List of examples of convergent evolution

Convergent evolution—the evolution of similar traits in unrelated lineages (usually geographically distant)—is rife in nature, as illustrated by the examples below. The ultimate cause of convergence is usually a similar evolutionary biome, as similar environments will select for similar traits in any species occupying the same ecological niche, even if those species are only distantly related. In the case of cryptic species, it can create species which are only distinguishable by analysing their genetics. Unrelated organisms often develop analogous structures by adapting to similar environments.

In animals

The skulls of the Thylacine (left) and the Grey Wolf, Canis lupus, are similar, although the species are only very distantly related (different infraclasses). The skull shape of the Red Fox, Vulpes vulpes, is even closer to that of the Thylacine.

Mammals

• Several groups of ungulates have independently reduced or lost side digits on their feet, often leaving one or two digits for walking. That name comes from their hooves, which have evolved from claws several times. Among familiar animals, horses have one walking digit and domestic bovines two. Various other land vertebrates have also reduced or lost digits.
• The pronghorn of North America, while not a true antelope and only distantly related to them, closely resembles the true antelopes of the Old World, both behaviorally and morphologically. It also fills a similar ecological niche and is found in the same biomes.
• Members of the two clades Australosphenida and theria evolved tribosphenic molars independently.
• The marsupial thylacine (Tasmanian Tiger) had many resemblances to the placental canids.
• Several mammal groups have independently evolved prickly protrusions of the skin – echidnas (monotremes), the insectivorous hedgehogs, some tenrecs (a diverse group of shrew-like Madagascan mammals), Old World porcupines (rodents) and New World porcupines (another biological family of rodents). In this case, because the two groups of porcupines are closely related, they would be considered to be examples of parallel evolution; however, neither echidnas, nor hedgehogs, nor tenrecs are close relatives of the Rodentia. In fact, the last common ancestor of all of these groups was a contemporary of the dinosaurs.
• Cat-like sabre-toothed predators evolved in three distinct lineages of mammals – sabre-toothed cats, Nimravids ("false" sabre-tooths), and the marsupial "lion" Thylacosmilus. Gorgonopsids and creodonts also developed long canine teeth, but with no other particular physical similarities.
• A number of mammals have developed powerful fore claws and long, sticky tongues that allow them to open the homes of social insects (e.g., ants and termites) and consume them (myrmecophagy). These include the four species of anteater, more than a dozen armadillos, eight species of pangolin (plus fossil species), the African aardvark, one echidna (an egg-laying monotreme), the enigmatic Fruitafossor, the singular Australian marsupial known as the numbat, the aberrant aardwolf, and possibly also the sloth bear of South Asia, all not related.
• Koalas of Australasia have evolved fingerprints, indistinguishable from those of humans.
• The Australian honey possums acquired a long tongue for taking nectar from flowers, a structure similar to that of butterflies, some moths, and hummingbirds, and used to accomplish the very same task.
• Marsupial sugar glider and squirrel glider of Australia are like the placental flying squirrel. Both lineages have independently developed wing-like flaps (patagia) for leaping from trees, and big eyes for foraging at night.
• The North American kangaroo rat, Australian hopping mouse, and North African and Asian jerboa have developed convergent adaptations for hot desert environments; these include a small rounded body shape with very large hind legs and long thin tails, a characteristic bipedal hop, and nocturnal, burrowing and seed-eating behaviours. These rodent groups fill similar niches in their respective ecosystems.
• Opossums have evolved an opposable thumb, a feature which is also commonly found in the non-related primates.
• Marsupial mole has many resemblances to the placental mole.
• Marsupial mulgara has many resemblances to the placental mouse.
• Planigale has many resemblances to the deer mouse.
• Marsupial Tasmanian devil has many resemblances to the placental hyena. Similar skull morphology, large canines and crushing carnasial molars.
• Kangaroo has many resemblances to the Patagonian cavy and Hares.
• The Marsupial lion had retractable claws, the same way the placental felines (cats) do today.
• Microbats, toothed whales and shrews developed sonar-like echolocation systems used for orientation, obstacle avoidance and for locating prey. Modern DNA phylogenies of bats have shown that the traditional suborder of echolocating bats (Microchiroptera) is not a true clade, and instead some echolocating bats are more related to non-echolocating Old World fruit bats than to other echolocating species. The implication is that echolocation in at least two lineages of bats, Megachiroptera and Microchiroptera has evolved independently or been lost in Old World fruit bats.
• Echolocation in bats and whales also both necessitate high frequency hearing. The protein prestin, which confers high hearing sensitivity in mammals, shows molecular convergence between the two main clades of echolocating bats, and also between bats and dolphins. Other hearing genes also show convergence between echolocating taxa.
• Both the aye-aye lemur and the striped possum have an elongated finger used to get invertebrates from trees. There are no woodpeckers in Madagascar or Australia where the species evolved, so the supply of invertebrates in trees was large.
• Castorocauda and beaver both have webbed feet and a flattened tail, but are not related.
• Prehensile tails came in to a number of unrelated species New World monkeys, kinkajous, porcupines, tree-anteaters, marsupial opossums, and the salamander Bolitoglossa pangolins, treerats, skinks and chameleons.
• Pig form, large-headed, pig-snouted and hoofs are independent in true pigs in Eurasia and peccary and entelodonts.
• Tapirs and pigs look much alike, but tapirs are perissodactyls (odd-toed ungulates) and pigs are artiodactyls (even-toed ungulates).
• Plankton feeding filters, like baleen: baleen whales like the humpback and blue whale, the whale shark, the basking shark, and the Mesozoic bony fish Leedsichthys have separately evolved ways of sifting plankton from marine waters.
• There are five species of river/freshwater dolphins, which are not closely related.
• Platypus have what looks like a bird's beak (hence its scientific name “Ornithorhynchus”), but is a mammal.
• Red blood cells in mammals lack a cell nucleus. In comparison, the red blood cells of other vertebrates have nuclei; the only known exceptions are salamanders of the Batrachoseps genus and fish of the Maurolicus genus.

