See if you can see a nova and an asteroid in these star pictures taken in April and May.
It is easier to see them by using the bats' method! Nova—easy; asteroid—bottom right.
Contents Updated: Tuesday, August 24, 1999
"Heaven held his hand, the likeness must be true."
The Church justified its resistance to the idea of evolution in the 18th and 19th Centuries by quoting Ecclesiastes 3:14: I know that everything God does will last for ever. Had it chosen verse 3:15 instead it would have supported Cuvier and anticipated this book: Whatever happens or can happen has already happened before. God makes the same thing happen again and again. (Quotations from the Good News Bible.)
What is the evidence that God has made the same thing happen again and again. In evolution it is the way different species have evolved along paths that lead them to essentially the same outcome. But surely there is a law that evolution is always progressive, onwards and divergent, not regressive, backwards or convergent—Dollo's law says evolution is irreversible.
But many people take it too literally. Except in a very strict sense, it is not true. A fish, hundreds of millions of years ago, left the water ultimately to evolve into terrestrial vertebrates, but land vertebrates have often taken to the water since, and have assumed the shape and lifestyles of fish. The strict sense is that they have not become fish again.
Dollo's rule can be thought of in terms of a motion picture of someone walking up a street. The man could perfectly well turn round at the top of the street and begin to walk down again and there would be nothing unusual in that. If, however, he got to the end of the street only to retrace his steps precisely but walking stiffly backwards, finishing exactly where he started and in the same posture, then we should consider it unusual. We might suspect that the motion picture had been played backward rather than that it had accurately portrayed a real event.
Dollo's rule is saying the same thing about evolution. It is vanishingly unlikely that any evolutionary path could be traced twice in either direction. But it is far from impossible, indeed it is very likely, that different organisms subject to the same evolutionary constraints will evolve marked similarities. An orange jelly made in the same mould as a strawberry blancmange will have the same shape because it has been subject to the same constraints of shape. But it remains a jelly and not a blancmange. The dolphin is not a fish but it looks very like a fish because it has been subject to the evolutionary constraints imposed by the dense aqueous medium that determined the shape of fish. This is convergent evolution.
Richard Dawkins explains that if a design principle is good enough to be used once in evolution it is good enough to be used twice—though not in exactly the same way.
Convergent evolution explains the way similar features are found in different creatures though no ancestor had those common features. Parallel evolution describes the case when both species or genera have a common ancestry but where both have independently evolved common solutions in response to the same or similar evolutionary problems. Evolutionary convergence is the more extreme in the sense that the two species or genera have no immediate common ancestor. In other words, they diverged long before either type of creature evolved and yet converged towards a common set of features in response to the same environmental constraints. Examples are powered flight which has evolved separately several times in insects, pterosaurs, birds and bats; the camera eye of vertebrates and the cephalopods; echolocation in bats, birds and whales; use of electricity in some fish; convergence in placental and marsupial mammals; and even convergence in self-replicating molecules grown in test tubes.
Generally speaking uncomplicated features evolve repeatedly, but more complicated ones less often. Intelligence is such a feature. People find it incredible that the camera eye evolved once let alone twice in the cephalopods and the vertebrates . Octopus eyes are amazingly similar in construction to the eyes of vertebrates yet have evolved quite separately. An octopus's eyes arose from skin cells, a vertebrate's from brain cells. In one respect octopus eyes are superior. The nerves taking the signals from the photoreceptors in the eye to the brain are placed behind the receptors so they do not interfere with the incoming light. In the vertebrate eye the nerves are in front of the photoreceptors interfering with the incoming light's path. The differences are fundamental and confirm that the two forms could not possibly have come from any common ancestor.
Echolocation has evolved several times, in two types of bat, two types of bird, in cetaceans, in seals and, to lesser extents in rats, shrews and humans. (Dawkins asserts that echolocation, used unconsciously, explains how some blind people can see with their cheeks. They are hearing subtle changes in sounds allowing them to sense obstacles in their paths.) It is well known that bats and cetaceans use echo sounding but not so well known that birds do. Two types of birds have the ability and yet must have evolved it independently (they are unrelated and live half a world apart)—the cave swifts of the Orient and the oil birds of South America. Echolocation has evolved in both these types of bird because they nest in deep, dark caves. Their echolocation system is, however, relatively crude being based on audible sounds rather than ultrasound as used by our native bats.
Bats have quite amazingly sophisticated sonar systems. They make very loud noises, although we cannot hear them because our ears are not sensitive to the ultrasound they use, and they have large and highly sensitive ears to enable them to receive the very weak echoes that they get from the small insects they seek as food. This combination of loud noises and sensitive ears creates its own problem—equivalent to putting a sensitive microphone next to a firing cannon—the power of the sound is liable to damage the bats' ears. To overcome this difficulty they decouple the delicate bones in their ears when they make the noise. Evolution has provided them a means of switching off their ears when they issue their ultrasonic blasts.
