The Ancestor's Tale Part 6

The islands or island continents that have shaped the evolution of mammals are, in the order we shall visit them, Madagascar, Laurasia (the great northern continent which was once isolated from its southern counterpart, Gondwana), South America, Africa, and Australia. Gondwana itself might be added to the list, for, as we shall discover at Rendezvous 15 Rendezvous 15, it too bred its own unique fauna, before it broke up into all our Southern Hemisphere continents. The Aye-Aye's Tale has shown us the faunistic and floristic extravagance of Madagascar. Laurasia is the ancient home, and Darwinian proving-ground, of the huge influx of pilgrims we shall meet at Rendezvous 11 Rendezvous 11, the laurasiatheres. At Rendezvous 12 Rendezvous 12 we shall be joined by a strange band of pilgrims, the xenarthrans, who served their evolutionary apprenticeship on the then island continent of South America, and who will tell us the tale of the others who shared it. At we shall be joined by a strange band of pilgrims, the xenarthrans, who served their evolutionary apprenticeship on the then island continent of South America, and who will tell us the tale of the others who shared it. At Rendezvous 13 Rendezvous 13 we find the afrotheres, another hugely varied group of mammals, whose diversity was honed on the island continent of Africa. Then, at we find the afrotheres, another hugely varied group of mammals, whose diversity was honed on the island continent of Africa. Then, at Rendezvous 14 Rendezvous 14 it is the turn of Australia and the marsupials. Madagascar is the microcosm which sets the pattern large enough to follow it, small enough to display it in exemplary clarity. it is the turn of Australia and the marsupials. Madagascar is the microcosm which sets the pattern large enough to follow it, small enough to display it in exemplary clarity.

1 The same habit, with the same long finger (except it is the fourth finger instead of the third), has convergently evolved in a group of New Guinean marsupials, the striped possum and the trioks ( The same habit, with the same long finger (except it is the fourth finger instead of the third), has convergently evolved in a group of New Guinean marsupials, the striped possum and the trioks (Dactylopsila). These marsupials seem to be champion convergers, by the way. They are striped in the same pattern as skunks. And like skunks, they emit a powerful smell in defence.

THE GREAT CRETACEOUS CATASTROPHE.

Rendezvous 8, where our pilgrims meet the lemurs 63 million years ago, was our last rendezvous 'before', in our backward journey, we burst through the 65-million-year barrier, the so-called K/T boundary, which separates the Age of Mammals from the much longer Age of Dinosaurs that preceded it.1 The K/T was a watershed in the fortunes of the mammals. They had been small, shrew-like creatures, nocturnal insectivores, their evolutionary exuberance held down under the weight of reptilian hegemony for more than 100 million years. Suddenly the pressure was released and, in a geologically very short time, the descendants of those shrews expanded to fill the ecological s.p.a.ces left by the dinosaurs. The K/T was a watershed in the fortunes of the mammals. They had been small, shrew-like creatures, nocturnal insectivores, their evolutionary exuberance held down under the weight of reptilian hegemony for more than 100 million years. Suddenly the pressure was released and, in a geologically very short time, the descendants of those shrews expanded to fill the ecological s.p.a.ces left by the dinosaurs.

What caused the catastrophe itself? A controversial question. At the time there was extensive volcanic activity in India, spewing out lava flows covering well over a million square kilometres (the 'Deccan Traps') which must have had a radical effect on the climate. However, a variety of evidence is building a consensus that the final deathblow was more sudden and more drastic. It seems that a projectile from s.p.a.ce a large meteorite or comet hit Earth. Detectives proverbially reconstruct events from cigar ash and footprints. The ash in this case is a worldwide layer of the element iridium at just the right place in the geological strata. Iridium is normally rare in the Earth's crust but common in meteorites. The sort of impact we are talking about would have pulverised the incoming bolide, and scattered its remains as dust throughout the atmosphere, from which it would eventually have rained down all over the Earth's surface. The footprint 100 miles wide and 30 miles deep is a t.i.tanic impact crater, Chicxulub, at the tip of the Yucatan peninsula in Mexico.