Prehistoric reptiles

• Ornithischian (bird-hipped) dinosaurs had a pelvis shape similar to that of birds, or avian dinosaurs, which evolved from saurischian (lizard-hipped) dinosaurs.
• The Heterodontosauridae evolved a tibiotarsus which is also found in modern birds. These groups aren't closely related.
• Ankylosaurs and glyptodont mammals both had spiked tails.
• The sauropods and giraffes independently evolved long necks.
• The horned snouts of ceratopsian dinosaurs like Triceratops have also evolved several times in Cenozoic mammals: rhinos, brontotheres, Arsinoitherium, and Uintatherium.
• Billed snouts on the duck-billed dinosaurs hadrosaurs strikingly convergent with ducks and duck-billed platypus.
• Ichthyosaurs a marine reptile of the Mesozoic era looked strikingly like dolphins.
• Beaks are independent in ceratopsian dinosaurs like Triceratops, birds and cephalopods like squid and octopus.
• The Pelycosauria and the Ctenosauriscidae bore striking resemblance to each other because they both had a sail-like fin on their back. The Pelycosaurs are more closely related to mammals while the Ctenosauriscids are closely related to pterosaurs and dinosaurs. Also, the Spinosaurids had sail-like fins on their backs, when they were not closely related to either.
  • Also, Acrocanthosaurus and Ouranosaurus, which are not closely related to either Pelycosaurs, Ctenosauriscids or Spinosaurids, also had similar, but thicker, spines on their vertebrae, and thus have humps, like the unrelated, mammalian camels and bison.
• Noasaurus, Baryonyx, and Megaraptor, all unrelated, all had an enlarged hand claw that were originally thought to be placed on the foot, as in dromaeosaurs. A similarly modified claw (or in this case, finger) is on the hand of Iguanodon.
• The Ornithopods had feet and beaks that resembled that of birds, but are only distantly related.
• Three groups of dinosaurs, the Tyrannosauridae, Ornithomimosauria, and the Troodontidae, all evolved an arctometatarsus, independently.

