The Ancestor's Tale Part 11

And so we come to the axolotl, whose tale this is. It is a strange creature, native to a mountain lake in Mexico. It is of the essence of its tale that it is hard to say exactly what an axolotl is. Is it a salamander? Well, sort of. Its name is Ambystoma mexicanum Ambystoma mexicanum, and it is a close relative of the tiger salamander Ambystoma tigrinum Ambystoma tigrinum, which is found in the same area and more widely in North America as well. The tiger salamander, named for obvious reasons, is an ordinary salamander with a cylindrical tail and dry skin, which walks around on land. The axolotl is not at all like an adult salamander. It is like a larval salamander. In fact it is a larval salamander except for one thing. It never turns into a proper salamander and never leaves the water, but mates and reproduces while still looking and behaving like a juvenile. I nearly said the axolotl mates and reproduces while still being being a juvenile, but this might violate the definition of a juvenile. a juvenile, but this might violate the definition of a juvenile.

Definitions apart, there seems little doubt about what happened in the evolution of the modern axolotl. A recent ancestor was just an ordinary land salamander, probably very like the tiger salamander. It had a swimming larva, with external gills and a deep-keeled tail. At the end of larval life it would metamorphose, as expected, into a dry-land salamander. But then a remarkable evolutionary alteration occurred. Probably under the control of hormones, something s.h.i.+fted in the embryological calendar such that the s.e.x organs and s.e.xual behaviour matured earlier and earlier (or it may even have been a sudden change). This evolutionary regression continued until s.e.xual maturity was arriving in what was, in other respects, clearly the larval stage. And the adult stage was chopped off the end of the life history. Alternatively, you may prefer to see the change not as an acceleration of s.e.xual maturity relative to the rest of the body ('progenesis'), but as a slowing down of everything else, relative to s.e.xual maturity ('neoteny').6 Whether the means is neoteny or progenesis, the evolutionary consequence is called paedomorphosis. It is not difficult to see its plausibility. Slowing-down or speeding-up of developmental processes, relative to other developmental processes, happens all the time in evolution. It is called heterochrony and it presumably, if you think about it, must underlie many, if not all, evolutionary changes in anatomical shape. When reproductive development varies heterochronically relative to the rest of development, what may evolve is a new species that lacks the old adult stage. This seems to be what happened with the axolotl.

The axolotl is just an extreme among salamanders. Many species seem to be, at least to some extent, paedomorphic. And others do other heterochronically interesting things. The various species of salamander colloquially called 'newts' have an especially revealing life history.7 A newt first lives as a gilled larva in water. Then it emerges from the water and lives for two or three years as a kind of salamander on dry land, having lost its gills and the keel on its tail. But, unlike other salamanders, newts don't reproduce on land. Instead, they return to water, regaining some, but not all, of their larval characteristics. Unlike axolotls, newts don't have gills, and their need to come to the surface to breathe air is an important and compet.i.tive constraint on their underwater courts.h.i.+p. Unlike the larval gills, they do regain the keel of the larval tail, and in other respects they resemble a larva. But unlike a typical larva, their reproductive organs develop and they court and mate underwater. The dry land phase never reproduces and, in this sense, one might prefer not to call it the 'adult'. A newt first lives as a gilled larva in water. Then it emerges from the water and lives for two or three years as a kind of salamander on dry land, having lost its gills and the keel on its tail. But, unlike other salamanders, newts don't reproduce on land. Instead, they return to water, regaining some, but not all, of their larval characteristics. Unlike axolotls, newts don't have gills, and their need to come to the surface to breathe air is an important and compet.i.tive constraint on their underwater courts.h.i.+p. Unlike the larval gills, they do regain the keel of the larval tail, and in other respects they resemble a larva. But unlike a typical larva, their reproductive organs develop and they court and mate underwater. The dry land phase never reproduces and, in this sense, one might prefer not to call it the 'adult'.

You might ask why newts bother to turn into a dry land form at all, given that they are going to return to water to breed. Why not just do what axolotls do: start in water and stay in water? The answer seems to be that there is an advantage to breeding in temporary ponds which form in the wet season and are destined to dry up, and you have to be good on dry land in order to reach them (shades of Romer). Having reached a pond, how do you then reinvent your aquatic equipment? Heterochrony comes to the rescue: but heterochrony of a peculiar kind, involving going into reverse after the 'dry adult' has served its purpose of dispersing to a new, temporary pond.

Newts serve to emphasise the flexibility of heterochrony. They remind us of the point I made about how genes in one part of the life cycle 'know' how to make other parts. Genes in dry land salamanders know how to make an aquatic form because that is what they once were; and, to prove it, that is precisely what newts do.

Axolotls are, in one respect, more straightforward. They have lopped the dry land phase off the end of the ancestral life cycle. But the genes for making a dry land salamander still lurk in every axolotl. It has long been known, from the cla.s.sic work of Laufberger and of Julian Huxley mentioned in the Epilogue to Little Foot's Tale, that they can be activated by a suitable dose of hormones in the laboratory. Axolotls treated with thyroxine lose their gills and become dry land salamanders, just as their ancestors once did naturally. Perhaps the same feat could be achieved by natural evolution, should selection favour it. One way might be a genetically mediated raising of the natural production of thyroxine (or an increase in sensitivity to the existing thyroxine). Maybe axolotls have undergone paedomorphic and reverse-paedomorphic evolutions repeatedly during their history. Maybe evolving animals in general are continually, though less dramatically than the axolotl, moving one way or another along an axis of paedomorphosis/reverse-paedomorphosis.

Paedomorphosis is one of those ideas of which, once you get the hang of it, you start seeing examples everywhere you look. What does an ostrich remind you of? During the Second World War my father was an officer in the King's African Rifles. His batman Ali, like many Africans of the time, had never seen most of the large wild animals for which their homeland is famous, and his first glimpse of an ostrich sprinting across the savannah elicited a shriek of astonishment: 'Big chicken, BIG CHICKEN!' Ali had it nearly right, but more penetrating would have been 'Big baby chicken!' The wings of an ostrich are silly little stubs, just like the wings of a newly hatched chick. Instead of the stout quills of a flying bird, ostrich feathers are coa.r.s.e versions of the fluffy down of a baby chick. Paedomorphosis illuminates our understanding of the evolution of flightless birds such as the ostrich and the dodo. Yes, the economy of natural selection favoured downy feathers and stubby wings in a bird that did not need to fly (see the Elephant Bird's Tale and the Dodo's Tale). But the evolutionary route that natural selection employed to achieve its advantageous outcome was paedomorphosis. An ostrich is an overgrown chick.