s.p.a.ce is full of moving objects, travelling in random directions and at a great variety of speeds relative to one another. There are many more ways in which objects can be travelling at high speeds relative to us than low speeds. So, most of the objects that hit our planet are travelling very fast indeed. Fortunately, most of them are small and burn up in our atmosphere as 'shooting stars'. A few are large enough to retain some solid ma.s.s all the way to the planet's surface. And, once in a few tens of millions of years, a very large one catastrophically collides with us. Because of their high velocity relative to Earth, these ma.s.sive objects release an unimaginably large quant.i.ty of energy when they collide. A gunshot wound is hot because of the velocity of the bullet. A colliding meteorite or comet is likely to be travelling even faster than a high-velocity rifle bullet. And where the rifle bullet weighs only ounces, the ma.s.s of the celestial projectile that ended the Cretaceous and slew the dinosaurs was measured in gigatons. The noise of the impact, thundering round the planet at a thousand kilometres per hour, probably deafened every living creature not burned by the blast, suffocated by the wind-shock, drowned by the 150-metre tsunami that raced around the literally boiling sea, or pulverised by an earthquake a thousand times more violent than the largest ever dealt by the San Andreas fault. And that was just the immediate cataclysm. Then there was the aftermath the global forest fires, the smoke and dust and ash which blotted out the sun in a two-year nuclear winter that killed off most of the plants and stopped dead the world's food chains.

No wonder all the dinosaurs, with the notable exception of the birds, perished and not just the dinosaurs, but about half of all other species too, particularly the marine ones.2 The wonder is that any life at all survives these cataclysmic visitations. By the way, the one that ended the Cretaceous and the dinosaurs is not the biggest that honour falls to the ma.s.s extinction that marks the end of the Permian, about a quarter of a billion years ago, in which some 95 per cent of all species went extinct. Recent evidence suggests that an even larger comet or meteorite may have been responsible for that mother of all extinctions. We are uneasily aware that a similar catastrophe could hit us at any moment. Unlike the dinosaurs in the Cretaceous, or the pelycosaurian (mammal-like) reptiles in the Permian, astronomers would give us several years' warning, or at least months. But this would not be a blessing for, at least with present-day technology, there is nothing we could do to prevent it. Fortunately, the odds that this will happen in any particular person's lifetime are, by normal actuarial standards, negligible. At the same time, the odds that it will happen in The wonder is that any life at all survives these cataclysmic visitations. By the way, the one that ended the Cretaceous and the dinosaurs is not the biggest that honour falls to the ma.s.s extinction that marks the end of the Permian, about a quarter of a billion years ago, in which some 95 per cent of all species went extinct. Recent evidence suggests that an even larger comet or meteorite may have been responsible for that mother of all extinctions. We are uneasily aware that a similar catastrophe could hit us at any moment. Unlike the dinosaurs in the Cretaceous, or the pelycosaurian (mammal-like) reptiles in the Permian, astronomers would give us several years' warning, or at least months. But this would not be a blessing for, at least with present-day technology, there is nothing we could do to prevent it. Fortunately, the odds that this will happen in any particular person's lifetime are, by normal actuarial standards, negligible. At the same time, the odds that it will happen in some some unfortunate individuals' lifetime are near certainty. Insurance companies are just not used to thinking that far ahead. And the unfortunate individuals concerned will probably not be human, for the statistical likelihood is that we shall be extinct before then anyway. unfortunate individuals' lifetime are near certainty. Insurance companies are just not used to thinking that far ahead. And the unfortunate individuals concerned will probably not be human, for the statistical likelihood is that we shall be extinct before then anyway.

A rational case can be mounted that humanity should start research into defensive measures now, to bring the technology up to the point where, if a credible warning were sounded, there would be time to put measures into effect. Present-day technology could only minimise the impact, by storing a suitable balance of seeds, domestic animals, machines including computers and databases full of acc.u.mulated cultural wisdom, in underground bunkers with privileged humans (now there's there's a political problem). Better would be to develop so far only dreamed-of technologies to avert the catastrophe by diverting or destroying the intruder. Politicians who invent external threats from foreign powers, in order to scare up economic or voter support for themselves, might find that a potentially colliding meteor answers their ign.o.ble purpose just as well as an Evil Empire, an Axis of Evil, or the more nebulous abstraction 'Terror', with the added benefit of encouraging international co-operation rather than divisiveness. The technology itself is similar to the most advanced 'star wars' weapons systems, and to that of s.p.a.ce exploration itself. The ma.s.s realisation that humanity as a whole shares common enemies could have incalculable benefits in drawing us together rather than, as at present, apart. a political problem). Better would be to develop so far only dreamed-of technologies to avert the catastrophe by diverting or destroying the intruder. Politicians who invent external threats from foreign powers, in order to scare up economic or voter support for themselves, might find that a potentially colliding meteor answers their ign.o.ble purpose just as well as an Evil Empire, an Axis of Evil, or the more nebulous abstraction 'Terror', with the added benefit of encouraging international co-operation rather than divisiveness. The technology itself is similar to the most advanced 'star wars' weapons systems, and to that of s.p.a.ce exploration itself. The ma.s.s realisation that humanity as a whole shares common enemies could have incalculable benefits in drawing us together rather than, as at present, apart.