Extant reptiles

• The thorny devil (Moloch horridus) is similar in diet and activity patterns to the Texas horned lizard (Phrynosoma cornutum), although the two are not particularly closely related.
• Modern Crocodilians resemble prehistoric phytosaurs, champsosaurs, certain labyrinthodont amphibians, and perhaps even the early whale Ambulocetus. The resemblance between the crocodilians and phytosaurs in particular is quite striking; even to the point of having evolved the graduation between narrow- and broad-snouted forms, due to differences in diet between particular species in both groups.
• The body shape of the prehistoric fish-like reptile Ophthalmosaurus is similar to those of other ichthyosaurians, dolphins (aquatic mammals), and tuna (scombrid fish).
• Death Adders strongly resemble true vipers, but are elapids.
• The Glass Snake is actually a lizard but is mistaken as a snake .
• Large Tegu lizards of South America have converged in form and ecology with monitor lizards, which are not present in the Americas.
• legless lizards-Pygopodidae are snake-like lizards that are much like true snakes.
• Mosasaurs of the Mesozoic era are like whales but are closely related to living monitor lizards and the Komodo Dragon.
• Anolis lizards are one of the best examples of both adaptive radiation and convergent evolution.
• Tuataras resemble lizards but in fact are in an order of their own, the Rhynchocephalia. The Tuatara has the sockets behind the eyes and has jagged extensions of the jaws instead of teeth.
• Asian sea snake, Enhydrina schistosa (beaked sea snake) look just like the Australian sea snake Enhydrina zweifeli, but in fact are not related.

Avian

• The Little Auk of the north Atlantic (Charadriiformes) and the diving-petrels of the southern oceans (Procellariiformes) are remarkably similar in appearance and habits.
• The Eurasian Magpie is a corvid, the Australian Magpie is not.
• Penguins in the Southern Hemisphere evolved similarly to flightless wing-propelled diving auks in the Northern Hemisphere: the Atlantic Great Auk and the Pacific mancallines.
• Vultures are a result of convergent evolution: both Old World vultures and New World vultures eat carrion, but Old World vultures are in the eagle and hawk family (Accipitridae) and use mainly eyesight for discovering food; the New World vultures are of obscure ancestry, and some use the sense of smell as well as sight in hunting. Birds of both families are very big, search for food by soaring, circle over sighted carrion, flock in trees, and have unfeathered heads and necks.

• 

  Nubian Vulture, an Old World vulture

• 

  Turkey Vulture, a New World vulture

• 

  Hummingbird, a New World bird, with a sunbird, an Old World bird

• Hummingbirds resemble sunbirds. The former live in the Americas and belong to an order or superorder including the swifts, while the latter live in Africa and Asia and are a family in the order Passeriformes.
• In an odd cross-phyla example, an insect, the Hummingbird Hawk-moth (Macroglossum stellatarum), also feeds by hovering in front of flowers and drinking their nectar in the same way as the above mentioned birds.
• Flightlessness has evolved in many different birds independently. However, taking this to a greater extreme, the terror birds, Gastornithiformes and dromornithids (ironically all extinct) all evolved the similar body shape (flightlessness, long legs, long necks, big heads), yet none of them were closely related. They also share the trait of being giant, flightless birds with vestigial wings, long legs, and long necks with the ratites, although they are not related.
• Certain longclaws (Macronyx) and meadowlarks (Sturnella) have essentially the same striking plumage pattern. The former inhabit Africa and the latter the Americas, and they belong to different lineages of Passerida. While they are ecologically quite similar, no satisfying explanation exists for the convergent plumage; it is best explained by sheer chance.
• Resemblances between swifts and swallows is due to convergent evolution. The Chimney Swift was originally identified as Chimney Swallow (Hirundo pelagica) by Carl Linnaeus in 1758, before being moved to the swift genus Chaetura by James Francis Stephens in 1825.
• Downy Woodpecker and Hairy Woodpecker look almost the same, as do some Chrysocolaptes and Dinopium flamebacks, the Smoky-brown Woodpecker and some Veniliornis species, and other Veniliornis species and certain "Picoides" and Piculus. In neither case are the similar species particularly close relatives.
• Many birds of Australia, like wrens and robins, look like northern hemisphere birds but are not related.
• Oilbird like microbats and toothed whales developed sonar-like echolocation systems used for locating prey.
• The brain structure, forebrain, of hummingbirds, songbirds, and parrots responsible for vocal learning (not by instinct) is very similar. These types of birds are not closely related.
• Seriemas very closely resemble the extinct troodontid dinosaurs. Both have evolved a retractable sickle-shaped claw on the second toe of each foot, and both have feathers, and are very similar, in their overall physical appearance.

