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When an octopus and a human being gaze at one another through the aquarium glass, they both do so through a camera-like eye. A human is a vertebrate and the occupant of the tank is a cephalopod mollusc. Their common ancestor lived more than one-half billion years ago, and since it did not have a camera-like eye the fact that humans can exchange gazes with octopuses can only mean that such an eye evolved independently. This is a classic example of convergent evolutioni.e., the emergence of a similar biological feature, not by descent from a common ancestor but from organisms that are effectively unrelated. Yet biologists also know that this eye-type has evolved independently at least four other times. For eyes to work the lens must be transparent. This property is conferred by employing particular proteins called crystallins. Their small molecular size enables a close packing in the watery medium of the lens, thus providing the necessary transparency. Yet the origins of human (and mammalian) and cephalopod crystallins are different. So here is an example of biochemical convergence. In both cases the crystallin is recruited from a protein originally involved with stress control; in mammals it came from a small heat-shock protein, but in cephalopods it derives from a detoxification protein. Both octopus and human end up seeing in much the same way, even though their respective ancestors could not.

Problems with the theory of convergent evolution

In the literature, such examples of convergence often provoke exclamations of "remarkable," "astonishing," and even "uncanny." It is almost as if there was a latent fear of the teleological principle being smuggled back into evolutionary biology. But are the eyes of humans and octopuses really convergent? After all, both employ the protein rhodopsin, which allows a chemical process whereby light is converted into an electrical signal that humans understand as vision. But this does not undermine the principle of convergence; it merely demonstrates that pre-existing structures will be recruited when necessary in a way analogous to the lens crystallins.

There is, however, a more serious obstacle in accepting convergence. This is in the form of the developmental gene known as Paired-Box (or Pax ) 6. This gene now has an almost iconic status: Pax-6 "makes" eyes in most, and perhaps all, groups of animals. Does this not undermine the principle of convergence? Hardly. In the developing embryo, the activity of Pax-6 is much more widespread. Originally it probably evolved in connection with the emerging needs for sensory systems in general: not only vision but also olfaction. Pax-6 is necessary but not sufficient; it is little more than a genetic switch. In human and octopus, it will ensure camera-eyes, but in flies and lobsters it "makes" compound eyes.

As already noted, camera-like eyes have evolved separately at least six times, while compound eyesmost familiar in insectshave evolved independently at least three times. These examples, involving vision, are surely more significant than the other familiar instances of convergence, such as the streamlining of aquatic vertebrates or warm-bloodedness in birds and mammals. This is because such sensory assimilation implies nervous activity and a brain, with the further link to cognition and sentience.

There are also striking instances of convergence in both hearing and olfaction. Even when a nose stops being used for olfaction, as in the star-nosed mole, its tactile sense is actually strongly convergent on the neurology of vision. Even senses that are decidedly alien to humans, such as echolocation (in bats and dolphins) and the generation of electric fields in fish, show splendid examples of convergence.

Scientific and theological implications

Few textbooks on evolutionary biology neglect to mention convergence, but curiously its wider implications are seldom addressed. These concern (1) its ubiquity, which implies (2) the reality of natural selection and thereby adaptation, and (3) the inevitability of evolutionary trends. Moreover, if the natural world is seen as part of God's great order, then convergence may also have theological implications. In brief, how different can this worldor any worldreally be? Put another way, if intelligent life exists elsewhere in the universe, will it be humanoid or, in Robert Bieri's phrase, the equivalent of a thinking pancake?

Convergence is, therefore, central to understanding organic evolution. First, it confirms its reality. The eyes of octopus and human are similar, but they are not identical. The structure of the lens and the position of the retina, for example, are different. Convergence does not guarantee the identical, only the emergence of particular biological properties. Second, the ubiquity of convergence implies the prevalence of selective pressure: how else could biological systems come so closely to resemble each other? So too with adaptation; it is a biological reality and not some incidental by-product of effectively random processes. Third, the reality of convergence has the implicit assumption that starting points will be disparate, but there will be defined and repeatable evolutionary trajectories in evolution. Trends are real, and if the end-point is not perfect, is it emphatically better than what came before.

Yet all this is strongly at odds with a widespread perception that contingent happenstance is the determining reality in evolution. Thus, to paraphrase American paleontologist Stephen Jay Gould's metaphor of the tape of life, if the history of the world were to be re-run, the end result would utterly different. Historians might meditate on the untimely demise of Hitler or the death at an advanced age of Alexander the Great, but the consensus amongst biologists is that even a nudge in one direction half a billion years ago would preclude entirely the emergence of humans. As individuals this must be true, given that all humans were conceived by their parents against the odds. Yet biologically this view is deeply credulous. It is no accident that those who suppose the emergence of humans to be the product of individual and contingent history, also believe that humans are not only free (as indeed they are) but may mold their morality to a scheme of their choosing.