Pekinese dogs are overgrown puppies.8 Pekinese adults have the domed forehead and the juvenile gait, even the juvenile appeal, of a puppy. Konrad Lorenz has wickedly suggested that Pekineses and other babyfaced breeds like King Charles spaniels appeal to the maternal instincts of frustrated mothers. The breeders may or may not have known what they were trying to achieve, but they surely didn't know that they were doing it through an artificial version of paedomorphosis. Pekinese adults have the domed forehead and the juvenile gait, even the juvenile appeal, of a puppy. Konrad Lorenz has wickedly suggested that Pekineses and other babyfaced breeds like King Charles spaniels appeal to the maternal instincts of frustrated mothers. The breeders may or may not have known what they were trying to achieve, but they surely didn't know that they were doing it through an artificial version of paedomorphosis.

Walter Garstang, a well-known English zoologist of a century ago, was the first to emphasise the importance of paedomorphosis in evolution. Garstang's case was later taken up by his son-in-law Alister Hardy, who was my professor when I was an undergraduate. Sir Alister delighted in reciting the comic verses which were Garstang's preferred medium for communicating his ideas. They were slightly funny at the time but not, I think, quite funny enough to justify the elaborate zoological glossary which would have to accompany a reprinting here.9 Garstang's idea of paedomorphosis, however, is today as interesting as ever which doesn't necessarily mean it is right. Garstang's idea of paedomorphosis, however, is today as interesting as ever which doesn't necessarily mean it is right.

We can think of paedomorphosis as a kind of evolutionary gambit: Garstang's Gambit. It can in theory herald a whole new direction in evolution: can even, Garstang and Hardy believed, permit a dramatic and, by geological standards, sudden breakout from an evolutionary dead end. This seems especially promising if the life cycle sports a distinct larval phase like a tadpole. A larva that is already adapted to a different way of life from the old adult is primed to swerve evolution into a whole new direction by the simple trick of accelerating s.e.xual maturity relative to everything else.

Among the cousins of the vertebrates are the tunicates or sea squirts. This seems surprising because adult sea squirts are sedentary filter-feeders anch.o.r.ed to rocks or seaweeds. How can these soft bags of water be cousins to vigorously swimming fishes? Well, the adult sea squirt may look like a bag, but the larva looks like a tadpole. It is even called a 'tadpole larva'. You can imagine what Garstang made of this, and we shall revisit the point, and unfortunately cast doubt on Garstang's theory, at Rendezvous 24 Rendezvous 24 when we meet the sea squirts. when we meet the sea squirts.

Bearing in mind the adult Pekinese as an overgrown puppy, think of the heads of juvenile apes. What do they remind you of? Wouldn't you agree that a juvenile chimpanzee or orang utan is more humanoid than an adult chimpanzee or orang utan? Admittedly it is controversial, but some biologists regard a human as a juvenile ape. An ape that never grew up. An ape axolotl. We have already met the idea in the Epilogue to Little Foot's Tale, and I shall not spell it out again here.

1 Sam Turvey tells me that the two frog species with the remotest island distribution, the Fijian frogs Sam Turvey tells me that the two frog species with the remotest island distribution, the Fijian frogs Platymantis vitiensis Platymantis vitiensis and and P. vitia.n.u.s P. vitia.n.u.s (closely related and presumably descended from a single colonising ancestor), develop completely in the egg rather than having a free-swimming tadpole. They appear more salt-tolerant than most frogs, with (closely related and presumably descended from a single colonising ancestor), develop completely in the egg rather than having a free-swimming tadpole. They appear more salt-tolerant than most frogs, with P. vitia.n.u.s P. vitia.n.u.s sometimes found on beaches. These unusual characteristics, if present in their colonising ancestor as seems likely, would have pre-adapted them for island-hopping. sometimes found on beaches. These unusual characteristics, if present in their colonising ancestor as seems likely, would have pre-adapted them for island-hopping.

2 Notably in recent years by Dr Jennifer Clack of Cambridge University and her colleagues. See her book, Notably in recent years by Dr Jennifer Clack of Cambridge University and her colleagues. See her book, Gaining Ground: The Origin and Evolution of Tetrapods Gaining Ground: The Origin and Evolution of Tetrapods.

3 The name lobefin is not used with universal agreement. Some authors exclude the lungfish and say that coelacanths are the only surviving lobefins. I follow the terminology of Professor Robert Carroll's The name lobefin is not used with universal agreement. Some authors exclude the lungfish and say that coelacanths are the only surviving lobefins. I follow the terminology of Professor Robert Carroll's Vertebrate Palaeontology and Evolution Vertebrate Palaeontology and Evolution and include lungfish as lobefins. and include lungfish as lobefins.

4 I am not a.s.serting that as a fact. I don't know if it is a fact, although I suspect that it is. It is an implication of our plausibly agreeing to give I am not a.s.serting that as a fact. I don't know if it is a fact, although I suspect that it is. It is an implication of our plausibly agreeing to give h.o.m.o ergaster h.o.m.o ergaster a different species name. a different species name.

5 I use this word advisedly. In 1999 the Mayor of Was.h.i.+ngton DC accepted the resignation of an official whose description of a budget proposal as n.i.g.g.ardly gave offence. Julian Bond, distinguished Chairman of the NAACP, correctly described the Mayor's judgement as n.i.g.g.ardly. Inspired by the case, a nasty little student at the University of Wisconsin brought an official complaint against her professor, who had used 'n.i.g.g.ardly' in a lecture on Chaucer. Such ignorant witch-hunting is not peculiar to the USA. In 2001, a mob of British vigilantes stoned the house of a consultant paediatrician, mistaking her for a paedophile. I use this word advisedly. In 1999 the Mayor of Was.h.i.+ngton DC accepted the resignation of an official whose description of a budget proposal as n.i.g.g.ardly gave offence. Julian Bond, distinguished Chairman of the NAACP, correctly described the Mayor's judgement as n.i.g.g.ardly. Inspired by the case, a nasty little student at the University of Wisconsin brought an official complaint against her professor, who had used 'n.i.g.g.ardly' in a lecture on Chaucer. Such ignorant witch-hunting is not peculiar to the USA. In 2001, a mob of British vigilantes stoned the house of a consultant paediatrician, mistaking her for a paedophile.

6 Stephen Jay Gould helpfully sorts out the terminology, in his cla.s.sic Stephen Jay Gould helpfully sorts out the terminology, in his cla.s.sic Ontogeny and Phylogeny Ontogeny and Phylogeny.

7 A. Fink-Nottle, A. Fink-Nottle, in litt in litt.

8 The pompous meddling with the English language that has given us 'Beijing', 'Mumbai' and 'cosmonaut' has so far spared us 'Beijinese dog'. The pompous meddling with the English language that has given us 'Beijing', 'Mumbai' and 'cosmonaut' has so far spared us 'Beijinese dog'.

9 A fragment of one heads the Lancelet's Tale. A fragment of one heads the Lancelet's Tale.

Rendezvous 18.