Evidently, since we exist, our ancestors survived the Permian extinction, and later the Cretaceous extinction. Both catastrophes, and the others that have also occurred, must have been extremely unpleasant for them, and they survived by the skin of their teeth, possibly deaf and blind but just capable of reproducing, otherwise we wouldn't be here. Perhaps they were hibernating at the time, and didn't wake up until after the nuclear winter that is thought to follow such catastrophes. And then, in the fullness of evolutionary time, they reaped the benefits. In the case of the Cretaceous survivors, there were now no dinosaurs to eat them, no dinosaurs to compete with them. You might think there was a down side: no dinosaurs for them to eat. But few mammals were large enough, and few dinosaurs small enough, to make that much of a loss. There can be no doubt that the mammals flowered ma.s.sively after the K/T, but the form of the flowering and how it relates to our rendezvous points is debatable. Three 'models' have been suggested, and now is the time to discuss them. The three shade into each other, and I shall present them in their extreme forms only for simplicity. For reasons of clarity, as I believe, I shall change their usual names to the Big Bang Model, the Delayed Explosion Model, and the Non-explosive Model. There are parallels in the controversy over the so-called Cambrian Explosion, to be discussed in the Velvet Worm's Tale.

1. The Big Bang Model, in its extreme form, sees a single mammal species surviving the K/T catastrophe, a sort of Palaeocene Noah. Immediately after the catastrophe, the descendants of this Noah started proliferating and diverging. On the Big Bang Model, most of the rendezvous points occurred in a bunch, just this side of the K/T boundary the backwards way of viewing the rapidly divergent branching of the Noah's descendants.2. The Delayed Explosion Model acknowledges that there was a major explosion of mammal diversity after the K/T boundary. But the mammals of the explosion were not descended from a single Noah, and most of the rendezvous points between mammal pilgrims pre-date the K/T boundary. When the dinosaurs suddenly left the scene, there were lots of little shrew-like lineages who survived to step into their shoes. One 'shrew' evolved into carnivores, a second 'shrew' evolved into primates, and so on. These different 'shrews', although probably quite similar to each other, traced their separate ancestry deep into the past, eventually to unite way back in the Age of Dinosaurs. Those ancestors followed, in parallel, their long fuses into the future through the Age of Dinosaurs to the K/T boundary. Then they all exploded in diversity, more or less simultaneously, when the dinosaurs disappeared. The consequence is that the concestors of modern mammals long pre-date the K/T boundary, although they only started diverging from each other in appearance and way of life after the death of the dinosaurs.3. The Non-explosive Model doesn't see the K/T boundary as marking any kind of sharp discontinuity in the evolution of mammalian diversity at all. Mammals just branched and branched, and this process went on before the K/T boundary in much the same way as it went on after it. As with the Delayed Explosion Model, the concestors of modern mammals pre-date the K/T boundary. But in this model they had already diverged considerably by the time the dinosaurs disappeared.

Of the three models, the evidence, especially molecular evidence but increasingly fossil evidence too, seems to favour the Delayed Explosion Model. Most of the major splits in the mammal family tree go way back, deep into dinosaur times. But most of those mammals that coexisted with dinosaurs were pretty similar to each other, and remained so until the removal of the dinosaurs freed them to explode into the Age of Mammals. A few members of those major lineages haven't changed much since those early times, and they consequently resemble each other, even though the common ancestors that they share are extremely ancient. Eurasian shrews and tenrec shrews, for example, are very similar to each other, probably not because they have converged from different starting points but because they haven't changed much since primitive times. Their shared ancestor, Concestor 13, is thought to have lived about 105 million years ago, nearly as long before the K/T boundary as the K/T is before the present.