The realities of biological evolution and the inevitability of convergence suggest, however, a new view of life. Creation presupposes a history and an end-point, but this does not constrain choice and acceptance (or the opposite). The universe is so arranged that sooner or later, somewhere or other, certain properties, biological and ultimately spiritual, will emerge. The quip by British geneticist and physiologist J.B.S. Haldane remarking upon the creator's inordinate fondness for beetles is thereby turned on its head. Creation is indeed rich, but the modalities of convergence suggest that ultimately it is otiose to speak of accidents. Seeded in the act of creation was the inevitability of sentience endowed with free will.

See also Adaptation; Chance; Design; Evolution; Evolution, Biological; Selection, Levels of


bieri, robert. "humanoids on other planets?" american scientist 52 (1964): 452458.

catania, kenneth c. "a nose that looks like a hand and acts like an eye: the unusual mechanosensory system of the star-nosed mole." journal of comparative physiology a 185 (1999): 367372

conway morris, simon. the crucible of creation: the burgess shale and the rise of animals. oxford: oxford university press, 1998

marino, lori. "what can dolphins tell us about primate evolution?" evolutionary anthropology 5, no. 3 (1996): 8185

moore, janet, and willmer, patricia. "convergent evolution in invertebrates." biological reviews 72 (1997): 160

mueller, ulrich g.; rehner, stephen a.; and schultz, ted r. "the evolution of agriculture in ants." science 281 (1998): 20342038

strausfeld, nicholas j., and hildebrand, john g. "olfactory systems: common design, uncommon origins." current opinion in neurobiology 1 9 (1999): 634639

wistow, graeme. "lens crystallins: gene recruitment and evolutionary dynamism." trends in biochemical sciences 18 (1993): 301306.

simon conway morris

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In the field of e-commerce and information technology, convergence typically refers to media convergence, and especially to the combination of television, telecommunications, and the personal computer into a single box that would deliver high-speed Internet access, traditional television programming, and interactive services. However, as of mid-2001, interactive television (iTV) was more vision than reality. Major providers included WebTV, owned by Microsoft since 1997 and re-branded in April 2001 as MSNTV; and AOLTV, which launched in October 2000 following tests in select markets. WebTV quickly gained some 1.1 million subscribers, and then found it difficult to acquire more customers. AOLTV had to convince AOL subscribers it was worth an additional $14.95 a month, while non-subscribers had to pay $24.95 a month for access to AOLTV. Short of iTV, some TV showsincluding Who Wants to Be a Millionaire? offered some measure of interactivity by directing viewers to their Web sites.

Proponents of convergence expected it to develop between 2000 and 2005. Convergence at a technological level will enable voice, video, and data to be transmitted to consumers through a single pipeline. The pipeline may be wired, such as cable or DSL, or it may be wireless. Instead of accessing the Internet from a single box, however, consumers will be able to access high-speed networks from a variety of access points, including PCs, TVs, mobile phones, mobile devices, public kiosks, and home appliances. Technological convergence also is expected to result in further convergence at the business level. The merger of AOL and Time Warner to form AOL Time Warner was expected to help accelerate the convergence of traditional and Internet media. Other telecommunications, cable, Internet, and traditional media companies also would join forces, either through mergers or strategic alliances, to form fewer and larger organizations.


Convergence has been an industry-driven rather than a consumer-driven phenomenon. Companies such as Microsoft and AOL Time Warner have invested heavily in developing iTV platforms. Consumers, on the other hand, have been less than enthusiastic. That reflects the fact that convergence is not simply a technological issue; it also is a matter of culture, lifestyle, and economics.

Televisions and personal computers represent different types of experiences in most consumers' lives. TV is visual with a strong sound component and emphasizes entertainment and news. Watching TV is a passive rather than an interactive experience, except for changing channels. TVs are easy to operate and require virtually no special training or education. Personal computers, on the other hand, tend to be more text-oriented and are highly interactive. They can be difficult to use and require some form of education or special training. They are used with more of a purpose in mind, and their content is geared more toward business and educational uses than entertainment.

Despite these differences, consumers appear to be integrating the personal computer into their home life. A study by the Gartner Group found that more than 44 million people used a personal computer and a television in the same room in 2000, compared to only 26 million in 1999. The study predicted that the number would grow to more than 50 million people by the end of 2001.


While cultural, lifestyle, and economic considerations may have slowed consumer acceptance, convergence had some technological obstacles to overcome before iTV became more widespread. Interactive applications can be offered by any type of TV operator, such as cable, satellite, or terrestrial broadcast. However, iTV also requires a return path, which may be cable, DSL, dial-up, or wireless. With so many different delivery networks, interactive platforms are expected to remain fragmented for many years to come. By the early 2000s, a single standard had yet to be developed.

Another obstacle involved the high cost of cable TV, both in term of the large investment required by providers and the subscription cost to consumers. Since iTV requires broadband technology to deliver interactive services, its penetration in the consumer market is dependent on broadband availability and acceptance. With limited broadband deployment, nearly half of all U.S. television households in late 2000 were without access to the high-speed service required for many iTV applications. As a result, the convergence represented by iTV was only available in select markets.