The bats get even more information from frequency modulating (changing the pitch of) their sounds and from the Doppler effect (the change in pitch of the echo caused by the motion of the source relative to the receiver) enabling them to follow the movement of the moths they prey upon.
They even filter out the babble of the other bats in their vicinity (in large caverns it could be millions) by relating the picture they get each time their ears switch on to the previous one. This is analogous to the method used by astronomers searching for new features in photographs of the night sky. A photographic plate of part of the sky is compared with an identical plate taken previously. Comparing the plates side by side is tedious and prone to error because of the millions of stars on the plates. Instead each photograph is projected into the astronomer's visual field in quick succession. Any object present on one but absent on the other immediately reveals itself by winking—thus is found a nova. If something has moved it seems to be jumping backwards and forwards—thus a comet or asteroid is found. The background noise of millions of fixed stars remains static and unnoticed, effectively removed by a technique perfected in its auditory context by the poor bat. All this is done quite spontaneously and unconsciously by the bat just as we are not conscious of the way we get pictures of the world via our eyes. Dawkins believes the bat has a mental picture of its surroundings based on its auditory sense in every way as complete as ours based on our visual sense.
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See if you can see a nova and an asteroid in these star pictures taken in April and May. It is easier to see them by using the bats' method! Nova—easy; asteroid—bottom right. |
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Some bats, the flying foxes, lack this elaborate sonar equipment though they may have a cruder version of it. They are also different from insectivorous bats in other ways. They are much larger with wingspans up to five feet, they live in trees rather than belfries and, of course, they eat fruit not insects. These differences suggest they might have evolved the bat wing independently of their insectivorous, echo-sounding brethren—another example of evolutionary convergence.
The whale has an unusually large brain probably because, like the bat, it has its own three dimensional map of the world's oceans which it constantly updates from its own echolocation system. It also talks to its fellow creatures. Tests on captive dolphins show the sophistication of their sonar system. They can distinguish objects of the same size by their shapes—a triangle from a square, for example. And they can detect a sphere half the size of a golf ball at 70 yards by sonar alone. All cetaceans use echolocation but baleen whales like the blue whale probably evolved independently of the toothed whales like the dolphin, further cases of convergence.
Alternative location systems evolve when the normal visual system cannot work efficiently—bats flying at night, birds nesting in deep caves, whales in deep waters and fish living in muddy estuaries. Two groups of fish have evolved location systems based on electric fields, yet are quite unrelated species. One group lives in African waters, the other in South American. Both groups of electric fish use the same design principles for their location systems. Muscles along the fish's flanks generate a weak electromagnetic field which surrounds the fish. Detectors also along their flanks monitor its intensity. In open water the field is symmetrical and corresponding detectors to the left and to the right register the same value. If any object comes within range of the field however it upsets the symmetry and the fish becomes aware of it from slight differences in voltage to the left and to the right.
But this solution to the problem of seeing in murky water generates another. Any movement of the body of the fish would create an asymmetry in the field yielding information only about the fish's bodily movements—the fish has to be rigid. How then is it to swim? A long fin running the length of its rigid body evolved—by undulating the fin the fish could swim while keeping its body stiff. Proof that the two fish evolved quite separately is that the fin of the African fish is on its back while that of the South American version is on its underside. Furthermore both groups have some members that regenerate their fields in discontinuous pulses and some that use a sort of alternating current. Not only have the two groups independently evolved the same location system but members of each group have separately evolved the same two methods of replenishing their electromagnetic fields.
There are other types of electric fish altogether more fearsome—the strongly electric fish which electrocute their victims by issuing a powerful electrical discharge. The electric eel is one such fish and the electric ray another. They are unrelated and must have developed their electrical weaponry independently. Indeed they have evolved quite different solutions to their other evolutionary problems. The electric eel is not an eel (it is related to the carp) but has adopted the eel shape because of its narrow habitat among rocks on the sea bed. The electric ray is a cartilaginous fish related to the shark but adapted for feeding on flat sandy ocean floors, whence its flat shape.
The African mormyrid's electric field allows it to sense its surroundings in muddy water. Fascinatingly, it has evolved a greatly enlarged cerebellum, strangely reminiscent of the mammal's neocortex, apparently because of its novel sense system. These electric fish might even communicate via their electric fields. Electric fields are really electromagnetic fields—they always have magnetic fields associated with them—and when they oscillate they generate electromagnetic radiation in a spectrum ranging from high frequency cosmic rays to low frequency radio waves. In its muddy waters the fish is using the low frequency end of the spectrum. We have a fish that is communicating by radio. And, to help it do it, it is developing a brain. What price the intelligent mammal being succeeded by an intelligent, telepathic fish?