1 K/T stands for CretaceousTertiary, with 'K' rather than 'C' because 'C' had already been granted by geologists to the Carboniferous Period. Cretaceous comes from K/T stands for CretaceousTertiary, with 'K' rather than 'C' because 'C' had already been granted by geologists to the Carboniferous Period. Cretaceous comes from creta creta, the Latin for chalk. The German for chalk is Kreide, hence the K. The 'Tertiary' was part of a now defunct system of nomenclature, and covered the first five epochs of the Cenozoic Era. The boundary is now called CretaceousPalaeogene (see the Geological Timescale in the General Prologue). Nevertheless, the abbreviation 'K/T' remains in common use, and I will use it here.

2 It is tempting to see the catastrophe as strangely selective. The deep sea Foraminifera (protozoa in tiny sh.e.l.ls which fossilise in enormous numbers and are therefore much used by geologists as indicator species) were almost entirely spared. It is tempting to see the catastrophe as strangely selective. The deep sea Foraminifera (protozoa in tiny sh.e.l.ls which fossilise in enormous numbers and are therefore much used by geologists as indicator species) were almost entirely spared.

Rendezvous 9.

COLUGOS AND TREE SHREWS.

Rendezvous 9 occurs 70 million years into the past, still in the time of the dinosaurs and before the flowering of mammalian diversity properly began. Actually, the flowering of flowers themselves had only just begun. Flowering plants, while diverse, had been previously restricted to disturbed habitats such as those uprooted by elephantine dinosaurs or ravaged by fire, but by now had gradually evolved to include a range of forest-canopy trees and understorey bushes. Concestor 9, which was something like our 10-million-greats-grandparent, was the common ancestor we share with a pair of squirrel-like mammal groups. Well, one of them is squirrel-like and the other more like a flying squirrel. They are the 18 species of tree shrews and the two species of colugos or 'flying lemurs', all from South East Asia.

The tree shrews are all very similar to each other, and are placed in the family Tupaiidae. Most live like squirrels, in trees, and some species resemble squirrels even down to having long, fluffy tails. The resemblance, however, is superficial. Squirrels are rodents. Tree shrews are certainly not rodents. As to what they are, well, that is partly what the next tale will be about. Are they shrews, as their common name would suggest? Are they primates, as certain authorities have long thought? Or are they something else altogether? The pragmatic solution has been to place them in their own, uncertainly placed, mammalian order, the Scandentia (Latin scandere scandere, to climb). But in seeking concestor points, we cannot avoid the problem so easily. The Colugo's Tale contains my justification or apology? for the solution I have adopted, which is to unite the colugos and the tree shrews 'before' they join our pilgrimage.

Colugos have long been known as flying lemurs, prompting the obvious put-down: they neither fly nor are lemurs. Recent evidence suggests that they are closer to lemurs than was realised even by those responsible for the misnomer. And, while they don't have powered flight like a bat or a bird, they are adept gliders. The two species, Cynocephalus volans Cynocephalus volans, the Philippine colugo, and C. variegatus C. variegatus, the Malayan colugo, have a whole order to themselves, the Dermoptera. It means 'wings of skin'. Like the flying squirrels of America and Eurasia, the more distantly related flying scaly-tailed squirrels of Africa, and the marsupial gliders of Australia and New Guinea, colugos have a single large flap of skin, the patagium, which works a bit like a controlled parachute. Unlike that of the other gliders, the colugo's patagium embraces the tail as well as the limbs, and it extends right to the tips of the fingers and toes. Colugos are also, with a 'wing' span of 70 centimetres, larger than any of those other gliders. Colugos can glide more than 70 metres through the forest at night, to a distant tree, with little loss of height.

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Tree shrews and colugos join. This is one of the most uncertain phylogenies in the book (see the Colugo's Tale). The scheme shown here, which groups the 16 species of tree shrew with the two colugos as a sister group to the primates, is advocated by some molecular taxonomists. The dates of this and the next rendezvous are not well established. This is one of the most uncertain phylogenies in the book (see the Colugo's Tale). The scheme shown here, which groups the 16 species of tree shrew with the two colugos as a sister group to the primates, is advocated by some molecular taxonomists. The dates of this and the next rendezvous are not well established.

