Evolution of tetrapods

From Wikipedia, the free encyclopedia

In Late Devonian vertebrate speciation, descendants of pelagic lobe-finned fish — like Eusthenopteron — exhibited a sequence of adaptations: *Panderichthys, suited to muddy shallows *Tiktaalik with limb-like fins that could take it onto land *Early tetrapods in weed-filled swamps, such as: **Acanthostega, which had feet with eight digits **Ichthyostega with limbs Descendants also included pelagic lobe-finned fish such as coelacanth species.

The evolution of tetrapods began about 395 million years ago in the Devonian Period with the earliest tetrapods derived from the lobe-finned fishes.^[1] Tetrapods are categorized as a biological superclass, Tetrapoda, which includes all living and extinct amphibians, reptiles, birds, and mammals. While most species today are terrestrial, little evidence supports the idea that any of the earliest tetrapods could move about on land, as their limbs could not have held their midsections off the ground and the known trackways do not indicate they dragged their bellies around. Presumably, the tracks were made by animals walking along the bottoms of shallow bodies of water.^[2] The specific aquatic ancestors of the tetrapods, and the process by which land colonization occurred, remain unclear, and are areas of active research and debate among palaeontologists at present.

Amphibians today generally remain semiaquatic, living the first stage of their lives as fish-like tadpoles. Several groups of tetrapods, such as the snakes and cetaceans, have lost some or all of their limbs. In addition, many tetrapods have returned to partially aquatic or fully aquatic lives throughout the history of the group (modern examples of fully aquatic tetrapods include cetaceans and sirenians). The first returns to an aquatic lifestyle may have occurred as early as the Carboniferous Period^[3] whereas other returns occurred as recently as the Cenozoic, as in cetaceans, pinnipeds,^[4] and several modern amphibians.^[5]

The change from a body plan for breathing and navigating in water to a body plan enabling the animal to move on land is one of the most profound evolutionary changes known.^[6] It is also one of the best understood, largely thanks to a number of significant transitional fossil finds in the late 20th century combined with improved phylogenetic analysis.^[7]

Origin

Evolution of fish

The Devonian period is traditionally known as the "Age of Fish", marking the diversification of numerous extinct and modern major fish groups.^[8] Among them were the early bony fishes, who diversified and spread in freshwater and brackish environments at the beginning of the period. The early types resembled their cartilaginous ancestors in many features of their anatomy, including a shark-like tailfin, spiral gut, large pectoral fins stiffened in front by skeletal elements and a largely unossified axial skeleton.^[9]
They did, however, have certain traits separating them from cartilaginous fishes, traits that would become pivotal in the evolution of terrestrial forms. With the exception of a pair of spiracles, the gills did not open singly to the exterior as they do in sharks; rather, they were encased in a gill chamber stiffened by membrane bones and covered by a bony operculum, with a single opening to the exterior. The cleithrum bone, forming the posterior margin of the gill chamber, also functioned as anchoring for the pectoral fins. The cartilaginous fishes do not have such an anchoring for the pectoral fins. This allowed for a movable joint at the base of the fins in the early bony fishes, and would later function in a weight bearing structure in tetrapods. As part of the overall armour of rhomboid cosmin scales, the skull had a full cover of dermal bone, constituting a skull roof over the otherwise shark-like cartilaginous inner cranium. Importantly, they also had a swim bladder/lung,^[10] a feature lacking in sharks and rays.

Lungs before land

The lung/swim bladder originated as an outgrowth of the gut, forming a gas-filled bladder above the digestive system. In its primitive form, the air bladder was open to the alimentary canal, a condition called physostome and still found in many fish.^[11] The primary function is not entirely certain. One consideration is buoyancy. The heavy scale armour of the early bony fishes would certainly weigh the animals down. In cartilaginous fishes, lacking a swim bladder, the open sea sharks need to swim constantly to avoid sinking into the depths, the pectoral fins providing lift.^[12] Another factor is oxygen consumption. Ambient oxygen was relatively low in the early Devonian, possibly about half of modern values.^[13] Per unit volume, there is much more oxygen in air than in water, and vertebrates are active animals with a high energy requirement compared to invertebrates of similar sizes.^[14][15] The Devonian saw increasing oxygen levels which opened up new ecological niches by allowing groups able to exploit the additional oxygen to develop into active, large-bodied animals.^[13] Particularly in tropical swampland habitats, atmospheric oxygen is much more stable, and may have prompted a reliance of lungs rather than gills for primary oxygen uptake.^[16][17] In the end, both buoyancy and breathing may have been important, and some modern physostome fishes do indeed use their bladders for both.

To function in gas exchange, lungs required a blood supply. In cartilaginous fishes and teleosts, the heart lies low in the body and pumps blood forward through the ventral aorta, which splits up in a series of paired aortic arches, each corresponding to a gill arch.^[18] The aortic arches then merge above the gills to form a dorsal aorta supplying the body with oxygenated blood. In lungfishes, bowfin and bichirs, the swim bladder is supplied with blood by paired pulmonary arteries branching off from the hindmost (6th) aortic arch.^[19] The same basic pattern is found in the lungfish Protopterus and in terrestrial salamanders, and was probably the pattern found in the tetrapods' immediate ancestors as well as the first tetrapods.^[20] In most other bony fishes the swim bladder is supplied with blood by the dorsal aorta.^[19]

External and internal nares

The nostrils in most bony fish differ from those of tetrapods. Normally, bony fish have four nares (nasal openings), one naris behind the other on each side. As the fish swims, water flows into the forward pair, across the olfactory tissue, and out through the posterior openings. This is true not only of ray-finned fish but also of the coelacanth, a fish included in the Sarcopterygii, the group that also includes the tetrapods. In contrast, the tetrapods have only one pair of nares externally but also sport a pair of internal nares, called choanae, allowing them to draw air through the nose. Lungfish are also sarcopterygians with internal nostrils, but these are sufficiently different from tetrapod choanae that they have long been recognized as an independent development.^[21]

The evolution of the tetrapods' internal nares was hotly debated in the 20th century. The internal nares could be one set of the external ones (usually presumed to be the posterior pair) that have migrated into the mouth, or the internal pair could be a newly evolved structure. To make way for a migration, however, the two tooth-bearing bones of the upper jaw, the maxilla and the premaxilla, would have to separate to let the nostril through and then rejoin; until recently, there was no evidence for a transitional stage, with the two bones disconnected. Such evidence is now available: a small lobe-finned fish called Kenichthys, found in China and dated at around 395 million years old, represents evolution "caught in mid-act", with the maxilla and premaxilla separated and an aperture—the incipient choana—on the lip in between the two bones.^[22] Kenichthys is more closely related to tetrapods than is the coelacanth,^[23] which has only external nares; it thus represents an intermediate stage in the evolution of the tetrapod condition. The reason for the evolutionary movement of the posterior nostril from the nose to lip, however, is not well understood.

Into the shallows

Devonian fishes, including an early shark Cladoselache, Eusthenopteron and other lobe-finned fishes, and the placoderm Bothriolepis (Joseph Smit, 1905).