LUNGFISH.

At Rendezvous 18 Rendezvous 18, around 417 million years ago, we are joined in the warm and shallow seas of the DevonianSilurian boundary by a tiny trickle of pilgrims who have plodded a lonely course from the present. They are the lungfish, and they join us to look at the common ancestor we share with them an experience that may seem less strange to them than to us, for they find they have much in common with Concestor 18. Approximately our 185-million-greats-grandparent, it was a sarcopterygian, a lobefin fish, certainly much more like a lungfish than like a tetrapod (see plate 22) (see plate 22).

There are only six species of lungfish today: Neoceratodus forsteri Neoceratodus forsteri from Australia, from Australia, Lepidosiren paradoxa Lepidosiren paradoxa from South America, and four species of from South America, and four species of Protopterus Protopterus from Africa. The Australian lungfish looks really quite excitingly like an ancient sarcopterygian, with fleshy lobe fins like a coelacanth. The African and South American species, which are closely related to each other, have their fins reduced to long trailing ta.s.sels, and they therefore look less like the lobe-finned fish from whom they are descended. All the lungfish breathe air using lungs. The Australian lungfish has a single lung, the others have two. The African and South American species use their lungs to withstand a dry season. They burrow into the mud and stay dormant, breathing air through a little breathing hole in the mud. The Australian species, by contrast, lives in permanent bodies of water filled with weed. It takes air into its lung to supplement its gills in oxygen-poor water. from Africa. The Australian lungfish looks really quite excitingly like an ancient sarcopterygian, with fleshy lobe fins like a coelacanth. The African and South American species, which are closely related to each other, have their fins reduced to long trailing ta.s.sels, and they therefore look less like the lobe-finned fish from whom they are descended. All the lungfish breathe air using lungs. The Australian lungfish has a single lung, the others have two. The African and South American species use their lungs to withstand a dry season. They burrow into the mud and stay dormant, breathing air through a little breathing hole in the mud. The Australian species, by contrast, lives in permanent bodies of water filled with weed. It takes air into its lung to supplement its gills in oxygen-poor water.

When first discovered in 1870, modern lungfish living in Queensland were united with fossil fish more than 200 million years old under the same name, Ceratodus Ceratodus. This gives an indication of how little they have changed during that time. Let's not get carried away, however. A cla.s.sic study published in 1949 by the British palaeontologist T. S. Westoll showed that, although the lungfish have indeed stagnated for the last 200 million years or so, they evolved much more rapidly before that. In the Carboniferous Period, from around 350 million years ago, they were really racing along, before they slowed down almost to a stop about 250 million years ago, towards the end of the Permian Period.

The Lungfish's Tale is a tale of 'living fossils'.

[image]

Lungfish join. Humans and the other 'tetrapods' could be said to be lobe-finned fish, whose arms, wings, or legs are modified lobed fins. The two other living lineages of lobefins are the coelacanths and the lungfish. The division of these three lineages at the end of the Silurian is thought to have happened in a very short s.p.a.ce of time. This makes the order of branching difficult to sort out, even using genetic data. Nevertheless, genetic and fossil studies are starting to agree that the three lungfish species are the closest living relatives of the tetrapods, as shown here. Humans and the other 'tetrapods' could be said to be lobe-finned fish, whose arms, wings, or legs are modified lobed fins. The two other living lineages of lobefins are the coelacanths and the lungfish. The division of these three lineages at the end of the Silurian is thought to have happened in a very short s.p.a.ce of time. This makes the order of branching difficult to sort out, even using genetic data. Nevertheless, genetic and fossil studies are starting to agree that the three lungfish species are the closest living relatives of the tetrapods, as shown here.

Image: Australian lungfish ( Australian lungfish (Neoceratodus forsteri).

THE LUNGFISH'S TALE.

Written with Yan Wong.

A living fossil is an animal that, while being as alive as you or me, strongly resembles its ancient ancestors. Not much evolutionary change has occurred down the line leading to the living fossil. It is one of those random, pointless facts that the four most famous living fossils all begin with L: Lungfish, Limulus, Latimeria Limulus, Latimeria (the coelacanth) and (the coelacanth) and Lingula. Limulus Lingula. Limulus, the so-called 'horseshoe crab' (not a crab at all, but its own thing, superficially resembling a large trilobite) is placed in the same genus as Limulus walchi Limulus walchi of the Jura.s.sic, 200 million years ago. of the Jura.s.sic, 200 million years ago. Lingula Lingula belongs to the phylum Brachiopoda, sometimes called lamp-sh.e.l.ls. The kind of lamp they resemble, if any, is the Aladdin variety with its wick coming out of a kind of teapot spout, but what belongs to the phylum Brachiopoda, sometimes called lamp-sh.e.l.ls. The kind of lamp they resemble, if any, is the Aladdin variety with its wick coming out of a kind of teapot spout, but what Lingula Lingula spectacularly resembles is its own ancestors of 400 million years ago. Its a.s.signment to the very same genus has been disputed, but the fossil forms are still remarkably similar to their modern representatives. Although the anatomies, and presumably the ways of life, of these living fossils have changed rather little, their DNA texts have not stopped evolving. We cousins of lungfish have been changing ma.s.sively during the hundreds of millions of years since we branched apart. But although lungfish bodies stagnated during the same time, you wouldn't guess it if you looked at the speed of evolution of their DNA. spectacularly resembles is its own ancestors of 400 million years ago. Its a.s.signment to the very same genus has been disputed, but the fossil forms are still remarkably similar to their modern representatives. Although the anatomies, and presumably the ways of life, of these living fossils have changed rather little, their DNA texts have not stopped evolving. We cousins of lungfish have been changing ma.s.sively during the hundreds of millions of years since we branched apart. But although lungfish bodies stagnated during the same time, you wouldn't guess it if you looked at the speed of evolution of their DNA.

The ray-finned fish (familiar fish, such as trout or perch) during this time have produced an amazing variety of forms. So, more familiarly, have the tetrapods we glorified lobe-finned fish who moved out onto the land. The bodies of the lobefins themselves have evolved extremely slowly. Yet at the same time here is the point this whole tale is leading up to their genetic molecules seem not to have stuck to this same slow pace. If they had, the DNA sequences of lungfish and coelacanths would be much more similar to each other (and presumably to ancient ancestors) than they are to us, and to ray-finned fish. Yet they are not.

We know from fossils the approximate timings of the ancestral splits between lungfish, coelacanths, ourselves and the ray-finned fish. The first split, at about 440 million years ago, is that between the ray-finned fish and all the rest of us. The next to split off were the coelacanths, about 425 million years ago. That left the lungfish and all the rest of us. About 5 or 10 million years later still, the lungfish split off, leaving the rest of us, now called tetrapods, to make our own evolutionary way. As evolutionary time goes, all three of these splits occurred at pretty nearly the same time, at least compared to the long time over which all four lineages have been evolving ever since.