Fish

• Goby dorsal finned like the lumpsuckers, yet they are not related.
• Sandlance fish and chameleons have independent eye movements and focusing by use of the cornea.
• Cichlids of South America and the "sunfish" of North America are strikingly similar in morphology, ecology and behavior.
• The Peacock Bass and Largemouth Bass are excellent examples.
• The Antifreeze protein of fish in the arctic and Antarctic, came about independently.
• Eel form are independent in the North American brook lamprey, neotropical eels, and the African spiny eel.
• Stickleback fish, there is widespread convergent evolution in Sticklebacks.
• Flying fish can fly up to 400 m (1,300 ft) at speeds of more than 70 kilometres per hour (43 mph) at a maximum altitude of more than 6 m (20 ft), much like other flying birds, bats and other gliders.
• The Cleaner Wrasse Labroides dimidiatus of the Indian Ocean is a small, longitudinally-striped black and bright blue cleaner fish, just like the Cleaner Goby Elacatinus evelynae of the Western Atlantic.

• 

  Cleaner wrasse Labroides dimidiatus servicing a Bigeye squirrelfish

• 

  Caribbean cleaning goby Elacatinus evelynae

Amphibians

• Plethodontid salamanders and Chameleons have evolved a harpoon-like tongue to catch insects.
• The Neotropical poison dart frog and the Mantella of Madagascar have independently developed similar mechanisms for obtaining alkaloids from a diet of mites and storing the toxic chemicals in skin glands. They have also independently evolved similar bright skin colors that warn predators of their toxicity (by the opposite of crypsis, namely aposematism).
• Caecilian are Lissamphibians that secondarly lost their limbs, resembling snakes

Arthropods

Pill bugs look like pill millipedes, but are actually wood lice that have converged on the same defenses, until they are difficult to tell apart

• Assassin spiders comprise two lineages that evolved independently. They have very long necks and fangs proportionately larger than those of any other spider, and they hunt other spiders by snagging them from a distance.
• The smelling organs of the terrestrial coconut crab are similar to those of insects.
• Pill bugs and pill millipedes have evolved not only identical defenses, but are even difficult tell apart at a glance.
• Silk: Spiders, silk moths, larval caddis flies, and the weaver ant all produce silken threads.
• The praying mantis body type – raptorial forelimb, prehensile neck, and extraordinary snatching speed - has evolved not only in mantid insects but also independently in neuropteran insects Mantispidae.
• Gripping limb ends have evolved separately in scorpions and in some decapod crustaceans, like lobsters and crabs. These chelae or claws have a similar architecture: the next-to-last segment grows a projection that fits against the last segment.
• Agriculture: Some kinds of ants, termites, and ambrosia beetles have for a long time cultivated and tend fungi for food. These insects sow, fertilize, and weed their crops. A damselfish also takes care of red algae carpets on its piece of reef; the damselfish actively weeds out invading species of algae by nipping out the newcomer.

Molluscs

• Bivalves and the gastropods in the family Juliidae have very similar shells.
• There are limpet-like forms in several lines of gastropods: "true" limpets, pulmonate siphonariid limpets and several lineages of pulmonate freshwater limpets.
• Cephalopod and vertebrate eyes are both lens-camera eyes with much overall similarity. A closer examination reveals lots of differences in detail. Embryonic development, innervation direction, how many lens parts, etc.
• Swim bladders – Buoyant bladders independently evolved in fishes, the Tuberculate Pelagic Octopus, and siphonophores such as the Portuguese Man o' War.
• Bivalves and brachiopods independently evolved paired hinged shells for protection. However, the anatomy of their soft parts is very dissimilar, which is why molluscs and brachiopods are put into different phyla.
• Jet propulsion in squids and in scallops: these two groups of mollusks have very different ways of squeezing water through their bodies in order to power rapid movement through a fluid. (Dragonfly larvae in the aquatic stage also use an anal jet to propel them, and Jellyfish have used jet propulsion for a very long time.)

Other

• The notochords in chordates are like the stomochords in hemichordates.
• Elvis taxon in the fossil record developed a similar morphology through convergent evolution.
• Venomous sting: To inject poison with a hypodermic needle, a sharppointed tube, has shown up independently 10+ times: jellyfish, spiders, scorpions, centipedes, various insects, cone shell, snakes, stingrays, stonefish, the male duckbill platypus, and stinging nettles plant.
• Bioluminescence: A symbiotic partnerships with light-emitting bacteria developed many times independently in deep-sea fish, jellyfish, and in fireflies and glow worms.
• Parthenogenesis: Some lizards and insects have independent the capacity for females to produce live young from unfertilized eggs. Some species are entirely female.
• extremely halophile archaeal Family Halobacteriaceae and the extremely halophilic bacterium Salinibacter ruber both can live in high salt environment.