The relatives of Kenichthys soon established themselves in the waterways and brackish estuaries and became the most numerous of the bony fishes throughout the Devonian and most of the Carboniferous. The basic anatomy of group is well known thanks to the very detailed work on Eusthenopteron by Erik Jarvik in the second half of the 20th century.^[24] The bones of the skull roof were broadly similar to those of early tetrapods and the teeth had an infolding of the enamel similar to that of labyrinthodonts. The paired fins had a build with bones distinctly homologous to the humerus, ulna, and radius in the fore-fins and to the femur, tibia, and fibula in the pelvic fins.^[25]

There were a number of families: Rhizodontida, Canowindridae, Elpistostegidae, Megalichthyidae, Osteolepidae and Tristichopteridae.^[26] Most were open-water fishes, and some grew to very large sizes; adult specimens are several meters in length.^[27] The Rhizodontid Rhizodus is estimated to have grown to 7 meters (23 feet), making it the largest freshwater fish known.^[28]

While most of these were open-water fishes, one group, the Elpistostegalians, adapted to life in the shallows. They evolved flat bodies for movement in very shallow water, and the pectoral and pelvic fins took over as the main propulsion organs. Most median fins disappeared, leaving only a protocercal tailfin. Since the shallows were subject to occasional oxygen deficiency, the ability to breath atmospheric air with the swim bladder became increasingly important.^[6] The spiracle became large and prominent, enabling these fishes to draw air.

Skull morphology

The tetrapods have their root in the early Devonian tetrapodomorph fish.^[29] Primitive tetrapods developed from an osteolepid tetrapodomorph lobe-finned fish (sarcopterygian-crossopterygian), with a two-lobed brain in a flattened skull. The coelacanth group represents marine sarcopterygians that never acquired these shallow-water adaptations. The sarcopterygians apparently took two different lines of descent and are accordingly separated into two major groups: the Actinistia (including the coelacanths) and the Rhipidistia (which include extinct lines of lobe-finned fishes that evolved into the lungfish and the tetrapodomorphs).

From fins to feet

The oldest known tetrapodomorph is Kenichthys from China, dated at around 395 million years old. Two of the earliest tetrapodomorphs, dating from 380 Ma, were Gogonasus and Panderichthys.^[30]
They had choanae and used their fins as to move through tidal channels and shallow waters choked with dead branches and rotting plants.^[31] Their fins could have been used to attach themselves to plants or similar while they were lying in ambush for prey. The universal tetrapod characteristics of front limbs that bend forward from the elbow and hind limbs that bend backward from the knee can plausibly be traced to early tetrapods living in shallow water. Pelvic bone fossils from Tiktaalik shows, if representative for early tetrapods in general, that hind appendages and pelvic-propelled locomotion originated in water before terrestrial adaptations.^[32]

Another indication that feet and other tetrapod traits evolved while the animals were still aquatic is how they were feeding. They did not have the modifications of the skull and jaw that allowed them to swallow prey on land. Prey could be caught in the shallows, at the water's edge or on land, but had to be eaten in water where hydrodynamic forces from the expansion of their buccal cavity would force the food into their esophagus.^[33]

It has been suggested that the evolution of the tetrapod limb from fins in lobe-finned fishes is related to expression of the HOXD13 gene or the loss of the proteins actinodin 1 and actinodin 2, which are involved in fish fin development.^[34][35] Robot simulations suggest that the necessary nervous circuitry for walking evolved from the nerves governing swimming, utilizing the sideways oscillation of the body with the limbs primarily functioning as anchoring points and providing limited thrust.^[36]

A 2012 study using 3d reconstructions of Ichthyostega concluded that it was incapable of typical quadrupedal gaits. The limbs could not move alternately as they lacked the necessary rotary motion range. In addition, the hind limbs lacked the necessary pelvic musculature for hindlimb-driven land movement. Their most likely method of terrestrial locomotion is that of synchronous "crutching motions", similar to modern mudskippers.^[37]

Denizens of the swamp

The first tetrapods probably evolved in coastal and brackish marine environments, and in shallow and swampy freshwater habitats.^[38] Formerly, researchers thought the timing was towards the end of the Devonian. In 2010, this belief was challenged by the discovery of the oldest known tetrapod tracks, preserved in marine sediments of the southern coast of Laurasia, now Świętokrzyskie (Holy Cross) Mountains of Poland. They were made during the Eifelian stage at the end of the Middle Devonian.
The tracks, some of which show digits, date to about 395 million years ago—18 million years earlier than the oldest known tetrapod body fossils.^[39] Additionally, the tracks show that the animal was capable of thrusting its arms and legs forward, a type of motion that would have been impossible in tetrapodomorph fish like Tiktaalik. The animal that produced the tracks is estimated to have been up to 2.5 metres (8.2 ft) long with footpads up to 26 centimetres (10 in) wide, although most tracks are only 15 centimetres (5.9 in) wide.^[40] The new finds suggest that the first tetrapods may have lived as opportunists on the tidal flats, feeding on marine animals that were washed up or stranded by the tide.^[39] Currently, however, fish are stranded in significant numbers only at certain times of year, as in alewife spawning season; such strandings could not provide a significant supply of food for predators. There is no reason to suppose that Devonian fish were less prudent than those of today.^[41] According to Melina Hale of University of Chicago, not all ancient trackways are necessarily made by early tetrapods, but could also be created by relatives of the tetrapods who used their fleshy appendages in a similar substrate-based locomotion.^[42][43]

Palaeozoic tetrapods

Devonian tetrapods

Research by Jennifer A. Clack and her colleagues showed that the very earliest tetrapods, animals similar to Acanthostega, were wholly aquatic and quite unsuited to life on land. This is in contrast to the earlier view that fish had first invaded the land — either in search of prey (like modern mudskippers) or to find water when the pond they lived in dried out — and later evolved legs, lungs, etc.

By the late Devonian, land plants had stabilized freshwater habitats, allowing the first wetland ecosystems to develop, with increasingly complex food webs that afforded new opportunities. Freshwater habitats were not the only places to find water filled with organic matter and choked with plants with dense vegetation near the water's edge. Swampy habitats like shallow wetlands, coastal lagoons and large brackish river deltas also existed at this time, and there is much to suggest that this is the kind of environment in which the tetrapods evolved. Early fossil tetrapods have been found in marine sediments, and because fossils of primitive tetrapods in general are found scattered all around the world, they must have spread by following the coastal lines — they could not have lived in freshwater only.