[image]

Evolutionary tree of various species from maximum likelihood a.n.a.lysis of DNA (see the Gibbon's Tale). Adapted from one of several trees put together by Zardoya and Meyer [ (see the Gibbon's Tale). Adapted from one of several trees put together by Zardoya and Meyer [324].

While working on a different problem, Rafael Zardoya of Spain and Axel Meyer of Germany drew the evolutionary tree above for the DNA of various species. The length of each branch is drawn to reflect the amount of evolutionary change, in mitochondrial DNA, along it.

If the DNA evolved at a constant rate, regardless of the species, then we would expect all the branches to finish lined up at the right hand edge. This clearly isn't the case. But neither do the organisms that show the least morphological change have the shortest branches. The DNA seems to have evolved at about the same rate in the lungfish and coelacanth as in the ray-finned fish. The vertebrates that colonised the land experienced a faster rate of DNA evolution, but even this is not obviously linked to morphological change. The winner and the runner-up of this molecular caucus race are the platypus and the alligator, neither of which have evolved morphologically as fast as, say, the blue whale or (vanity cannot help whispering) us.

The diagram ill.u.s.trates an important fact. The rate of DNA evolution is not always constant, but neither is it obviously correlated with morphological change. The tree on the previous page is just one example. Lindell Bromham of the University of Suss.e.x and her colleagues compared evolutionary trees based on morphological change against equivalent trees based on DNA change. And what they found confirmed the message of the Lungfish's Tale. The overall rate of genetic change is independent of morphological evolution.1 This is not to say that it is constant that would have been too good to be true. Certain lineages, such as the rodents and the nematode worms, seem to have a rather fast overall rate of molecular evolution compared to close relatives. In others, such as the cnidarians, the rate is much slower than related lineages. This is not to say that it is constant that would have been too good to be true. Certain lineages, such as the rodents and the nematode worms, seem to have a rather fast overall rate of molecular evolution compared to close relatives. In others, such as the cnidarians, the rate is much slower than related lineages.

The Lungfish's Tale encourages a hope that, a few years ago, no zoologist would have dared to entertain. With due caution in choosing genes, and with available methods of correcting for lineages that show variable rates of evolution, we should be able to put a figure, in millions of years, on the time of separation of any species from any other species. This bright hope is called the 'molecular clock', and it is the technique responsible for most of the quoted dates on our rendezvous points in this book. The principle of the molecular clock, and the controversies that still bedevil it, will be explained in the Epilogue to the Velvet Worm's Tale.

But now, on to Rendezvous 19 Rendezvous 19 and the mysterious coelacanth. and the mysterious coelacanth.

1 An earlier study had obtained a different result. But Bromham and her colleagues convincingly showed that the previous study had failed to allow for non-independence of data the multiple counting problem that we met in the Seal's Tale. An earlier study had obtained a different result. But Bromham and her colleagues convincingly showed that the previous study had failed to allow for non-independence of data the multiple counting problem that we met in the Seal's Tale.

Rendezvous 19.

COELACANTHS.

Concestor 19, perhaps our 190-million-greats-grandparent, lived around 425 million years ago, just as plants were colonising the land and coral reefs expanding in the sea. At this rendezvous we meet one of the spa.r.s.est, most tenuous bands of pilgrims in this story. We know of only one genus of coelacanth alive today, and its discovery was a huge surprise when it happened. The episode is well described by Keith Thomson in his Living Fossil: the Story of the Coelacanth Living Fossil: the Story of the Coelacanth.

The coelacanths were well known in the fossil record, but thought to have gone extinct before the dinosaurs. Then, astoundingly, a living coelacanth turned up in the catch of a South African trawler in 1938. By good fortune Captain Harry Goosen, skipper of the Nerita Nerita, was friendly with Marjorie Courtenay-Latimer, the enthusiastic young curator of the East London Museum. It was Goosen's habit to put aside interesting finds for her, and on 22 December 1938 he telephoned to tell her he had something. She went down to the quay, and an old Scotsman of the crew showed her a motley collection of discarded fish, which at first didn't seem of any interest. She was about to leave when: I saw a blue fin and pus.h.i.+ng off the fish, the most beautiful fish I had ever seen was revealed. It was 5 feet long and a pale mauve blue with iridescent silver markings.

She made a sketch of the fish, which she sent to South Africa's leading ichthyologist, Dr J. L. B. Smith, and it knocked him out. 'I would not have been more surprised if I had seen a dinosaur walking down the street.' (See plate 23) (See plate 23). Unfortunately, Smith took his time going to the scene, for reasons that are hard to fathom. By Keith Thomson's account, Smith didn't trust his judgement until he had sent off for a particular reference book, from Dr Barnard, a colleague in Cape Town. Smith hesitantly confessed his secret to Barnard, who was immediately sceptical. It seems that it was weeks before Smith could bring himself to go to East London and actually see the fish. Meanwhile, poor Miss Courtenay-Latimer was coping with its noi-some decay. Too large to fit in a formalin jar, she wrapped it in formalin-soaked cloths. These were inadequate to stave off decay, and eventually she had to have it stuffed. It was in this form that Smith finally saw it: Coelacanth, yes, by G.o.d! Although I had come prepared, that first sight hit me like a white-hot blast and made me feel shaky and queer, my body tingled. I stood as if stricken to stone ... I forgot everything else and then almost fearfully went close up and touched and stroked, while my wife watched in silence ... It was only then that speech came back, the exact words I have forgotten, but it was to tell them that it was true, it really was true, it was unquestionably a coelacanth. Not even I could doubt any more.

[image]

Coelacanths join. Growing consensus places coelacanths (of which there are two living species known) as the earliest diverging of the three extant lineages of lobefins. Growing consensus places coelacanths (of which there are two living species known) as the earliest diverging of the three extant lineages of lobefins.

Image: Comorean coelacanth ( Comorean coelacanth (Latimeria chalumnae).

Smith named it Latimeria Latimeria after Marjorie. Since then, many more have been found in deep waters around the Comoros Islands near Madagascar, and a second species turned up on the other side of the Indian Ocean, off Sulawesi. The genus has now been studied in detail, although not without the acrimony and accusations of fakery that seem regrettably but I suppose understandably to go with rare and very important discoveries. after Marjorie. Since then, many more have been found in deep waters around the Comoros Islands near Madagascar, and a second species turned up on the other side of the Indian Ocean, off Sulawesi. The genus has now been studied in detail, although not without the acrimony and accusations of fakery that seem regrettably but I suppose understandably to go with rare and very important discoveries.

Rendezvous 20.

RAY-FINNED FISH.

