism but evolved separately...
structures, in contrast to
homologousIn evolutionary biology, homology refers to any similarity between characteristics of organisms that is due to their shared ancestry. The word homologous derives from the ancient Greek ομολογειν, 'to agree'. There are examples in different branches of biology...
structures, which have a common origin. Bat and pterodactyl wings are an example of analogous structures, while the bat wing is homologous to human and other mammal forearms, sharing an ancestral state despite serving different functions. Similarity in species of different ancestry which is the result of convergent evolution, is called
homoplasy.
The opposite of convergent evolution is
divergent evolutionDivergent evolution is the accumulation of differences between groups which can lead to the formation of new species, usually a result of diffusion of the same species adapting to different environments, leading to natural selection defining the success of specific mutations...
, whereby related species evolve different traits. On a molecular level, this can happen due to random mutation unrelated to adaptive changes; see
long branch attractionLong branch attraction is a phenomenon in phylogenetic analyses when rapidly evolving lineages are inferred to be closely related, regardless of their true evolutionary relationships. For example, in DNA sequence-based analyses, the problem arises when sequences from two lineages evolve rapidly...
.
Convergent evolution is similar to, but distinguishable from, the phenomena of evolutionary relay and
parallel evolutionParallel evolution is the development of a similar trait in different not closely related species , but descending from the same ancestor.-Parallel vs...
. Evolutionary relay describes how independent species acquire similar characteristics through their evolution in similar ecosystems, at different times: for example the
dorsal finA dorsal fin is a fin located on the backs of some fish, whales, dolphins, and porpoises, as well as the ichthyosaurs. Depending on the species, an animal can have up to three of them. Its main purpose is to stabilize the animal against rolling and assist in sudden turns...
s of
ichthyosaurIchthyosaurs were giant marine reptiles that resembled fish and dolphins...
s and
sharkSharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago, before the time of the dinosaurs....
s. Parallel evolution occurs when two independent species evolve together
at the same time in the same ecospace and acquire similar characteristics—for instance extinct
browsing-horseThe evolution of the horse involves the gradual development of the modern horse from the fox-sized, forest-dwelling Hyracotherium. Paleozoologists have been able to piece together a more complete picture of the modern horse's evolutionary lineage than that of any other animal.The horse belongs to...
s and
paleotherePalaeotherium is an extinct genus of primitive perissodactyl ungulate. George Cuvier originally described them as being a kind of tapir, and as such, Palaeotherium is popularly reconstructed as a tapir-like animal...
s.
Causes
Similarity can also result if organisms occupy similar
ecological nicheIn ecology, a niche is a term describing the relational position of a species or population in its ecosystem to each other; e.g. a dolphin could potentially be in another ecological niche from one that travels in a different school if the members of these schools utilize significantly different...
s—that is, a distinctive way of life. A classic comparison is between the marsupial fauna of Australia and the placental mammals of the Old World. The two lineages are
cladeA clade is a term used in modern alpha taxonomy, the scientific classification of living and fossil organisms, to describe a monophyletic group, defined as a group consisting of a single common ancestor and all its descendants.[The term "monophyletic group" is used in this article...]
s—that is, they each share a common ancestor that belongs to their own group, and are more closely related to one another than to any other clade—but very similar forms evolved in each isolated population. Many "body plans," for instance sabre-toothed cats and flying squirrels, evolved independently in both populations.
Examples
One of the most famous examples of convergent evolution is the camera eye of
cephalopod{Taxobox| name = Cephalopods| fossil_range = | image = Tafel 054 300.jpg| image_caption = A variety of cephalopod forms from Ernst Haeckel's 1904 Kunstformen der Natur| regnum = Animalia| image_width = 220px| phylum = Mollusca| classis = Cephalopoda...
s (e.g. squid),
vertebrateVertebrates are members of the subphylum Vertebrata, chordates with backbones or spinal columns. About 58,000 species of vertebrates have been described. Vertebrata is the largest subphylum of chordates, and contains many familiar groups of large land animals. Vertebrates comprise cyclostomes, bony...
s (e.g. mammals) and
cnidariaCnidaria is a phylum containing over 10,000 species of animals found exclusively in aquatic, mostly marine, environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey...
(e.g. box jellies). Their last common ancestor had at most a very simple photoreceptive spot, but
a range of processesThe evolution of the eye has been a subject of significant study, as a distinctive example of a homologous organ present in a wide variety of taxa. Certain components of the eye, such as the visual pigments, appear to have a common ancestry – that is, they evolved once, before the animals radiated...
led to the progressive refinement of this structure to the advanced camera eye - with one subtle 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. The similarity of the structures in other respects, despite the complex nature of the organ, illustrates how there are some biological challenges (vision) that have an optimal solution.
Significance
The degree to which convergence affects the products of evolution is the subject of a popular controversy. In his book
Wonderful LifeWonderful Life: The Burgess Shale and the Nature of History is a book on the evolution of Cambrian fauna by Harvard paleontologist Stephen Jay Gould...
,
Stephen Jay GouldStephen Jay Gould was an American paleontologist, evolutionary biologist, and historian of science. He was also one of the most influential and widely read writers of popular science of his generation. Gould spent most of his career teaching at Harvard University and working at the American Museum...
argues that if the tape of life were re-wound and played back, life would have taken a very different course.
Simon Conway MorrisSimon Conway Morris FRS is a British paleontologist. He was born on 6th November 1951 in Carshalton, Surrey and brought up in London, England. He made his reputation with a very detailed and careful study of the Burgess Shale fossils, an exploit celebrated in Stephen Jay Gould's Wonderful Life,...
counters this argument by arguing that convergence is a dominant force in evolution, and that since the same environmental and physical constraints act on all life, there is an "optimum" body plan which life will inevitably evolve towards, with evolution bound to stumble upon intelligence - a trait of primates, crows and dolphins - at some point. Convergence is difficult to quantify, so progress on this issue may require exploitation of engineering specifications (e.g. of wing aerodynamics) and comparably rigorous measures of "very different course" in terms of phylogenetic (molecular) distances.
Cultural convergence
The term
convergence is also used to describe phenomena in the theory of cultural evolution.
Further reading
- Rasmussen, L.E.L., Lee, T.D., Roelofs, W.L., Zhang, A., Doyle Davies Jr, G. (1996). Insect pheromone in elephants. Nature. 379: 684.
- Convergent Evolution Examples- Ecological Equivalents, Department of Biology, Bellarmine University.
- Stearns, S. & Hoekstra, R. 2005. Evolution: An introduction.