One analysis from the University of Oregon suggests no evidence for the "shrinking waterhole" theory - transitional fossils are not associated with evidence of shrinking puddles or ponds - and indicates that such animals would probably not have survived short treks between depleted waterholes.^[44] The new theory suggests instead that proto-lungs and proto-limbs were useful adaptations to negotiate the environment in humid, wooded floodplains.^[45]

The Devonian tetrapods went through two major bottlenecks during what is known as the Late Devonian extinction; one at the end of the Frasnian stage, and one twice as large at the end of the following Famennian stage. These events of extinctions led to the disappearance of primitive tetrapods with fish-like features like Ichthyostega and their primary more aquatic relatives.^[46] When tetrapods reappear in the fossil record again after the Devonian extinctions, the adult forms are all fully adapted to a terrestrial existence, with later species secondary adapted to an aquatic lifestyle.^[47]

Excretion in tetrapods

The common ancestor of all present gnathostomes lived in freshwater, and later migrated back to the sea. To deal with the much higher salinity in sea water, they evolved the ability to turn the nitrogen waste product ammonia into harmless urea, storing it in the body to give the blood the same osmolarity as the sea water without poisoning the organism. This is the system currently found in cartilaginous fishes. Ray-finned fishes (Actinopterygii) later returned to freshwater and lost this ability, while the fleshy-finned fishes (Sarcopterygii) retained it. Since the blood of ray-finned fishes contains more salt than freshwater, they could simply get rid of ammonia through their gills. When they finally returned to the sea again, they did not recover their old trick of turning ammonia to urea, and they had to evolve salt excreting glands instead. Lungfishes do the same when they are living in water, making ammonia and no urea, but when the water dries up and they are forced to burrow down in the mud, they switch to urea production. Like cartilaginous fishes, the coelacanth can store urea in its blood, as can the only known amphibians that can live for long periods of time in salt water (the toad Bufo marinus and the frog Rana cancrivora). These are traits they have inherited from their ancestors.

If early tetrapods lived in freshwater, and if they lost the ability to produce urea and used ammonia only, they would have to evolve it from scratch again later. Not a single species of all the ray-finned fishes living today has been able to do that, so it is not likely the tetrapods would have done so either. Terrestrial animals that can only produce ammonia would have to drink constantly, making a life on land impossible (a few exceptions exist, as some terrestrial woodlice can excrete their nitrogenous waste as ammonia gas). This probably also was a problem at the start when the tetrapods started to spend time out of water, but eventually the urea system would dominate completely. Because of this it is not likely they emerged in freshwater (unless they first migrated into freshwater habitats and then migrated onto land so shortly after that they still retained the ability to make urea), although some species never left, or returned to, the water could of course have adapted to freshwater lakes and rivers.

Lungs

It is now clear that the common ancestor of the bony fishes (Osteichthyes) had a primitive air-breathing lung—later evolved into a swim bladder in most actinopterygians (ray-finned fishes). This suggests that crossopterygians evolved in warm shallow waters, using their simple lung when the oxygen level in the water became too low.

Fleshy lobe-fins supported on bones rather than ray-stiffened fins seem to have been an ancestral trait of all bony fishes (Osteichthyes). The lobe-finned ancestors of the tetrapods evolved them further, while the ancestors of the ray-finned fishes (Actinopterygii) evolved their fins in a different direction. The most primitive group of actinopterygians, the bichirs, still have fleshy frontal fins.

Fossils of early tetrapods

Nine genera of Devonian tetrapods have been described, several known mainly or entirely from lower jaw material. All but one were from the Laurasian supercontinent, which comprised Europe, North America and Greenland. The only exception is a single Gondwanan genus, Metaxygnathus, which has been found in Australia.

The first Devonian tetrapod identified from Asia was recognized from a fossil jawbone reported in 2002. The Chinese tetrapod Sinostega pani was discovered among fossilized tropical plants and lobe-finned fish in the red sandstone sediments of the Ningxia Hui Autonomous Region of northwest China. This finding substantially extended the geographical range of these animals and has raised new questions about the worldwide distribution and great taxonomic diversity they achieved within a relatively short time.

Eusthenopteron

Panderichthys

Tiktaalik

Acanthostega

Ichthyostega

Hynerpeton

Tulerpeton

Crassigyrinus

Diadectes

These earliest tetrapods were not terrestrial. The earliest confirmed terrestrial forms are known from the early Carboniferous deposits, some 20 million years later. Still, they may have spent very brief periods out of water and would have used their legs to paw their way through the mud.

Why they went to land in the first place is still debated. One reason could be that the small juveniles who had completed their metamorphosis had what it took to make use of what land had to offer. Already adapted to breathe air and move around in shallow waters near land as a protection (just as modern fish (and amphibians) often spent the first part of their life in the comparative safety of shallow waters like mangrove forests), two very different niches partially overlapped each other, with the young juveniles in the diffuse line between. One of them was overcrowded and dangerous while the other was much safer and much less crowded, offering less competition over resources. The terrestrial niche was also a much more challenging place for primary aquatic animals, but because of the way evolution and the selection pressure works, those juveniles who could take advantage of this would be rewarded. Once they gained a small foothold on land, thanks to their preadaptations and being at the right place at the right time, favourable variations in their descendants would gradually result in continuing evolution and diversification.

At this time the abundance of invertebrates crawling around on land and near water, in moist soil and wet litter, offered a food supply. Some were even big enough to eat small tetrapods, but the land was free from dangers common in the water.

From water to land

Initially making only tentative forays onto land, tetrapods adapted to terrestrial environments over time and spent longer periods away from the water. It is also possible that the adults started to spend some time on land (as the skeletal modifications in early tetrapods such as Ichthyostega suggests) to bask in the sun close to the water's edge, while otherwise being mostly aquatic.

Carboniferous tetrapods

Until the 1990s, there was a 30 million year gap in the fossil record between the late Devonian tetrapods and the reappearance of tetrapod fossils in recognizable mid-Carboniferous amphibian lineages. It was referred to as "Romer's Gap", which now covers the period from about 360 to 345 million years ago (the Devonian-Carboniferous transition and the early Mississippian), after the palaeontologist who recognized it.
During the "gap", tetrapod backbones developed, as did limbs with digits and other adaptations for terrestrial life. Ears, skulls and vertebral columns all underwent changes too. The number of digits on hands and feet became standardized at five, as lineages with more digits died out. The very few tetrapod fossils found in the "gap" are all the more precious.

The transition from an aquatic lobe-finned fish to an air-breathing amphibian was a momentous occasion in the evolutionary history of the vertebrates. For an animal to live in a gravity-neutral, aqueous environment and then invade one that is entirely different required major changes to the overall body plan, both in form and in function. Eryops is an example of an animal that made such adaptations. It retained and refined most of the traits found in its fish ancestors. Sturdy limbs supported and transported its body while out of water. A thicker, stronger backbone prevented its body from sagging under its own weight. Also, by utilizing vestigial fish jaw bones, a rudimentary ear was developed, allowing Eryops to hear airborne sound.

By the Visean age of mid-Carboniferous times the early tetrapods had radiated into at least three main branches. Recognizable basal-group tetrapods are representative of the temnospondyls (e.g. Eryops) lepospondyls (e.g. Diplocaulus) and anthracosaurs, which were the relatives and ancestors of the Amniota. Depending on whichever authorities one follows, modern amphibians (frogs, salamanders and caecilians) are derived from either temnospondyls or lepospondyls (or possibly both, although this is now a minority position).

The first amniotes are known from the early part of the Late Carboniferous, and during the Triassic counted among their number the earliest mammals, turtles, crocodiles (lizards and birds appeared in the Jurassic, and snakes in the Cretaceous), and a fourth Carboniferous group, the baphetids, which are thought related to temnospondyls, left no modern survivors.