Rendezvous 20 is a big one, 440 million years ago in the earliest Silurian, still with a southern ice cap left over from the cold Ordovician. Concestor 20, which I am estimating to be our 195-million-greats-grandparent, is the one that unites us to the actinopterygian or ray-finned fish, most of whom belong to the large and successful group known as teleosts. The teleost fish are the great success story among modern vertebrates there are some 23,500 species of them. They are prominent at many levels of underwater food chains, in both salt and freshwater. They have managed to invade hot springs at one extreme, and the icy waters of the Arctic seas and high mountain lakes at the other. They thrive in acid streams, stinking marshes and saline lakes. is a big one, 440 million years ago in the earliest Silurian, still with a southern ice cap left over from the cold Ordovician. Concestor 20, which I am estimating to be our 195-million-greats-grandparent, is the one that unites us to the actinopterygian or ray-finned fish, most of whom belong to the large and successful group known as teleosts. The teleost fish are the great success story among modern vertebrates there are some 23,500 species of them. They are prominent at many levels of underwater food chains, in both salt and freshwater. They have managed to invade hot springs at one extreme, and the icy waters of the Arctic seas and high mountain lakes at the other. They thrive in acid streams, stinking marshes and saline lakes.

'Ray' refers to the fact that their fins have a skeleton similar to a Victorian lady's fan. Ray-fins lack the fleshy lobe at the base of each fin eponym for the lobefin fish like coelacanths and Concestor 18. Unlike our arms and legs, which have relatively few bones, and muscles that can move them relative to one another within the limb, actinopterygian fins are moved mostly by muscles in the main body wall. In this respect, we are more like lobefin fish as well we should be, for we are lobefins adjusted for life on land. Lobefin fish have muscles in the fleshy fins themselves, just as we have biceps and triceps muscles in our upper arms and Popeye muscles in our lower arms.

The ray-finned fish are mostly teleosts, plus a few odds and ends, including the sturgeons, and the paddlefish whom we met in the Duckbill's Tale. It is right and proper that such a hugely successful group should contribute several tales and I shall relegate most of what I have to say about them to the tales. The teleost pilgrims arrive in a jostling crowd, brilliant in their variety. The magnitude of that variety is the inspiration for the Leafy Sea Dragon's Tale.

[image]

Ray-finned fish join. The ray-finned fish are the closest relatives of we lobefins, and contain roughly the same number of described species about 25,000. Their phylogeny is not well resolved, although it is clear that the sturgeons and paddlefish, the b.i.+.c.hirs, the gars and the bowfin all branched off early. The phylogeny displayed here is particularly uncertain. For this reason, a few of the especially obscure groups have been omitted from this tree. The ray-finned fish are the closest relatives of we lobefins, and contain roughly the same number of described species about 25,000. Their phylogeny is not well resolved, although it is clear that the sturgeons and paddlefish, the b.i.+.c.hirs, the gars and the bowfin all branched off early. The phylogeny displayed here is particularly uncertain. For this reason, a few of the especially obscure groups have been omitted from this tree.

Images, left to right: plaice ( plaice (Pleuronectes platessa); snaggletooth (Astronesthes niger); pike (Esox lucius); red-bellied piranha (Serrasalmus nattereri); northern anchovy (Engraulis mordax); green moray (Gymnothorax prasinus); Florida gar (Lepisosteus platyrhincus); Siberian sturgeon (Acipenser baeri).

THE LEAFY SEA DRAGON'S TALE.

When my daughter was tiny, she loved to ask adults to draw fish for her. She would rush up to me when I was trying to write a book, thrust a pencil in my hand and clamour, 'Draw a fish. Daddy, draw a fis.h.!.+' The cartoon fish that I would immediately draw to keep her quiet and the only kind of fish she ever wanted me to draw was always the same: a regulation-issue fish like a herring or a perch, streamlined side view, pointed at the front end, triangular fin top and bottom, triangular tail at the back, finally dotted with an eye bracketed by the curve of a gill cover. I don't think I ran to pectoral or pelvic fins, which was remiss of me because they all have them. The standard fish is indeed an extremely common shape, one that obviously works well over the full range of sizes from minnow to tarpon.

What would Juliet have said if I had possessed the skill to draw for her a leafy sea dragon, Phycodurus equus Phycodurus equus? (See plate 24.) (See plate 24.)'NO, Daddy. NOT seaweed. Draw a fish. Draw a FISH.' The message of the Leafy Sea Dragon's Tale is that animal shapes are malleable like plasticine (see plate 25). A fish can change in evolutionary time to whatever unfishy shape is required for its way of life. Those fish that look like the standard-issue Juliet fish do so only because it suits them. It is a good shape for swimming through open water. But if survival is a matter of hanging motionless in beds of gently swaying kelp, the standard fish shape can be twisted and kneaded, pulled out in fantastically branched projections whose resemblance to the fronds of brown seaweed is so great that a botanist might be tempted to narrow it down to species (perhaps of the genus Fucus Fucus).

The shrimpfish, Aeoliscus strigatus Aeoliscus strigatus, which lives on reefs in the western Pacific, is again much too cunningly disguised for Juliet to have been satisfied, had I drawn it as a 'fish'. Its extremely elongated body is further prolonged by a long snout, and the effect is enhanced by a dark stripe running right through the eye and straight to the very untail-like tail. The fish looks like a long shrimp, or a little like a cut-throat razor which accounts for its other name of razorfish. It is covered with a transparent armour which, my colleague George Barlow who has watched them in the wild tells me, even feels like that of a shrimp. The resemblance to a shrimp is probably, however, no part of their camouflage. Like many teleosts, shrimpfish swim around in coordinated groups, and with military synchrony. But unlike any other teleost you might think of, shrimpfish swim with the body pointing straight down. I don't mean they swim in a vertical direction. They swim in a horizontal direction, but with the body vertical. The whole effect of this synchronised swimming is a resemblance to a stand of weeds, or, even more strikingly, to the tall spines of a giant sea urchin, among which they often take refuge. Swimming head down is a deliberate decision. When alarmed, they are perfectly capable of flipping into more conventional, horizontal mode and they then flee with surprising speed.

Or, what would Juliet have said if I had drawn for her a snipe eel (Nemichthyidae) or a gulper eel ( (Eurypharynx pelecanoides), two deep-sea eels with birds in their names? The snipe eel looks like a joke, ludicrously long and thin, with bird-like jaws that curve away from each other like a megaphone. So dysfunctional do these diverging jaws look, I can't help wondering how many of the fish have been seen alive. Could the megaphone jaws be a distortion in a dried-up museum specimen?