Images, left to right: Malayan colugo ( Malayan colugo (Cynocephalus variegatus); northern tree shrew (Tupaia belangeri).

The fact that the patagium stretches right to the tip of the tail, and to the tips of the fingers and toes, suggests that the colugos are more deeply committed to the gliding way of life than other mammalian gliders. And indeed, they are pretty inept on the ground. They more than make up for it in the air, where their huge parachute gives them the run of large areas of forest at high speed. This necessitates good stereoscopic vision for steering accurately at night towards a target tree, avoiding fatal collisions, and then making a precision landing. And indeed they have large stereoscopic eyes, excellent for night vision.

Colugos and tree shrews have unusual reproductive systems, but in very different directions. Colugos resemble marsupials in that their young are born early in embryonic development. Having no marsupial pouch, the mother presses the patagium into service. The tail region of the patagium is folded forwards to form a makeshift pouch in which the (usually single) young sits. The mother often hangs upside down from a branch like a sloth, and the patagium then looks and feels like a hammock for the baby.

To be a baby colugo peeping over the edge of a warm, furry hammock sounds appealing. A baby tree shrew, on the other hand, receives perhaps less maternal care than any other baby mammal. The mother tree shrew, at least in several of the species, has two nests, one in which she herself lives, the other in which the babies are deposited. She visits them only to feed them, and then only for the briefest possible time, between five and ten minutes. And she visits them for this brief feed only once in every 48 hours. In the meantime, with no mother to keep them warm as any other baby mammal would have, the little tree shrews need to heat themselves from their food. To this end, the mother's milk is exceptionally rich.

The affinities of the tree shrews and the colugos, to each other and to the rest of the mammals, are subject to dispute and uncertainty. There is a lesson in that very fact, and it is the lesson of the Colugo's Tale.

THE COLUGO'S TALE.

The colugo could tell a tale of nocturnal gliding through the forests of South East Asia. But for the purposes of our pilgrimage it has a more down-to-earth tale to tell, whose moral is a warning. It is the warning that our apparently tidy story of concestors, rendezvous points, and the sequence in which pilgrims join us, is heavily subject to disagreement and revision as new research is done. The phylogeny diagram at Rendezvous 9 Rendezvous 9 shows one recently supported view. According to this view, which I am provisionally accepting here, the pilgrims we primates greet at shows one recently supported view. According to this view, which I am provisionally accepting here, the pilgrims we primates greet at Rendezvous 9 Rendezvous 9 are an already united band consisting of the colugos and the tree shrews. A few years ago, the colugos would not have entered into this picture. Orthodox taxonomy would have had the tree shrews alone joining the primates at this rendezvous: the colugos would have joined us further down the road, not even very close. are an already united band consisting of the colugos and the tree shrews. A few years ago, the colugos would not have entered into this picture. Orthodox taxonomy would have had the tree shrews alone joining the primates at this rendezvous: the colugos would have joined us further down the road, not even very close.

There is no guarantee that our present picture will stay settled. New evidence may resurrect our previous view, or it may prompt a completely different one. Some researchers even think the colugos are closer to the primates than the tree shrews are. If they are right, Rendezvous 9 Rendezvous 9 is where we primates are joined by the colugos. We'd have to wait for the tree shrews at is where we primates are joined by the colugos. We'd have to wait for the tree shrews at Rendezvous 10 Rendezvous 10, and the numbering of concestors from then on would need to be increased by one. But that is not the view I have adopted. Doubt and uncertainty may seem rather unsatisfactory as the moral for a tale, but it is an important lesson that must be taken on board before our pilgrimage to the past proceeds much further. The lesson will apply to many other rendezvous.

I could have signalled my uncertainty by having multi-way splits ('polytomies': see the Gibbon's Tale) in my phylogenetic trees. This is the solution adopted by certain authors, notably Colin Tudge in his masterly phylogenetic summary of all life on Earth, The Variety Of Life The Variety Of Life. But having polytomies on some branches risks giving false confidence in the others. The revolution in mammalian systematics involving the laurasiatheres and afrotheres (Rendezvous 11 to to 13 13) happened after Tudge's book was published, as recently as 2000, and so some areas of his cla.s.sification which he considered resolved have now been transformed. Were he to bring out a new edition, it would surely be radically changed. Very possibly the same will happen with this book, and it isn't just the colugos and tree shrews. The position of tarsiers (Rendezvous 7), and the grouping of lampreys with hagfishes (Rendezvous 22) are unsure. The affinities of the afrotheres (Rendezvous 13) and the coelacanths (Rendezvous 19) are still slightly unsure. The ordering of our rendezvous with cnidarians and ctenoph.o.r.es (Rendezvous 28 and and 29 29) could be the wrong way round.