Amphibians and reptiles were affected by the Carboniferous Rainforest Collapse (CRC), an extinction event that occurred ~300 million years ago. The sudden collapse of a vital ecosystem shifted the diversity and abundance of major groups. Several large groups, labyrinthodont amphibians were particularly devastated, while the first reptiles fared better, being ecologically adapted to the drier conditions that followed. Amphibians must return to water to lay eggs, in contrast, reptiles - whose amniote eggs have a membrane ensuring gas exchange out of water and can therefore be laid on land - were better adapted to the new conditions. Reptiles invaded new niches at a faster rate and began diversifying their diets, developing herbivory and carnivory, previously only having been insectivores and piscivores.^[48]

Permian tetrapods

In the Permian period, in addition to temnospondyl and anthracosaur clades among the early "amphibia" (labyrinthodonts), there were two important clades of amniotes, the Sauropsida and the Synapsida. The latter were the most important and successful Permian animals.
The end of the Permian saw a major turnover in fauna during the Permian–Triassic extinction event. There was a protracted loss of species, due to multiple extinction pulses.^[49] Many of the once large and diverse groups died out or were greatly reduced.

Mesozoic tetrapods

Life on Earth seemed to recover quickly after the Permian extinctions, but this was mostly in the form of disaster taxa, such as the hardy Lystrosaurus; specialized animals that formed complex ecosystems, with high biodiversity, complex food webs and a variety of niches, took much longer to recover.^[49] Current research indicates that this long recovery was due to successive waves of extinction, which inhibited recovery, and to prolonged environmental stress to organisms that continued into the Early Triassic. Recent research indicates that recovery did not begin until the start of the mid-Triassic, 4M to 6M years after the extinction;^[50] and some writers estimate that the recovery was not complete until 30M years after the P-Tr extinction, i.e. in the late Triassic.^[49]

A small group of reptiles, the diapsids, began to diversify during the Triassic, notably the dinosaurs. By the late Mesozoic, the large labyrinthodont groups that first appeared during the Paleozoic such as temnospondyls and reptile-like amphibians had gone extinct. All current major groups of sauropsids evolved during the Mesozoic, with birds first appearing in the Jurassic as a derived clade of theropod dinosaurs. Many groups of synapsids such as anomodonts and therocephalians that once comprised the dominant terrestrial fauna of the Permian also became extinct during the Mesozoic; during the Triassic, however, one group (Cynodontia) gave rise to the descendant taxon Mammalia, which survived through the Mesozoic to later diversify during the Cenozoic.

Extant (living) tetrapods

Following the great faunal turnover at the end of the Mesozoic, only six major groups of tetrapods were left, all of which also include many extinct groups:

• Lissamphibia: frogs and toads, newts and salamanders, and caecilians
• Testudines: turtles and tortoises
• Lepidosauria: tuataras, lizards, amphisbaenians and snakes
• Crocodilia: crocodiles, alligators, caimans and gharials
• Neornithes: modern birds
• Mammalia: mammals

Evolution of birds

From Wikipedia, the free encyclopedia

Archaeopteryx at Paläontologisches Museum München

The evolution of birds is thought to have begun in the Jurassic Period, with the earliest birds derived from a clade of theropoda dinosaurs named Paraves. Birds are categorized as a biological class, Aves. The earliest known is Archaeopteryx lithographica, from the Late Jurassic period, though Archaeopteryx is not commonly considered to have been a true bird. Modern phylogenies place birds in the dinosaur clade Theropoda. According to the current consensus, Aves and a sister group, the order Crocodilia, together are the sole living members of an unranked "reptile" clade, the Archosauria.

Phylogenetically, Aves is usually defined as all descendants of the most recent common ancestor of a specific modern bird species (such as the House Sparrow, Passer domesticus), and either Archaeopteryx,^[1] or some prehistoric species closer to Neornithes (to avoid the problems caused by the unclear relationships of Archaeopteryx to other theropods).^[2] If the latter classification is used then the larger group is termed Avialae. Currently, the relationship between dinosaurs, Archaeopteryx, and modern birds is still under debate.

On 31 July 2014, scientists reported details of the evolution of birds from theropod dinosaurs.^[3][4]

Origins

The mounted skeleton of a Velociraptor, showing the very bird-like quality of the smaller theropod dinosaurs

There is significant evidence that birds emerged within theropod dinosaurs, specifically, that birds are members of Maniraptora, a group of theropods which includes dromaeosaurs and oviraptorids, among others.^[5] As more non-avian theropods that are closely related to birds are discovered, the formerly clear distinction between non-birds and birds becomes less so. This was noted already in the 19th century, with Thomas Huxley writing:

We have had to stretch the definition of the class of birds so as to include birds with teeth and birds with paw-like fore limbs and long tails. There is no evidence that Compsognathus possessed feathers; but, if it did, it would be hard indeed to say whether it should be called a reptilian bird or an avian reptile.^[6]

Discoveries in northeast China (Liaoning Province) demonstrate that many small theropod dinosaurs did indeed have feathers, among them the compsognathid Sinosauropteryx and the microraptorian dromaeosaurid Sinornithosaurus. This has contributed to this ambiguity of where to draw the line between birds and reptiles.^[7] Cryptovolans, a dromaeosaurid found in 2002 (which may be a junior synonym of Microraptor) was capable of powered flight, possessed a sternal keel and had ribs with uncinate processes. Cryptovolans seems to make a better "bird" than Archaeopteryx which lacks some of these modern bird features. Because of this, some paleontologists have suggested that dromaeosaurs are actually basal birds whose larger members are secondarily flightless, i.e. that dromaeosaurs evolved from birds and not the other way around. Evidence for this theory is currently inconclusive, but digs continue to unearth fossils (especially in China) of feathered dromaeosaurs. At any rate, it is fairly certain that flight utilizing feathered wings existed in the mid-Jurassic theropods. The Cretaceous unenlagiine Rahonavis also possesses features suggesting it was at least partially capable of powered flight.

Although ornithischian (bird-hipped) dinosaurs share the same hip structure as birds, birds actually originated from the saurischian (lizard-hipped) dinosaurs if the dinosaurian origin theory is correct. They thus arrived at their hip structure condition independently. In fact, a bird-like hip structure also developed a third time among a peculiar group of theropods, the Therizinosauridae.

An alternate theory to the dinosaurian origin of birds, espoused by a few scientists, notably Larry Martin and Alan Feduccia, states that birds (including maniraptoran "dinosaurs") evolved from early archosaurs like Longisquama.^[8] This theory is contested by most other paleontologists and experts in feather development and evolution.^[9]

Mesozoic birds

Reconstruction of Iberomesornis romerali, a toothed enantiornithe

The basal bird Archaeopteryx, from the Jurassic, is well known as one of the first "missing links" to be found in support of evolution in the late 19th century. Though it is not considered a direct ancestor of modern birds, it gives a fair representation of how flight evolved and how the very first bird might have looked. It may be predated by Protoavis texensis, though the fragmentary nature of this fossil leaves it open to considerable doubt whether this was a bird ancestor. The skeleton of all early bird candidates is basically that of a small theropod dinosaur with long, clawed hands, though the exquisite preservation of the Solnhofen Plattenkalk shows Archaeopteryx was covered in feathers and had wings.^[6] While Archaeopteryx and its relatives may not have been very good fliers, they would at least have been competent gliders, setting the stage for the evolution of life on the wing.