The gulper looks like a nightmare. With jaws ludicrously too large for its body, or so it would seem, it is capable of swallowing whole prey larger than itself one of several deep-sea fish with this remarkable talent. It is not unusual, of course, for predators to kill kill prey larger than themselves, and then eat them in bits. Lions do it; so do spiders. prey larger than themselves, and then eat them in bits. Lions do it; so do spiders.1 But to But to swallow swallow a larger animal than yourself whole is hard to imagine. The gulper eel, and other deep-sea fish such as the closely related swallower eel, and the unrelated black swallower, which is not an eel achieve the trick. They do it by a combination of disproportionately oversized jaws and a slack distensible stomach that hangs down only when full, looking rather like some gross external tumour. After the long digestion period, the stomach shrinks again. Why the trick of prodigious swallowing should be peculiar to snakes a larger animal than yourself whole is hard to imagine. The gulper eel, and other deep-sea fish such as the closely related swallower eel, and the unrelated black swallower, which is not an eel achieve the trick. They do it by a combination of disproportionately oversized jaws and a slack distensible stomach that hangs down only when full, looking rather like some gross external tumour. After the long digestion period, the stomach shrinks again. Why the trick of prodigious swallowing should be peculiar to snakes2 and deep-sea fish is not obvious to me. The gulper and the swallower eel lure prey into the vicinity of their mouth with a luminous lure at the tip of the tail. and deep-sea fish is not obvious to me. The gulper and the swallower eel lure prey into the vicinity of their mouth with a luminous lure at the tip of the tail.

The teleost body plan seems almost indefinitely malleable over evolutionary time, tolerant of being pulled or squashed into any shape, however distantly removed from the 'standard' fish shape. The oceanic sunfish's Latin name, Mola mola Mola mola, means millstone, and it is easy to see why. Seen from the side, it looks like a huge disc, up to an astonis.h.i.+ng four metres in diameter and weighing up to two tonnes. The circularity of its outline is broken only by two gigantic fins on top and underneath, each one up to two metres long.

The Hippo's Tale invoked, in explanation of its dramatic difference from its whale cousins, the liberation from gravity that whales must have enjoyed, as soon as they severed all ties with the land. No doubt something similar explains the great variety of shapes that the teleost fish display. But in exploiting that liberation, teleosts have one other advantage over, for example, sharks. Teleosts cope with buoyancy in a very special way, and the pike will tell the tale.

THE PIKE'S TALE.

In the sad province of Ulster, where 'the Mountains of Mourne sweep down to the sea', I know a beautiful lake. A party of children were swimming naked there one day, when somebody shouted that they had seen a large pike. Instantly all the boys but not the girls fled to dry land. The northern pike, Esox lucius Esox lucius, is a formidable predator of small fish. It is beautifully camouflaged, not against predators but to help it steal up on its prey. A stealth predator, and not particularly fast over a distance, it hangs almost motionless in the water, creeping imperceptibly forwards until within striking distance. During the deadly creep, it propels itself with imperceptible movements of the rear-mounted dorsal fin.

This whole hunting technique depends upon the ability to hang in the water at the desired level, like a drifting dirigible, without any effort, in perfect hydrostatic equilibrium. All locomotor work is concentrated on the clandestine business of creeping forwards. If a pike needed to swim in order to maintain its level, as many sharks do, its ambush technique would not work. Effortless maintenance, and adjustment, of hydrostatic equilibrium is what teleost fish are supremely good at, and it may be the single most significant key to their success. How do they do it? By means of the swim bladder: a modified lung filled with gas, which provides sensitive dynamic control of the animal's buoyancy. Except for some bottom-dwellers who have secondarily lost the swim bladder, all teleosts have it not just pike and not just their prey.

The swim bladder is often explained as working like a Cartesian Diver, but I think that is not quite correct. A Cartesian Diver is a miniature diving bell containing a bubble of air, which hangs at hydrostatic equilibrium in a bottle of water. When the pressure is increased (usually by squeezing down the cork in the neck of the bottle), the bubble is compressed and less water is displaced by the diver as a whole. Therefore, by Archimedes' Principle, the diver sinks. If the cork is eased slightly upwards so that the pressure in the bottle decreases, the bubble in the diver expands, more water is displaced, and the diver floats a little higher. So, with your thumb on the cork, you can exert fine control over the level at which the diver finds its equilibrium.

The key point about a Cartesian Diver is that the number of air molecules in the bubble remains fixed, while the volume and the pressure are changed (in inverse proportions, following Boyle's Law). If fish worked like Cartesian Divers, they would use muscle power to squeeze, or relax, the swim bladder, thereby changing the pressure and volume but leaving the number of molecules the same. That would work in theory, but it isn't what happens. Instead of keeping the number of molecules fixed and adjusting the pressure, the fish adjust the number of molecules. To sink, the fish absorbs some molecules of gas from its swim bladder into the blood, thereby reducing the volume. To rise, it does the reverse, releasing molecules of gas into the swim bladder.

In some teleosts, the swim bladder is also used to a.s.sist in hearing. The fish's body being mostly water, sound waves propagate through it pretty much as they did through the water before they hit the fish. When they strike the wall of the swim bladder, however, they suddenly reach a different medium, gas. The swim bladder therefore acts as a kind of eardrum. In some species it lies right against the inner ear. In others it is connected to the inner ear by a series of small bones called the Weberian ossicles. These do a similar job to our own hammer, anvil and stirrup, but are completely different bones.

The swim bladder seems to have evolved been 'co-opted' from a primitive lung, and some surviving teleosts, such as bowfins, gars and b.i.+.c.hirs, still use it for breathing. This perhaps comes as a little surprise to us, for whom breathing air seems like a significant 'advance' that went with leaving the water for the land. One might have supposed the lung to be a modified swim bladder. On the contrary, it seems that the primitive breathing lung forked in evolution and went two ways. On the one hand, it carried its old breathing function out onto the land, and we use it still. The other branch of the fork was the new and exciting one: the old lung became modified to form a genuine innovation the swim bladder.

THE MUDSKIPPER'S TALE.

On an evolutionary pilgrimage it is fitting that some of the tales, though told by surviving pilgrims, should deal with recent re-enactments of ancient evolutionary events. Teleost fish are so variable and so versatile, it is only to be expected that some of them might replay parts of the lobefins' history, and come out onto the land. The mudskipper is just such a fish out of water, and it lives to tell the tale.

A number of teleost fish species live in swampy water, poor in oxygen. Their gills cannot extract enough, and they need help from the air. Familiar aquarium fish from the swamps of South-East Asia, such as the Siamese fighting fish Betta splendens Betta splendens, frequently come to the surface to gulp air, but they still use their gills to extract the oxygen. I suppose, since the gills are wet, you could say the gulping is equivalent to locally oxygenating their gill water, as you might bubble air through your aquarium. It goes further than that, however, because the gill chamber is furnished with an auxiliary air s.p.a.ce, richly supplied with blood vessels. This cavity is not a true lung. The true h.o.m.ologue of the lung in teleost fish is the swim bladder which, as the Pike's Tale has shown, they use for keeping their buoyancy neutral.