Of the many other examples of convergent features are mundane ones like webbed toes in a wide variety of vertebrates adapted to watery lives such as ducks, beavers, otters, frogs, crocodiles and quite often in humans. Then there are highly specialized ones like the filtering devices used by some creatures for sieving tiny shrimps and krill from water. Species as different as whales, sharks and flamingos all use the same technique today but there are also extinct examples. A long legged duck developed the filtering system 45 million years ago in North America and in Argentina a pterodactyl fed in the same way.
So much for the evolution of particular specialized features in different animals but the general features of animals, in common or similar environments, can also evolve convergently. Grass or scrub is poor in readily assimilated nutrients. To get sufficient nutrition the animals have to eat a lot, and they need a way of breaking down the tough food. To help their digestion, many, like the cow, have bacteria in their gut which ferment cellulose. To process large amounts of poor quality food and intimidate potential predators some grow to large sizes—like the rhino and elephant. To evade predators many become very nimble growing long legs to improve their speed and effectively lengthening their limbs by standing on their toes—like horses and gazelle.
The cracking apart of the Southern supercontinent of Gondwanaland starting about 130 million years ago provided us with a huge natural experiment in evolution. Ultimately South America, Africa and Australia were to separate and remain separate for countless aeons allowing evolution to take its course on three large landmasses independently. We can see the results of this experiment both in the flesh and in the fossil record. A pronounced example was that of the litopterns which evolved on the grasslands of South America. Faced with an identical environment to that of horses, the litopterns evolved legs so astonishingly like horses' legs that one expert was convinced that the horse actually evolved in South America. Grassland life is much the same the world over, and horses and litopterns independently evolved the same qualities to cope, observes Richard Dawkins.
South America also produced marsupial equivalents of rhinos, elephants and camels but most of the native South American species were out-competed when a link was formed with the North American continent allowing placental mammals to invade. And we have met these features in studying the dinosaurs. The problems faced by herbivorous dinosaurs then were similar to those posed to grassland herbivores today. Sauropods were dinosaur elephants; ceratopsians were dinosaur rhinos; hadrosaurs were dinosaur cattle or horses, as shown by their obvious features as well as by the ratios of the bones in their legs.
In Australia the grassland herbivores, the kangaroos and wallabies, quite different from horses or gazelles are uncannily similar in appearance to some dinosaurs. The marsupial sugar glider is very similar to the primitive coluga or the North American flying squirrel, while the marsupial mole looks remarkably similar to the placental mole. Australia and South America had no true native cats or dogs but parallels between some marsupial and placental carnivores are so close that it is not easy to tell them apart without a careful examination. There have been marsupial wolves in Australia and marsupial cats in South America. The marsupial wolf or thylacine only died out 50 years ago. Indeed there are repeated claims that breeding groups still live in remote parts of Tasmania and Australia. Newsreels from the 1930s show us exactly what it was like—virtually indistinguishable from a dog.
Some lowly creatures, the anteaters, were moulded in the Gondwanaland experiment. These creatures developed long noses with long sticky tongues and strong claws for breaking into ants' and termites' nests. Australia has a marsupial anteater with just these characteristics and also has the spiny anteater, a burrowing monotreme like the duck-billed platypus. Africa and Asia have placental anteaters like the aardvark and the pangolin. South America has placental anteaters suited to different habitats but which are more similar in external appearance to the marsupial editions in Australia than their closer cousins in Africa. A interesting common feature is their low metabolic rate which gives them an unusually low body temperature. We have noted this in the platypus, with its only semi-warm blood. But a low metabolic rate in this variant group of creatures, differing in many ways other than in their adaptation to a diet of ants, suggests that their diet provides some other evolutionary constraint. Convergent evolution responded with a low metabolic rate.
Within ants themselves there are many cases of parallel evolution but particularly striking is the parallel between the army ants of South America and the driver ants of tropical Africa. Both types periodically move camp lock, stock and barrel, swarming across the countryside carrying their queen or queens, larvae and eggs and sweeping all before them. The African variety moves in the largest numbers—about 20 million ants weighing some 45 pounds take to the road. In South America the colonies might have about a million individuals. These species of ants have evolved their peculiar habits quite separately on two different continents.
Convergent evolution is also exemplified by comparing the lifestyles and appearances of ants and termites. Although termites are totally unrelated to ants, being more the kin of cockroaches, their popular name, white ants, betrays the perceived similarities. Both ants and termites have evolved into many biological niches. Some species of both groups have become mushroom farmers taking decaying vegetation into their colonies to grow fungus which they then use as food. Each fungus has become so specialized that it can only survive with the help of the insects. Workers of both ants and termites are sterile to maintain the cohesion of the colony. By allowing only the queen and her consorts to procreate there can be no division of loyalty within the nest. If more evidence were needed that the habits of both groups do not stem from a common ancestor, it is that all ants' workers are sterile females whereas the termites' workers are both males and females.