The evolutionary trend among birds has been the reduction of anatomical elements to save weight. The first element to disappear was the bony tail, being reduced to a pygostyle and the tail function taken over by feathers. Confuciusornis is an example of their trend. While keeping the clawed fingers, perhaps for climbing, it had a pygostyle tail, though longer than in modern birds. A large group of birds, the Enantiornithes, evolved into ecological niches similar to those of modern birds and flourished throughout the Mesozoic. Though their wings resembled those of many modern bird groups, they retained the clawed wings and a snout with teeth rather than a beak in most forms. The loss of a long tail was followed by a rapid evolution of their legs which evolved to become highly versatile and adaptable tools that opened up new ecological niches.^[10]

The Cretaceous saw the rise of more modern birds with a more rigid ribcage with a carina and shoulders able to allow for a powerful upstroke, essential to sustained powered flight. Another improvement was the appearance of an alula, used to achieve better control of landing or flight at low speeds. They also had a more derived pygostyle, with a ploughshare-shaped end. An early example is Yanornis. Many were coastal birds, strikingly resembling modern shorebirds, like Ichthyornis, or ducks, like Gansus. Some evolved as swimming hunters, like the Hesperornithiformes – a group of flightless divers resembling grebes and loons. While modern in most respects, most of these birds retained typical reptilian-like teeth and sharp claws on the manus.

The modern toothless birds evolved from the toothed forefathers in the Cretaceous.^[11] While the earlier primitive birds, particularly the Enantiornithes, continued to thrive and diversify alongside the pterosaurs, all but a few groups of the toothless Neornithes were cut short at the Chicxulub impact. The surviving lineages of birds were the comparatively primitive Paleognathae (ostrich and its allies), the aquatic duck lineage, the terrestrial fowl, and the highly volant Neoaves.

Adaptive radiation of modern birds

Haast's eagle and New Zealand moa, the eagle is a Neognath, the moas are Paleognaths.

Modern birds are classified in Neornithes, which are now known to have evolved into some basic lineages by the end of the Cretaceous (see Vegavis). The Neornithes are split into the paleognaths and neognaths.

The paleognaths include the tinamous (found only in Central and South America) and the ratites, which nowadays are found almost exclusively on the Southern Hemisphere. The ratites are large flightless birds, and include ostriches, rheas, cassowaries, kiwis and emus. A few scientists propose that the ratites represent an artificial grouping of birds which have independently lost the ability to fly in a number of unrelated lineages.^[12] In any case, the available data regarding their evolution is still very confusing, partly because there are no uncontroversial fossils from the Mesozoic.

The basal divergence from the remaining Neognathes was that of the Galloanserae, the superorder containing the Anseriformes (ducks, geese and swans), and the Galliformes (chickens, turkeys, pheasants, and their allies). The presence of basal anseriform fossils in the Mesozoic and likely some galliform fossils implies the presence of paleognaths at the same time, in spite of the absence of fossil evidence.

The dates for the splits are a matter of considerable debate amongst scientists. It is agreed that the Neornithes evolved in the Cretaceous and that the split between the Galloanserae and the other neognaths - the Neoaves - occurred before the Cretaceous–Paleogene extinction event, but there are different opinions about whether the radiation of the remaining neognaths occurred before or after the extinction of the other dinosaurs.^[13] This disagreement is in part caused by a divergence in the evidence, with molecular dating suggesting a Cretaceous radiation, a small and equivocal neoavian fossil record from Cretaceous, and most living families turning up during the Paleogene. Attempts made to reconcile the molecular and fossil evidence have proved controversial.^[13][14]

On the other hand, two factors must be considered: First, molecular clocks cannot be considered reliable in the absence of robust fossil calibration, whereas the fossil record is naturally incomplete. Second, in reconstructed phylogenetic trees, the time and pattern of lineage separation corresponds to the evolution of the characters (such as DNA sequences, morphological traits etc.) studied, not to the actual evolutionary pattern of the lineages; these ideally should not differ by much, but may well do so in practice.

Considering this, it is easy to see that fossil data, compared to molecular data, tends to be more accurate in general, but also to underestimate divergence times: morphological traits, being the product of entire developmental genetics networks, usually only start to diverge some time after a lineage split would become apparent in DNA sequence comparison - especially if the sequences used contain many silent mutations.

Classification of modern species

The diversity of modern birds

The phylogenetic classification of birds is a contentious issue. Sibley & Ahlquist's Phylogeny and Classification of Birds (1990) is a landmark work on the classification of birds (although frequently debated and constantly revised). A preponderance of evidence suggests that most modern bird orders constitute good clades. However, scientists are not in agreement as to the precise relationships between the main clades. Evidence from modern bird anatomy, fossils and DNA have all been brought to bear on the problem but no strong consensus has emerged. As of the mid-2000s, new fossil and molecular data provide an increasingly clear picture of the evolution of modern bird orders, and their relationships. For example, the Charadriiformes seem to consititute an ancient and distinct lineage, while the Mirandornithes and Cypselomorphae are supported by a wealth of anatomical and molecular evidence. The understanding of the interrelationships of lower level taxa also continues to increase, particularly in the massively diverse perching bird group Passeriformes.
Bird classification and phylogenetic analysis is still under debate and requires more research. A recent 2008 study published in Science examined DNA sequences from 169 species of birds that represented all of the major extant groups. The findings may necessitate a wholesale restructuring of the avian phylogenetic tree. The findings also supported unestablished relationships between orders and confirmed disputes over particular groupings.^[15]

Current evolutionary trends in birds

Evolution generally occurs at a scale far too slow to be witnessed by humans. However, bird species are currently going extinct at a far greater rate than any possible speciation or other generation of new species. The disappearance of a population, subspecies, or species represents the permanent loss of a range of genes.

Another concern with evolutionary implications is a suspected increase in hybridization. This may arise from human alteration of habitats enabling related allopatric species to overlap. Forest fragmentation can create extensive open areas, connecting previously isolated patches of open habitat. Populations that were isolated for sufficient time to diverge significantly, but not sufficient to be incapable of producing fertile offspring may now be interbreeding so broadly that the integrity of the original species may be compromised. For example, the many hybrid hummingbirds found in northwest South America may represent a threat to the conservation of the distinct species involved.^[16]

Several species of birds have been bred in captivity to create variations on wild species. In some birds this is limited to color variations, while others are bred for larger egg or meat production, for flightlessness or other characteristics.

Evolutionary theories

• Lack's principle
• Survival of the fittest

Darwin's Dangerous Idea

From Wikipedia, the free encyclopedia

Darwin's Dangerous Idea

Jacket painting by Henri Rousseau (image)
Author	Daniel C. Dennett
Subject	Evolution, ethics
Published	1995 (Simon & Schuster)
Media type	Print
Pages	586
ISBN	0-684-82471-X
OCLC	34925327
Preceded by	Consciousness Explained
Followed by	Kinds of Minds: Toward an Understanding of Consciousness

Darwin's Dangerous Idea: Evolution and the Meanings of Life is a 1995 book by Daniel Dennett, which looks at some of the repercussions of Darwinian theory. The crux of the argument is that, whether or not Darwin's theories are overturned, there is no going back from the dangerous idea that design (purpose or what something is for) might not need a designer. Dennett makes this case on the basis that natural selection is a blind process, which is nevertheless sufficiently powerful to explain the evolution of life. Darwin's discovery was that the generation of life worked algorithmically, that processes behind it work in such a way that given these processes the results that they tend toward must be so.