Convergence is also taking place in other areas of commerce and technology. An auto industry conference called "Convergence 2000" promoted the convergence of technology and automobiles, including in-car navigation systems, entertainment systems, and always-on Internet connections. Convergence has become more of a reality in the financial services market, as banking, investing, and insurance were consolidated under one roof at some financial institutions.


Berger, Robin. "TV and Web Can Already Get Together." Electronic Media. January 22, 2001.

"The Big iTV Five." Variety. January 15, 2001.

Cheng, Kipp. "AOL-Time Warner Deal Hastens Convergence." Mediaweek. January 17, 2000.

"Convergence." Whatis?.com July 25, 2001. Available from

Gilman, Brian. "I Want My ITV!" eMarketer. June 5, 2000. Available from

. "Is ITV Ready for Prime Time?" eMarketer. October 16, 2000. Available from

Greenberg, Daniel. "AOLTV: Tuning in to Channel Zero." The Washington Post. December 8, 2000.

Jenkins, Henry. "Convergence? I Diverge." Technology Review. June, 2001.

Macklin, Ben. "The Broadband-iTV Blend." eMarketer. April 3, 2001. Available from

SEE ALSO: Broadband Technology; Connectivity, Internet; Home Networking

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The term "convergence" is used to describe the presence of a similar feature in two or more taxa that are not closely related. Convergent features evolve independently often as a result of natural selection operating on unrelated taxa that occupy similar environments. The recognition of convergence requires an accurate phylogeny. With a phylogenetic hypothesis, homologous characters can be distinguished from analogous traits. Convergent structures are usually derived from different morphological features or by different developmental pathways (although this is not the case in one special class of convergence described below).

Convergence can be further broken down into specific phenomena. Analogy describes the convergent modifications of a nonhomologous trait. For example, analogous organs may share a common function but develop from different tissue types in unrelated organisms. The wings of insects and the wings of birds have the same functional role (flight) but they are derived from nonhomologous structures and are structurally very different. Therefore, they are considered analogous structures. The term "parallelism" refers to apomorphic, or derived, traits. Apomorphies are identical traits that are found in different taxa but that do not share a common evolutionary origin. Apomorphies may arise independently (even in closely related taxa) as consequences of similarities in development among species (often due to developmental constraints imposed by similarities in genes that regulate developmental processes). Parallelism differs from other types of convergent evolution in that parallel traits are the product of the same genes and developmental processes operating in different taxa. In this case, convergence is found not only in physical traits but also in the developmental processes that produce them.

A striking example of convergent evolution in animals is the evolution of flight in three different vertebrate taxa: pterosaurs (extinct flying reptiles), birds, and bats. Structural similarities in the wings of each of these groups are indications of common constraints imposed by phylogeny and biomechanics. All vertebrate forelimbs have similar developmental patterns, regardless of whether they will become limbs or wings in the adult. In order to achieve flight, the ratio of the surface area of the wings relative to body mass must be great enough to provide sufficient lift to overcome gravity. Differences among them indicate that in each lineage, unique solutions have evolved under particular historical and functional constraints, resulting in different structural patterns with similar functions. Pterosaurs, like birds, had hollow bones and keeled sterna (breastbones), a short and stout humerus (upper arm bone), and wing fibers that were analogous to bird feathers. The pterosaur wing was supported primarily by an elongated fourth digit. In birds, digits of the forelimb are reduced and the wing is supported primarily by the radius and ulna (bones of the lower arm) and bones of the wrist. Feathers provide rigidity and increased surface area to the wing. In pterosaurs and bats the digits are elongated and provide support for patagia (thin membranes of skin). In birds, feathers provide a unique structural solution to the challenge of flight while elongate digits and patagia have evolved convergently in other groups that lack feathers.

see also Biological Evolution.

Andrew G. Gluesenkamp


Cockburn, Andrew. An Introduction to Evolutionary Ecology. Oxford, U.K.: Blacwell Scientific Publications, 1991.

Futuyma, D. J. Evolutionary Biology. Sunderland, MA: Sinauer Associates, 1986.

McKinney, M. L. Evolution of Life. Englewood Cliffs, NJ: Prentice Hall, 1993.

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1. The situation in which, over a given lapse of time, more air flows into a given region than flows out of it. It is commonly accompanied by confluence of the streamlines, but may be caused by differences of velocity (e.g. where the wind comes against a coast or a mountain wall). Surface friction can produce convergence. Compare divergence.

2. The point, line, or region where two oceanic water masses or surface currents meet. This leads to the denser water from one side sinking beneath the lighter water of the other side.

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1. Situation in which, over a given lapse of time, more air flows into a given region than flows out of it. It is commonly accompanied by confluence of the streamlines, but may be caused by differences of velocity, e.g. where the wind comes against a coast or a mountain wall. Surface friction can produce convergence. Compare DIVERGENCE.

2. The point, line, or region where two oceanic water masses or surface currents meet. This leads to the denser water from one side sinking beneath the lighter water of the other side.

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convergence of an algorithm. See error analysis.

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convergence (kŏn-ver-jĕns) n. inward turning of the eyes to achieve fusion of separate images during near vision.