In plants

• Leaves have evolved multiple times - see Evolutionary history of plants. They have evolved not only in land plants, but also in various algae, like kelp.
• Prickles, thorns and spines are all modified plant tissues that have evolved to prevent or limit herbivory, these structures have evolved independently a number of times.
• Stimulant toxins: Plants which are only distantly related to each other, such as coffee and tea, produce caffeine to deter predators.
• The aerial rootlets found in ivy (Hedera) are similar to those of the climbing hydrangea (Hydrangea petiolaris) and some other vines. These rootlets are not derived from a common ancestor but have the same function of clinging to whatever support is available.
• Flowering plants (Delphinium, Aerangis, Tropaeolum and others) from different regions form tube-like spur which contains nectar (that's why insect from one place sometimes can feed on plant from other which has such structure like the flower which is the traditional source of food for the animal).
• Both some dicots (Anemone) and monocots (Trillium) in inhospitable environments are able to form underground organs such as corms, bulbs and rhizomes for reserving of nutrition and water till the conditions become better.
• Insectivorous plants: Nitrogen-deficient plants have in at least 7 distinct times become insectivorous, like: flypaper traps\sundew, spring traps-Venus fly trap, and pitcher traps in order to capture and digest insects to obtain scarce nitrogen.
• Similar-looking rosette succulents have arisen separately among plants in the families Asphodelaceae (formerly Liliaceae) and Crassulaceae.
• The Orchids, the Birthwort family and Stylidiaceae have evolved independently the specific organ known as gynostemium, more popular as column.
• The Euphorbia of deserts in Africa and southern Asia, and the Cactaceae of the New World deserts have similar modifications (see picture below for one of many possible examples).
• Sunflower: some types of Sunflower and Pericallis are due to convergent evolution.
• C4 photosynthesis is estimated to have evolved over 60 times within plants. C4 plants use a different metabolic pathway to capture carbon dioxide but also have differences in leaf anatomy and cell biology compared to most other plants.

• 

  Euphorbia obesa

• 

  Astrophytum asterias

In fungi

There are a variety of saprophytic and parasitic organisms that have evolved the habit of growing into their substrates as thin strands. This is most typical of the "true" fungi, but it has also evolved in actinobacteria (bacteria), oomycetes (stramenopiles, like kelp), parasitic plants, and rhizocephalans (parasitic barnacles).

Proteins including enzymes and biochemical pathways

Proteins undergoing functional convergence

Here is a list of examples in which unrelated proteins have similar functions with different structure.

• The existence of distinct families of carbonic anhydrase is believed to illustrate convergent evolution.

• The use of (Z)-7-dodecen-1-yl acetate as a sex pheromone by the Asian elephant (Elephas maximus) and by more than 100 species of Lepidoptera.

• The biosynthesis of plant hormones such as gibberellin and abscisic acid by different biochemical pathways in plants and fungi.

• The protein prestin that drives the cochlea amplifier and confers high auditory sensitivity in mammals, shows numerous convergent amino acid replacements in bats and dolphins, both of which have independently evolved high frequency hearing for echolocation. This same signature of convergence has also been found in other genes expressed in the mammalian cochlea

• The repeated independent evolution of nylonase in two different strains of Flavobacterium and one strain of Pseudomonas.

• The myoglobin from the abalone Sulculus diversicolor has a different structure from normal myoglobin but serves a similar function — binding oxygen reversibly. “The molecular weight of Sulculus myoglobin is 41kD, 2.5 times larger than other myoglobins.” Moreover, its amino acid sequence has no homology with other invertebrate myoglobins or with hemoglobins, but is 35% homologous with human indoleamine dioxygenase (IDO), a vertebrate tryptophan-degrading enzyme. Interestingly, it does not share similar function with IDO. “The IDO-like myoglobin is unexpectedly widely distributed among gastropodic molluscs, such as Sulculus, Nordotis, Battilus, Omphalius and Chlorostoma.”