Dennett says, for example, that by claiming that minds cannot be reduced to purely algorithmic processes, many of his eminent contemporaries are claiming that miracles can occur. These assertions have generated a great deal of debate and discussion in the general public. The book was a finalist for the 1995 National Book Award in non-fiction^[1] and the 1996 Pulitzer Prize for General Non-Fiction.^[2]

Background

Dennett's previous book was Consciousness Explained (1991). Dennett noted discomfort with Darwinism among not only lay people but also even academics and decided it was time to write a book dealing with the subject.^[3] Darwin's Dangerous Idea is not meant to be a work of science, but rather an interdisciplinary book; Dennett admits that he does not understand all of the scientific details himself. He goes into a moderate level of detail, but leaves it for the reader to go into greater depth if desired, providing references to this end.

In writing the book, Dennett wanted to "get thinkers in other disciplines to take evolutionary theory seriously, to show them how they have been underestimating it, and to show them why they have been listening to the wrong sirens." To do this he tells a story; one that is mainly original but includes some material from his previous work.

Dennett taught an undergraduate seminar at Tufts University on Darwin and philosophy, which included most of the ideas in the book. He also had the help of fellow staff and other academics, some of whom read drafts of the book.^[4] It is dedicated to W. V. O. Quine, "teacher and friend".^[5]

Synopsis

Part I: Starting in the Middle

"Starting in the Middle", Part I of Darwin's Dangerous Idea, gets its name from a quote by Willard Van Orman Quine: "Analyze theory-building how we will, we all must start in the middle. Our conceptual firsts are middle-sized, middle-distance objects, and our introduction to them and to everything comes midway in the cultural evolution of the race."

The first chapter "Tell Me Why" is named after a song.

Tell me why the stars do shine,
Tell me why the ivy twines,
Tell me why the sky's so blue.
Then I will tell you just why I love you.
Because God made the stars to shine,
Because God made the ivy twine,
Because God made the sky so blue.
Because God made you, that's why I love you.

Before Charles Darwin, and still today, a majority of people see God as the ultimate cause of all design, or the ultimate answer to 'why?' questions. John Locke argued for the primacy of mind before matter,^[6] and David Hume, while exposing problems with Locke's view,^[7] could not see any alternative.

Darwin's Dangerous Idea makes extensive use of cranes as an analogy.

Darwin provided just such an alternative: evolution.^[8] Besides providing evidence of common descent, he introduced a mechanism to explain it: natural selection. According to Dennett, natural selection is a mindless, mechanical and algorithmic process—Darwin's dangerous idea. The third chapter introduces the concept of "skyhooks" and "cranes" (see below). He suggests that resistance to Darwinism is based on a desire for skyhooks, which do not really exist. According to Dennett, good reductionists explain apparent design without skyhooks; greedy reductionists try to explain it without cranes.

Chapter 4 looks at the tree of life, such as how it can be visualized and some crucial events in life's history. The next chapter concerns the possible and the actual, using the 'Library of Mendel' (the space of all logically possible genomes) as a conceptual aid.

In the last chapter of part I, Dennett treats human artifacts and culture as a branch of a unified Design Space. Descent or homology can be detected by shared design features that would be unlikely to appear independently. However, there are also "Forced Moves" or "Good Tricks" that will be discovered repeatedly, either by natural selection (see convergent evolution) or human investigation.

Part II: Darwinian Thinking in Biology

Tree diagram in Origin

The first chapter of part II, "Darwinian Thinking in Biology", asserts that life originated without any skyhooks, and the orderly world we know is the result of a blind and undirected shuffle through chaos.

The eighth chapter's message is conveyed by its title, "Biology is Engineering"; biology is the study of design, function, construction and operation. However, there are some important differences between biology and engineering. Related to the engineering concept of optimization, the next chapter deals with adaptationism, which Dennett endorses, calling Gould and Lewontin's "refutation" of it^[9] an illusion. Dennett thinks adaptationism is, in fact, the best way of uncovering constraints.

The tenth chapter, entitled "Bully for Brontosaurus", is an extended critique of Stephen Jay Gould, who Dennett feels has created a distorted view of evolution with his popular writings; his "self-styled revolutions" against adaptationism, gradualism and other orthodox Darwinism all being false alarms.
The final chapter of part II dismisses directed mutation, the inheritance of acquired traits and Teilhard's "Omega Point", and insists that other controversies and hypotheses (like the unit of selection and Panspermia) have no dire consequences for orthodox Darwinism.

Part III: Mind, Meaning, Mathematics and Morality

The frontispiece to Thomas Hobbes' Leviathan, which appears at the beginning of chapter 16 "On the Origin of Morality".

"Mind, Meaning, Mathematics and Morality" is the name of Part III, which begins with a quote from Nietzsche.^[10] Chapter 12, "The Cranes of Culture", discusses cultural evolution. It asserts that the meme has a role to play in our understanding of culture, and that it allows humans, alone among animals, to "transcend" our selfish genes.^[11] "Losing Our Minds to Darwin" follows, a chapter about the evolution of brains, minds and language. Dennett criticizes Noam Chomsky's perceived resistance to the evolution of language, its modeling by artificial intelligence, and reverse engineering.

The evolution of meaning is then discussed, and Dennett uses a series of thought experiments to persuade the reader that meaning is the product of meaningless, algorithmic processes.

Von Kempelen's chess automaton, discussed in chapter 15.

Chapter 15 asserts that Gödel's Theorem does not make certain sorts of artificial intelligence impossible. Dennett extends his criticism to Roger Penrose.^[12] The subject then moves on to the origin and evolution of morality, beginning with Thomas Hobbes^[13] (who Dennett calls "the first sociobiologist") and Friedrich Nietzsche.^[14] He concludes that only an evolutionary analysis of ethics makes sense, though he cautions against some varieties of 'greedy ethical reductionism'. Before moving to the next chapter, he discusses some sociobiology controversies.

The penultimate chapter, entitled "Redesigning Morality", begins by asking if ethics can be 'naturalized'. Dennett does not believe there is much hope of discovering an algorithm for doing the right thing, but expresses optimism in our ability to design and redesign our approach to moral problems. In "The Future of an Idea", the book's last chapter, Dennett praises biodiversity, including cultural diversity. In closing, he uses Beauty and the Beast as an analogy; although Darwin's idea may seem dangerous, it is actually quite beautiful.

Central concepts

Design Space

Dennett believes there is little or no principled difference between the naturally generated products of evolution and the man-made artifacts of human creativity and culture. For this reason he indicates deliberately that the complex fruits of the tree of life are in a very meaningful sense "designed"—even though he does not believe evolution was guided by a higher intelligence.

Dennett supports using the notion of memes to better understand cultural evolution. He also believes even human creativity might operate by the Darwinian mechanism.^[15] This leads him to propose that the "space" describing biological "design" is connected with the space describing human culture and technology.