Other rendezvous, such as that with the orang utans, are as near certain as it is possible to be, and there are many more in that happy category. There are also some borderline cases. So, rather than make what comes close to a subjective judgement about which groups deserve fully resolved trees and which do not, I have nailed my more-or-less uncertain colours to the mast in 2004, explaining the doubts in the text whenever possible (apart from a single rendezvous, number 37, where the order is so unsure that even the experts are not willing to hazard a guess). In the fullness of time, I fear that some (but relatively few, I hope) of my rendezvous points and their phylogenies will turn out to be wrong, in the light of new evidence.1 Earlier systems of taxonomy that were not tied to the evolution-standard might be controversial, in the way that matters of taste or judgement are controversial. A taxonomist might argue that, for reasons of convenience in exhibiting museum specimens, tree shrews should be grouped with shrews and colugos with flying squirrels. In such judgements there is no absolutely right answer. The phyletic taxonomy adopted in this book is different. There is a correct tree of life,2 but we don't yet know what it is. There is still room for human judgement, but it is judgement about what will eventually turn out to be the undisputable truth. It is only because we haven't looked at enough details yet, especially molecular details, that we are still unsure what that truth is. The truth really is hanging up there waiting to be discovered. The same cannot be said for judgements of taste or of museum convenience. but we don't yet know what it is. There is still room for human judgement, but it is judgement about what will eventually turn out to be the undisputable truth. It is only because we haven't looked at enough details yet, especially molecular details, that we are still unsure what that truth is. The truth really is hanging up there waiting to be discovered. The same cannot be said for judgements of taste or of museum convenience.

1 Creationist misquotation alert: Creationists, please do not quote this as indicating that 'the evolutionists can't agree about anything' with the implication that the whole ma.s.sive underlying theory can therefore be thrown out. Creationist misquotation alert: Creationists, please do not quote this as indicating that 'the evolutionists can't agree about anything' with the implication that the whole ma.s.sive underlying theory can therefore be thrown out.

2 With the slight reservation that this tree will actually be a majority consensus among gene trees, as explained in the closing paragraphs of the Gibbon's Tale. With the slight reservation that this tree will actually be a majority consensus among gene trees, as explained in the closing paragraphs of the Gibbon's Tale.

Rendezvous 10.

RODENTS AND RABBITKIND.

Rendezvous 10 occurs 75 million years into our journey. It is here that our pilgrims are joined overwhelmed, rather by a teeming, scurrying, gnawing, whisker-quivering plague of rodents. For good measure, we also greet at this point the rabbits, including the very similar hares and jack-rabbits, and the rather more distant pikas. Rabbits were once cla.s.sified as rodents, because they also have very prominent gnawing teeth at the front indeed they outpoint the rodents, with an extra pair. They were then separated off, and are still placed in their own order, Lagomorpha, as opposed to Rodentia. But modern authorities group the lagomorphs together with the rodents in a 'cohort' called Glires. In the terms of this book, the lagomorph pilgrims and the rodent pilgrims joined up with each other 'before' the whole lot of them joined our pilgrimage. Concestor 10 is approximately our 15-million-greats-grandparent. It is the latest ancestor we share with a mouse, but the mouse is connected to it through a very much larger number of greats, because of short generation times. occurs 75 million years into our journey. It is here that our pilgrims are joined overwhelmed, rather by a teeming, scurrying, gnawing, whisker-quivering plague of rodents. For good measure, we also greet at this point the rabbits, including the very similar hares and jack-rabbits, and the rather more distant pikas. Rabbits were once cla.s.sified as rodents, because they also have very prominent gnawing teeth at the front indeed they outpoint the rodents, with an extra pair. They were then separated off, and are still placed in their own order, Lagomorpha, as opposed to Rodentia. But modern authorities group the lagomorphs together with the rodents in a 'cohort' called Glires. In the terms of this book, the lagomorph pilgrims and the rodent pilgrims joined up with each other 'before' the whole lot of them joined our pilgrimage. Concestor 10 is approximately our 15-million-greats-grandparent. It is the latest ancestor we share with a mouse, but the mouse is connected to it through a very much larger number of greats, because of short generation times.