• The hemocyanin from arthropods and molluscs evolved from different ancestors, tyrosinase and insect storage proteins, respectively. They have different molecular weight and structure. However, the proteins both use copper binding sites to transport oxygen.

• The hexokinase, ribokinase, and galactokinase families of sugar kinases have similar enzymatic functions of sugar phosphorylation, but they evolved from three distinct nonhomologous families since they all have distinct three-dimensional folding and their conserved sequence patterns are strikingly different.

• Hemoglobins in jawed vertebrates and jawless fish evolved independently. The oxygen-binding hemoglobins of jawless fish evolved from an ancestor of cytoglobin which has no oxygen transport function and is expressed in fibroblast cells.

• Toxic agent, serine protease BLTX, in the venom produced by two distinct species, the North American short-tailed shrew Blarina brevicauda and the Mexican beaded lizard, undergo convergent evolution. Although their structures are similar, it turns out that they increased the enzyme activity and toxicity through different way of structure changes. These changes are not found in the other non-venomous reptiles or mammals.

• Another toxin BgK, a K+ channel-blocking toxin from the sea anemone Bunodosoma granulifera and scorpions adopt distinct scaffolds and unrelated structures, however, they have similar functions.

• Antifreeze proteins are a perfect example of convergent evolution. Different small proteins with a flat surface which is rich in threonine from different organisms are selected to bind to the surface of ice crystals. “These include two proteins from fish, the ocean pout and the winter flounder, and three very active proteins from insects, the yellow mealworm beetle, the spruce budworm moth, and the snow flea.”

• RNA-binding proteins which contain RNA-binding domain(RBD) and the cold-shock domain (CSD) protein family are also an interesting example of convergent evolution. Except that they both have concerved RNP motifs, other protein sequence are totally different. However, they have a similar function.

• Blue-light-receptive cryptochrome expressed in the sponge eyes likely evolved convergently in the absence of opsins and nervous systems. The fully sequenced genome of Amphimedon queenslandica, a demosponge larvae, lacks one vital visual component: opsin-a gene for a light-sensitive opsin pigment which is essential for vision in other animals.

• The structure of Immunoglobin G-binding bacterial proteins A and H do not contain any sequences homologous to the constant repeats of IgG antibodies, but they have similar functions. Both protein G, A, H are inhibited in the interactions with IgG antibodies (IgGFc) by a synthetic peptide corresponding to an 11-amino-acid-long sequence in the COOH-terminal region of the repeats.

Proteins undergoing structural convergence

Here is a list of examples in which unrelated proteins have similar tertiary structures but different functions.

• The independent development of the catalytic triad in serine proteases independently with subtilisin in prokaryotes and the chymotrypsin clan in eukaryotes.

• The independent development of three distinct hydrogenases exemplifies convergent evolution.

• Two functionally unrelated, sequentially non-homologous proteins: the immunoglobulin domain and the copper, zinc superoxide dismutase subunit shared a striking similarity in three-dimensional structure and overall shape. They all contain the same topological folding pattern --- a bilayer structure or flattened cylinder composed by antiparallel β-strands and external loops occur in places equivalent to hypervariable region loops.

• The lactate and malate dehydrogenase is an example of functional different but structurally related proteins. By superimpose the two similar structures in a way to minimize the sum of squared distances between all N atoms. Rossman et al. concluded that dehydrogenase region subtilisin and flavodoxin have similar tertiary structures such as the nucleotide-binding domain. The factors which contribute to such a functional convergence are favor of α-helix for certain amino acids and moreover, the supersecondary structure such as hydrophobic patches or pocket at the end of the parallel sheet.

• The structure of Globin family bear resemblance to each other and they developed different but related functions through evolution. Cullis et al. firstly compared the tertiary structure of horse oxyhemoglobin and whale myoglobin. Then with higher resolution, Huber et al. found optimum relation between complete structures of sperm whale myoglobin and insect (Chironomous thummi) hemoglobin.
• ABAC is a database of convergently evolved protein interaction interfaces. Examples comprise fibronectin/long chain cytokines, NEF/SH2, cyclophilin/capsid proteins. Details are described here.

See also

• McGhee, G.R. (2011) Convergent Evolution: Limited Forms Most Beautiful. Vienna Series in Theoretical Biology: Massachusetts Institute of Technology Press, Cambridge (MA). 322 pp.