A precise mathematical definition of Design Space is not given in Darwin's Dangerous Idea. Dennett acknowledges this and admits he is offering a philosophical idea rather than a scientific formulation.^[16]

Natural selection as an algorithm

Dennett describes natural selection as a substrate-neutral, mindless algorithm for moving through Design Space.

Universal acid

Dennett writes about the fantasy of a "universal acid" as a liquid that is so corrosive that it would eat through anything that it came into contact with, even a potential container. Such a powerful substance would transform everything it was applied to; leaving something very different in its wake. This is where Dennett draws parallels from the “universal acid” to Darwin’s idea:

“it eats through just about every traditional concept, and leaves in its wake a revolutionized world-view, with most of the old landmarks still recognizable, but transformed in fundamental ways.”

While there are people who would like to see Darwin’s idea contained within the field of biology, Dennett asserts that this dangerous idea inevitably “leaks” out to transform other fields as well.

Skyhooks and cranes

Dennett uses the term "skyhook" to describe a source of design complexity that does not build on lower, simpler layers—in simple terms, a miracle.

In philosophical arguments concerning the reducibility (or otherwise) of the human mind, Dennett's concept pokes fun at the idea of intelligent design emanating from on high, either originating from one or more gods, or providing its own grounds in an absurd, Munchausen-like bootstrapping manner.

Dennett also accuses various competing neo-Darwinian ideas of making use of such supposedly unscientific skyhooks in explaining evolution, coming down particularly hard on the ideas of Stephen Jay Gould.

Dennett contrasts theories of complexity that require such miracles with those based on "cranes", structures that permit the construction of entities of greater complexity but are themselves founded solidly "on the ground" of physical science.

Reception

In The New York Review of Books, John Maynard Smith praised Darwin's Dangerous Idea:

It is therefore a pleasure to meet a philosopher who understands what Darwinism is about, and approves of it. Dennett goes well beyond biology. He sees Darwinism as a corrosive acid, capable of dissolving our earlier belief and forcing a reconsideration of much of sociology and philosophy. Although modestly written, this is not a modest book. Dennett argues that, if we understand Darwin's dangerous idea, we are forced to reject or modify much of our current intellectual baggage...^[17]

Writing in the same publication, Stephen Jay Gould criticised Darwin's Dangerous Idea for being an "influential but misguided ultra-Darwinian manifesto":

Daniel Dennett devotes the longest chapter in Darwin's Dangerous Idea to an excoriating caricature of my ideas, all in order to bolster his defense of Darwinian fundamentalism. If an argued case can be discerned at all amid the slurs and sneers, it would have to be described as an effort to claim that I have, thanks to some literary skill, tried to raise a few piddling, insignificant, and basically conventional ideas to "revolutionary" status, challenging what he takes to be the true Darwinian scripture. Since Dennett shows so little understanding of evolutionary theory beyond natural selection, his critique of my work amounts to little more than sniping at false targets of his own construction. He never deals with my ideas as such, but proceeds by hint, innuendo, false attribution, and error.^[18]

Gould was also a harsh critic of Dennett's idea of the "universal acid" of natural selection and of his subscription to the idea of memetics; Dennett responded, and the exchange between Dennett, Gould, and Robert Wright was printed in the New York Review of Books.^[19]

Biologist H. Allen Orr wrote a critical review emphasizing similar points in the Boston Review.^[20]

The Selfish Gene

From Wikipedia, the free encyclopedia

Original cover, with details from the painting The Expectant Valley by zoologist Desmond Morris.
Author	Richard Dawkins
Subject	Evolutionary biology
Publisher	Oxford University Press
Publication date	1976 Second edition in 1989 Third edition in 2006
Pages	224
ISBN	ISBN 0-19-857519-X
OCLC	2681149
Followed by	The Extended Phenotype

The Selfish Gene is a book on evolution by Richard Dawkins, published in 1976. It builds upon the principal theory of George C. Williams's first book Adaptation and Natural Selection. Dawkins used the term "selfish gene" as a way of expressing the gene-centred view of evolution as opposed to the views focused on the organism and the group, popularising ideas developed during the 1960s by W. D. Hamilton and others. From the gene-centred view follows that the more two individuals are genetically related, the more sense (at the level of the genes) it makes for them to behave selflessly with each other. Therefore the concept is especially good at explaining many forms of altruism. This should not be confused with misuse of the term along the lines of a selfishness gene.

An organism is expected to evolve to maximise its inclusive fitness—the number of copies of its genes passed on globally (rather than by a particular individual). As a result, populations will tend towards an evolutionarily stable strategy. The book also coins the term meme for a unit of human cultural evolution analogous to the gene, suggesting that such "selfish" replication may also model human culture, in a different sense. Memetics has become the subject of many studies since the publication of the book.

In the foreword to the book's 30th-anniversary edition, Dawkins said he "can readily see that [the book's title] might give an inadequate impression of its contents" and in retrospect thinks he should have taken Tom Maschler's advice and called the book The Immortal Gene.^[1]

"Selfish" genes

In describing genes as being "selfish", the author does not intend (as he states unequivocally) to imply that they are driven by any motives or will, but merely that their effects can be metaphorically and pedagogically described as if they were. The contention is that the genes that get passed on are the ones whose evolutionary consequences serve their own implicit interests (to continue being replicated), not necessarily those of the organism. Bringing the level of evolutionary dynamics down to the single gene, or complementary genes which work well together in a given type of organism, Dawkins categorically rejects the school of thought which tells that evolution operates on the level of social group.

This view is said to explain altruism at the individual level in nature, especially in kinship relationships: when an individual sacrifices its own life to protect the lives of kin, it is acting in the interest of its own genes. Some people find this metaphor entirely clear, while others find it confusing, misleading or simply redundant to ascribe mental attributes to something that is mindless.

For example, Andrew Brown has written:

"Selfish", when applied to genes, doesn't mean "selfish" at all. It means, instead, an extremely important quality for which there is no good word in the English language: "the quality of being copied by a Darwinian selection process." This is a complicated mouthful. There ought to be a better, shorter word—but "selfish" isn't it.^[2]

Donald Symons also finds it inappropriate to use everyday language in conveying scientific meaning in general and particularly for the present instance:

In summary, the rhetoric of The Selfish Gene exactly reverses the real situation: through metaphor genes are endowed with properties only sentient beings can possess, such as selfishness, while sentient beings are stripped of these properties and called machines (robots).^[3]

Genes and selection

Dawkins proposes the idea of the "replicator,"^[4] the initial molecule which first managed to reproduce itself and thus gained an advantage over other molecules within the primordial soup.^[5] As replicating molecules became more complex, Dawkins postulates, the replicators became the genes within organisms, with each organism's body serving the purpose of a 'survival machine' for its genes.
Dawkins writes that gene combinations which help an organism to survive and reproduce tend to also improve the gene's own chances of being passed on and, as a result, frequently "successful" genes will also be beneficial to the organism. An example of this might be a gene that protects the organism against a disease, which helps the gene spread and also helps the organism.