Those fish that breathe air through their gill chamber have rediscovered air breathing by a completely different route. Perhaps the most advanced exponents of the air-breathing gill chamber are the climbing perches Anabas Anabas. These fish also live in poorly oxygenated water and they have the habit of walking over land looking for water when their previous home has dried up. They can survive out of water for days at a time. Anabas Anabas is, indeed, a living, breathing example of what Romer was talking about in his (now less fas.h.i.+onable) theory of how fish came out onto the land. is, indeed, a living, breathing example of what Romer was talking about in his (now less fas.h.i.+onable) theory of how fish came out onto the land.

Another group of walking teleost fish are the mudskippers, for example Periophthalmus Periophthalmus, whose tale this is. Some mudskippers actually spend more time out of water than in it. They eat insects and spiders, which are not normally found in the sea. It is possible that our Devonian ancestors enjoyed similar benefits when they first left the water, for they were preceded onto the land by both insects and spiders. A mudskipper flaps its body across the mudflats, and it can also crawl using its pectoral (arm) fins, whose muscles are so well developed that they can support the fish's weight. Indeed, mudskipper courts.h.i.+p takes place partly on land, and a male may do pushups, as some male lizards do, to show off his golden chin and throat to females. The fin skeleton, too, has evolved convergently to resemble that of a tetrapod such as a salamander.

Mudskippers can jump more than half a metre by bending the body to one side and suddenly straightening it hence some of their many vernacular names, including 'mud-hopper', 'johnny jumper', 'frogfish' and 'kangaroo fish'. Another common name, 'climbing fish', comes from their habit of climbing mangrove trees looking for prey. They cling to the trees with the pectoral fins, aided by a kind of sucker which is made by bringing the pelvic fins together under the body.

Like the swamp fish already mentioned, mudskippers breathe by taking air into their moist gill chambers. They also take in oxygen through the skin, which has to be kept moist. If a mudskipper is in danger of drying out, it will roll about in a puddle. Their eyes are especially vulnerable to dryness, and they sometimes wipe them with a wet fin. The eyes bulge close together near the top of the head, where, as with frogs and crocodiles, they can be used as periscopes to see above the surface when the fish is underwater. When out on land, a mudskipper will frequently withdraw its bulging eyes into their sockets to moisten them. Before leaving the water on a land sortie, the fish will fill its gill cavities with water.

In a popular book on the conquest of the land, the author mentions an account by an eighteenth-century artist living in Indonesia who kept a 'frogfish' alive for three days in his house: It followed me everywhere with great familiarity, much like a little dog.

The book has a cartoon of a 'frogfish' walking like a little dog, but what it actually depicts is clearly an angler angler fish: a deep-sea fish with a lure on the end of a spine sticking up above the head, used to catch smaller fish. I suspect that the cartoonist has been the victim of a misunderstanding: an instructive one because it shows what can happen if we rely on colloquial common names for animals rather than the scientific names that, whatever their faults, are designed to be unique. It is true that some people call angler fish frogfish. But it is highly implausible that the fish that followed the artist around like a dog could have been a deep-sea angler fish. It could easily have been a mudskipper, however. They do live in Indonesia, and frogfish is one of their colloquial names. A mudskipper looks, to my eyes at least, far more like a frog than an angler fish ever could, and it leaps like a frog. I conjecture that the artist's pet 'frogfish', which followed him around like a little dog, was a mudskipper. fish: a deep-sea fish with a lure on the end of a spine sticking up above the head, used to catch smaller fish. I suspect that the cartoonist has been the victim of a misunderstanding: an instructive one because it shows what can happen if we rely on colloquial common names for animals rather than the scientific names that, whatever their faults, are designed to be unique. It is true that some people call angler fish frogfish. But it is highly implausible that the fish that followed the artist around like a dog could have been a deep-sea angler fish. It could easily have been a mudskipper, however. They do live in Indonesia, and frogfish is one of their colloquial names. A mudskipper looks, to my eyes at least, far more like a frog than an angler fish ever could, and it leaps like a frog. I conjecture that the artist's pet 'frogfish', which followed him around like a little dog, was a mudskipper.

I like the idea that we are descended from some creature which, even if it was different from a modern mudskipper in many other respects, was as adventurous and enterprising as a little dog: the nearest thing, perhaps, to a dog that the Devonian had to offer? A girlfriend of mine from long ago explained why she loved dogs: 'Dogs are such good sports.' I think the first fish to venture out onto the land must have been an archetypal good sport, whom it would be a pleasure to call ancestor.

THE CICHLID'S TALE.

Lake Victoria is the third largest lake in the world, but it is also one of the youngest. Geological evidence indicates that it is only about 100,000 years old. It is home to a huge number of endemic cichlid (p.r.o.nounced 'sick-lid') fish. Endemic means that they are found nowhere else than in Lake Victoria, and presumably evolved there. Depending on whether your ichthyologist is a lumper or a splitter, the number of species of cichlid in Lake Victoria is somewhere between 200 and 500, and a recent authoritative estimate puts it at 450. Of these endemic species, the great majority belong to one tribe, the haplochromines. It looks as though they all evolved, as a single 'species flock', during the last hundred thousand years or so.

As we saw in the Narrowmouth's Tale, the evolutionary splitting of one species into two is called speciation. What surprises us about the young age of Lake Victoria is that it suggests an astonis.h.i.+ngly high rate of speciation. There is also evidence that the lake dried up completely about 15,000 years ago, and some people even drew the conclusion that the 450 endemic species must have evolved from a single founder in this astonis.h.i.+ngly short time. As we shall see, this is probably an exaggeration. But in any case a little calculation helps to get these short times into perspective. What sort of speciation rate would it take to generate 450 species in 100,000 years? The most prolific pattern of speciation in theory would be a succession of doublings. In this idealised pattern, one ancestral species gives rise to two daughter species, each of those splits into two, then each of those splits into two, and so on. Following this most productive ('exponential') pattern of speciation, an ancestral species could easily generate 450 species in 100,000 years, with what seems like the rather long interval of 10,000 years between speciations within any one lineage. Starting with any one modern cichlid pilgrim and going backwards, there would be only ten rendezvous points in 100,000 years.