We have seen that animals similar to modern ones evolved by convergence among extinct species too. Besides triceratops and the rhino there are examples like struthiomimus and the ostrich, ichthyosaurs and whales, and pterosaurs and birds or bats. Convergence at the time of the dinosaurs to solutions which are still suitable today confirms that many evolutionary problems were broadly the same, even though elements of the environment characteristic of today, such as grass, had not emerged in the Cretaceous period. Many of the solutions discovered by the mammals had already been discovered by the dinosaurs before them. Perhaps more than we have imagined!
Strong support for evolution—its fundamental machinery—came with the discovery by Watson and Crick of the double helical structure of the DNA (deoxyribonucleic acid) molecule. DNA is the blueprint of life. It contains in its sequences of nucleotides (nitrogen containing molecules able to form weak but specific bonds with each other) the factors or genes which influence variation and pass on characteristics from one generation to the next. The discovery that DNA was a double helix demonstrated in the most obvious way the basis for reproduction—the helix simply split down the center of its coil forming two separate halves. Each half then reformed the complete molecule from the surrounding nutrient molecules. Thus two complete molecules are formed from one and these can in turn divide and reform, multiplying the molecule as long as there are enough nutrient molecules remaining in the environment.
Astonishingly enough, experiments have been carried out showing that convergence can occur in the replication of molecules like these.
RNA (ribonucleic acid) is a fundamental molecule of life related in structure to DNA although simpler. It too is a replicating molecule. Sol Spiegelman extracted RNA from a virus and put it into a test tube with an enzyme (a molecule which helps a biochemical reaction to occur) and some nutrient molecules. The RNA replicated itself. A drop of the resulting liquid was extracted and put into another test tube containing only nutrient molecules and the enzyme. The drop contained some of the RNA molecules so these replicated as before. The same procedure was then repeated exhaustively. The RNA was analyzed at intervals to check its structure. It was found that the RNA evolved! Occasionally an error in the replication process occurred to yield a slightly different RNA molecule. If that proved not to be as good at replicating as the rest of them then it soon got diluted to such an extent that the drop extracted from the last test tube contained none of the mutant molecules and that species had died out. If however the mutant was better at reproducing itself in the test tube environment than its parent, the converse occurred. Before long the original RNA had become so diluted by the mutant's offspring that the drop taken from the last test tube contained none of the original and the mutant RNA had survived the extinction of the old species. After many trials a stable species seemed to evolve—the one best suited to the test tube world in which it lived!
Remarkably, the experimenters went on to provide the most spectacular example of convergence. They found that the same RNA molecule evolved from different RNA taken from different sources!
The story is yet more amazing. Manfred Eigen, a Nobel prizewinning chemist, carried out a complementary and even more unlikely experiment. He used no RNA as a seed molecule. He just used the enzyme and the nutrient molecular broth. After many trials he found that RNA built itself spontaneously from the nucleotides and other molecules present. It then evolved into essentially the same test tube species as before. Its size and general structure were the same but there were some minor variations. Just as you would expect of convergence from considerably different starting points.
So, with overpopulation, variation and inheritance transmitting the effects of the environment to successive generations of creatures, evolutionary change occurs. Because there are a large number of environmental factors, evolution effectively takes place in a multidimensional space in which the factors are the dimensions. C.H.Waddington introduced the concept of epigenetic landscapes moulded by physical laws which determine the possible paths of evolution just as streams flow down valleys in our three dimensional physical landscapes. There are only certain routes through the multidimensional space that are likely, small deviations from them being unstable and reverting to the original path.
A large deviation, though, could displace motion through the space to an alternative route. In physical terms, a small displacement of a spring will not alter the main course of the stream which it generates. It rapidly assumes the lowest point in the valley as it did before and then flows along the same riverbed. But a large displacement could take the outlet of the spring to the other side of the watershed and the course of the stream would then be quite different. It might flow out into a different ocean. Or it might flow down a valley which eventually runs into its previous course as a tributary, convergent evolution—though starting in different places the two rivers eventually flow together.
In this picture, features like echolocation or the camera eye, correspond to lakes in depressions in the landscape perhaps surrounded by high lands cut through by only a few narrow valleys. If evolution should lead into one of these valleys then it is very likely to lead to the lake which is the biological feature, camera eye or whatever. The landscape image offers a way of illustrating convergent evolution. It also offers a useful way of visualizing evolutionary paths as valleys, punctuated evolution as a succession of hanging valleys or tarns and stable features as lakes.
At least one such lake in the landscape must correspond to the development of intelligence. We, in our arrogance, believe we are the only species to have bathed in that particular lake, but evolutionary convergence warrants that other species will do so sooner or later. Perhaps they already have. One route there, is down the evolutionary rapids.