Genes can reproduce at the expense of the organism

There are other times when the implicit interests of the vehicle and replicator are in conflict, such as the genes behind certain male spiders' instinctive mating behaviour, which increase the organism's inclusive fitness by allowing it to reproduce, but shorten its life by exposing it to the risk of being eaten by the cannibalistic female. Another good example is the existence of segregation distortion genes that are detrimental to their host but nonetheless propagate themselves at its expense. Likewise, the existence of junk DNA that provides no benefit to its host, once a puzzle, can be more easily explained.^[6]

Power struggles are rare

These examples might suggest that there is a power-struggle between genes and their host. In fact, the claim is that there isn't much of a struggle because the genes usually win without a fight. Only if the organism becomes intelligent enough to understand its own interests, as distinct from those of its genes, can there be true conflict.

An example of this conflict might be a person using birth control to prevent fertilisation, thereby inhibiting the replication of his or her genes.

But that may not be a conflict of the 'self-interest' of the organism with his or her genes, since a person using birth control may also be enhancing the survival chances of their genes by limiting family size to conform with available resources, thus avoiding extinction as predicted under the Malthusian model of population growth.

Many phenomena explained

When examined from the standpoint of gene selection, many biological phenomena that, in prior models, were difficult to explain become easier to understand. In particular, phenomena such as kin selection and eusociality, where organisms act altruistically, against their individual interests (in the sense of health, safety or personal reproduction) to help related organisms reproduce, can be explained as gene sets "helping" copies of themselves (or sequences with the same phenotypic effect) in other bodies to replicate. Interestingly, the "selfish" actions of genes lead to unselfish actions by organisms.

Prior to the 1960s, it was common for such behaviour to be explained in terms of group selection, where the benefits to the organism or even population were supposed to account for the popularity of the genes responsible for the tendency towards that behaviour. This was shown not to be an evolutionarily stable strategy, in that it would only take a single individual with a tendency towards more selfish behaviour to undermine a population otherwise filled only with the gene for altruism towards non-kin.

Reception

The book was extremely popular when first published, caused "a silent and almost immediate revolution in biology",^[7] and continues to be widely read. It has sold over a million copies, and has been translated into more than 25 languages.^[8]

Proponents argue that the central point, that the gene is the unit of selection, usefully completes and extends the explanation of evolution given by Charles Darwin before the basic mechanisms of genetics were understood. Critics argue that it oversimplifies the relationship between genes and the organism. Mathematical biologists' initial relationship with the ideas in the book was, according to Alan Grafen, "at best difficult" due to what Grafen postulates is a reliance solely on Mendelian genetics by these biologists.^[9]

In 1976, Arthur Cain, one of Dawkins's tutors at Oxford in the 1960s, called it a "young man's book" (which Dawkins points out was a deliberate quote of a commentator on A.J. Ayer's Language, Truth, and Logic); Dawkins later noted he had been "flattered by the comparison, [but] knew that Ayer had recanted much of his first book and [he] could hardly miss Cain's pointed implication that [he] should, in the fullness of time, do the same."^[1]

Other types of selection suggested

Most modern evolutionary biologists accept that the idea is consistent with many processes in evolution. However, the view that selection on other levels, such as organisms and populations, seldom opposes selection on genes is more controversial. While naïve versions of group selectionism have been disproved, more sophisticated formulations make accurate predictions in some cases while positing selection at higher levels.^[10] Nevertheless, the explanatory gains of using sophisticated formulations of group selectionism as opposed to Dawkins's gene-centred selectionism are still under dispute. Both sides agree that very favourable genes are likely to prosper and replicate if they arise and both sides agree that living in groups can be an advantage to the group members. The conflict arises not so much over disputes on hard facts but over what is the best way of viewing evolutionary selection in animals.

In "The Social Conquest of Earth," E. O. Wilson contends that kin selection as described in "The Selfish Gene" is a largely ineffective model of social evolution. Chapter 18 of "The Social Conquest of Earth" describes the deficiencies of kin selection and outlines group selection, which Wilson argues is a more realistic model of social evolution. He writes, "...unwarranted faith in the central role of kinship in social evolution has led to the reversal of the usual order in which biological research is conducted. The proven best way in evolutionary biology, as in most of science, is to define a problem arising during empirical research, then select or devise the theory that is needed to solve it. Almost all research in inclusive-fitness theory [such as in "The Selfish Gene"] has been the opposite: hypothesize the key roles of kinship and kin selection, then look for evidence to test that hypothesis."^[11]

Unit of selection or of evolution

Some biologists have criticised the idea for describing the gene as the unit of selection, but suggest describing the gene as the unit of evolution, on the grounds that selection is a "here and now" event of reproduction and survival, while evolution is the long-term trend of shifting allele frequencies.^[12]

Stephen Jay Gould also took issue with the gene as the unit of selection, arguing that genes are not directly 'visible' to natural selection. Rather, the unit of selection is the phenotype, not the genotype, because it is phenotypes which interact with the environment at the natural selection interface.^[13] As Kim Sterelny^[14] summarises Gould's view, "Gene differences do not cause evolutionary changes in populations, they register those changes". This is also Niles Eldredge's view. Eldredge^[15] notes that in Dawkins' book A Devil's Chaplain, which was published just before Eldredge's book, "Richard Dawkins comments on what he sees as the main difference between his position and that of the late Stephen Jay Gould. He concludes that it is his own vision that genes play a causal role in evolution", while Gould (and Eldredge) "sees genes as passive recorders of what worked better than what".

Moral arguments

Another criticism of the book, made by the philosopher Mary Midgley in her book Evolution as a Religion, is that it discusses philosophical and moral questions that go beyond the biological arguments that Dawkins makes. For instance, humanity finally gaining power over the "selfish replicators" is a major theme at the end of the book. This view is criticised by primatologist Frans de Waal, who refers to it as the "veneer theory". Dawkins has pointed out that he is only describing how things are under evolution, not endorsing them as morally good.^[16][17] Mary Midgley's essential argument is that what separates humans from other aspects of nature, is that humans have the ability to reconstruct nature through the tools of what humans call "society" and "culture". She argues that Richard Dawkins account of The Selfish Gene is in fact a moral and ideological justification for nature's behavior of selfishness to be adopted by modern human societies. She argues further that humanly organized social and political institutions and structures in society have been created to counteract the selfish tendencies of nature and that Dawkins's conception of selfishness as the engine of genetic behavior will have disastrous consequences to future human society.

Editions

The Selfish Gene was first published in 1976^[18] in eleven chapters with a preface by the author and a foreword by Robert Trivers. A second edition was published in 1989. This edition added two extra chapters, and substantial endnotes to the preceding chapters, reflecting new findings and thoughts. It also added a second preface by the author, but the original foreword by Trivers was dropped.

30th anniversary

In 2006, a 30th anniversary edition^[8] was published which reinstated the Trivers foreword and contained a new introduction by the author (alongside the previous two prefaces), with some selected extracts from reviews at the back. It was accompanied by a festschrift entitled Richard Dawkins: How a Scientist Changed the Way We Think. In March 2006, a special event entitled The Selfish Gene: Thirty Years On was held at the London School of Economics. The event was organised by Helena Cronin, and chaired by Melvyn Bragg. In March 2011, Audible Inc published an audiobook edition narrated by Richard Dawkins and Lalla Ward.