Of course, it is highly unlikely that real-life speciation would actually follow the ideal pattern of successive doubling. The opposite extreme would be a pattern in which the founder species successively threw one daughter species after another, with none of the daughter species subsequently speciating. Following this least 'efficient' pattern of speciation, in order to generate 450 species in 100,000 years, the interval between speciation events would need to be a couple of centuries. Even that doesn't sound ridiculously short. The truth surely lies between the two extreme patterns: say one or a few millennia as the average interval between speciation events in any one lineage. When you put it like that, the speciation rate doesn't seem so spectacularly high after all, especially in the light of the sorts of evolutionary rates that we saw in the Galapagos Finch's Tale. Nevertheless, as a sustained feat of speciation, it is very fast and prolific by the standards evolutionists have come to expect, and the cichlid fish of Lake Victoria have become legendary among biologists for this reason.3 Lakes Tanganyika and Malawi are only slightly smaller than Victoria smaller in area, that is. But where Victoria is a wide, shallow basin, Tanganyika and Malawi are Rift Valley lakes: long, narrow and very deep. They are not so young as Victoria. Lake Malawi, which I have already nostalgically mentioned as the site of my first 'seaside' holidays, is between 1 and 2 million years old. Lake Tanganyika is the oldest, at 1214 million. Despite these differences, all three lakes share the remarkable feature that inspires this tale. All are teeming with hundreds of endemic cichlid fish, unique to the particular lake. Victoria cichlids are a completely different set of species from Tanganyika cichlids, and Malawi cichlids are a completely different set from either. Yet, each of the three flocks of hundreds of species has produced, by convergent evolution in its own lake, an extremely similar range of types. It looks as though a single founding haplochromine species (or very few) entered each infant lake, perhaps through a river. From such small beginnings, successive evolutionary subdivisions 'speciation events' generated hundreds of species of cichlids, whose range of types closely paralleled those in each of the other great lakes. This sort of rapid diversification into many different types is called 'adaptive radiation'. Darwin's finches are another famous example of an adaptive radiation, but African cichlids are particularly special because it has happened in triplicate.4 Much of the variation within each lake is concerned with diet. Each of the three lakes has its specialists in plankton feeding, its specialists in grazing algae off rocks, its predators on other fish, its scavengers, its food robbers, its fish-egg eaters. There are even parallels to the cleaner fish habit, which is better known from tropical coral reef fish (see the Polypifer's Tale). Cichlid fish have a complicated system of double jaws. In addition to the 'ordinary' outer jaws that we can see, there is a second set of 'pharyngeal jaws' buried deep in the throat. It is likely that this innovation primed the cichlids for their dietary versatility and hence their ability to diversify repeatedly in the great African lakes.

Despite their greater age, Lakes Tanganyika and Malawi don't have a noticeably larger number of species than Victoria. It is as though each lake achieves a sort of closure, at an equilibrium number of species, that doesn't go on getting larger as time goes by. Indeed, it may even get smaller. Lake Tanganyika, the oldest of the three lakes, has the fewest species. Lake Malawi, of intermediate age, has the most. It seems likely that all three lakes followed the Victoria pattern of extremely rapid speciation from very small beginnings, generating several hundred new endemic species within the first few hundred thousand years.

The Narrowmouth's Tale touched upon the favoured theory of how speciation happens, the geographical isolation theory. It is not the only theory, and more than one may be right in different cases. 'Sympatric speciation', the separation of populations into separate species in the same geographical area, can happen under some conditions, especially in insects where it may even be the norm. There is some evidence for sympatric speciation of cichlid fish in small African crater lakes. But the geographical isolation model of speciation is still the dominant one, and it will prevail through the rest of this tale.

According to the geographical isolation theory, speciation begins with the accidental geographical division of a single ancestral species into separate populations. No longer able to interbreed, the two populations drift apart, or are pushed by natural selection in different evolutionary directions. Then, if they subsequently meet after this divergence, they either can't interbreed or don't want to. They often recognise their own species by some particular feature, and studiously avoid similar species who lack it. Natural selection penalises mating with the wrong species, especially where the species are close enough for it to be a temptation, and close enough for hybrid offspring to survive, to consume costly parental resources, and then turn out to be sterile, like mules. Many zoologists have interpreted courts.h.i.+p displays as aimed mainly against miscegenation. This may be an exaggeration, and there are other important selection pressures bearing upon courts.h.i.+p. But it is still probably correct to interpret some courts.h.i.+p displays, and some bright colours and other conspicuous advertis.e.m.e.nts, as 'reproductive isolation mechanisms' evolved through selection against hybridisation.

As it happens, a particularly neat experiment was done on cichlid fish by Ole Seehausen, now at the University of Hull, and his colleague Jacques van Alphen at the University of Leiden. They took two related species of Lake Victoria cichlids, Pundamilia pundamilia Pundamilia pundamilia and and P. nyererei P. nyererei (named after one of Africa's great leaders, Julius Nyerere of Tanzania). The two species are very similar, except that (named after one of Africa's great leaders, Julius Nyerere of Tanzania). The two species are very similar, except that P. nyererei P. nyererei has a reddish colour, whereas has a reddish colour, whereas P. pundamilia P. pundamilia is bluish. Under normal conditions, females in choice tests prefer to mate with males of their own species. But now, Seehausen and van Alphen did their critical test. They gave females the same choice, but in artificial monochromatic light. This does dramatic things to perceived colour, as I remember vividly from schooldays in Salisbury, a city whose streets happened to be lit by sodium lights. Our bright red caps, and the bright red buses, all looked dirty brown. This is what happened to both the red and the blue is bluish. Under normal conditions, females in choice tests prefer to mate with males of their own species. But now, Seehausen and van Alphen did their critical test. They gave females the same choice, but in artificial monochromatic light. This does dramatic things to perceived colour, as I remember vividly from schooldays in Salisbury, a city whose streets happened to be lit by sodium lights. Our bright red caps, and the bright red buses, all looked dirty brown. This is what happened to both the red and the blue Pundamilia Pundamilia males in Seehausen and van Alphen's experiment. Red or blue in white light, they all went dirty brown. And the result? The females no longer distinguished between them, and mated indiscriminately. Offspring of these matings were fully fertile, indicating that female choice is the only thing that stands between these species and hybridisation. The Gra.s.shopper's Tale gives a similar example. If the two species were a bit more different, their offspring would probably be infertile, like mules. Later still in the process of divergence, isolated populations reach the point where they couldn't hybridise even if they wanted to. males in Seehausen and van Alphen's experiment. Red or blue in white light, they all went dirty brown. And the result? The females no longer distinguished between them, and mated indiscriminately. Offspring of these matings were fully fertile, indicating that female choice is the only thing that stands between these species and hybridisation. The Gra.s.shopper's Tale gives a similar example. If the two species were a bit more different, their offspring would probably be infertile, like mules. Later still in the process of divergence, isolated populations reach the point where they couldn't hybridise even if they wanted to.

Whatever the basis of the separation, failure to hybridise defines a pair of populations as belonging to different species. Each of the two species is now free to evolve separately, free from contamination by the genes of the other, even though the original geographical barrier to such contamination is no more. Without the initial intervention of geographical barrriers (or some equivalent), species could never become specialised to particular diets, habitats or behaviour patterns. Notice that 'intervention' does not necessarily mean it is geography itself that made the active change as when a valley floods or a volcano er