Rodents are one of the great success stories of mammaldom. More than 40 per cent of all mammal species are rodents, and there are said to be more individual rodents in the world than all other mammals combined. Rats and mice have been the hidden beneficiaries of our own Agricultural Revolution, and they have travelled with us across the seas to every land in the world. They devastate our granaries and our health. Rats and their cargo of fleas were responsible for the Great Plague (traditionally, but now controversially, the Black Death may also have been bubonic plague), they have spread typhus, and have been blamed for more human deaths in the second millennium than all wars and revolutions put together. When even the four hors.e.m.e.n are laid low by the apocalypse, it will be rats that scavenge their remains, rats that will swarm like lemmings over the ruins of civilisation. And, by the way, lemmings are rodents, too northern voles who, for reasons that are not entirely clear, build up their populations to plague proportions in so-called 'lemming years', and then indulge in frantic though not wantonly suicidal as is falsely alleged ma.s.s migrations.

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Rodents and rabbits join. Experts generally accept that the 70 or so species of rabbit relatives and the approximately 2,000 rodents (two-thirds of which are in the mouse family) group together. Recent genetic studies place this group as the sister to the primates, colugos, and tree shrews. Parts of the branching order within the rodents are not entirely established, but a phylogeny similar to this is supported by most molecular data. Experts generally accept that the 70 or so species of rabbit relatives and the approximately 2,000 rodents (two-thirds of which are in the mouse family) group together. Recent genetic studies place this group as the sister to the primates, colugos, and tree shrews. Parts of the branching order within the rodents are not entirely established, but a phylogeny similar to this is supported by most molecular data.

Images, left to right: capybara ( capybara (Hydrochaeris hydrochaeris); Cape mole rat (Georychus capensis); Cape porcupine (Hystrix africaeaustralis); red squirrel (Sciurus vulgaris); common dormouse (Muscardinus avellanarius); springhare (Pedetes capensis); European beaver (Castor fiber); bank vole (Clethrionomys glareolus); northern birch mouse (Sicista betulina); Arctic hare (Lepus arcticus); American pika (Ochotona princeps).

Rodents are gnawing machines. They have a pair of very prominent incisor teeth at the front, perpetually growing to replace ma.s.sive wear and tear. The gnawing ma.s.seter muscles are especially well developed in rodents. They don't have canine teeth, and the large gap or diastema that separates their incisors from their back teeth improves the efficiency of their gnawing. Rodents can gnaw their way through almost anything. Beavers fell substantial trees by gnawing through their trunks. Mole rats live entirely underground, tunnelling, not with their front paws like moles, but purely with their incisor teeth.1 Different species of rodents have penetrated the deserts of the world (gundis, gerbils), the high mountains (marmots, chinchillas), the forest canopy (squirrels, including flying squirrels), rivers (water voles, beavers, capybaras), rainforest floor (agoutis), savannah (maras, springhares), and Arctic tundra (lemmings). Different species of rodents have penetrated the deserts of the world (gundis, gerbils), the high mountains (marmots, chinchillas), the forest canopy (squirrels, including flying squirrels), rivers (water voles, beavers, capybaras), rainforest floor (agoutis), savannah (maras, springhares), and Arctic tundra (lemmings).

Most rodents are mouse-sized, but they range up through marmots, beavers, agoutis and maras to the sheep-sized capybaras of the South American waterways. Capybaras are prized for meat, not just because of their large size but because, bizarrely, the Roman Catholic Church traditionally deemed them honorary fish for Fridays, presumably because they live in water. Large as they are, modern capybaras are dwarfed by various giant South American rodents that went extinct only quite recently. The giant capybara, Protohydroch.o.e.rus Protohydroch.o.e.rus, was the size of a donkey. Telicomys Telicomys was an even larger rodent the size of a small rhinoceros which, like the giant capybara, went extinct at the time of the Great American Interchange, when the Isthmus of Panama ended South America's island status. These two groups of giant rodents were not particularly closely related to each other, and seem to have evolved their gigantism independently. was an even larger rodent the size of a small rhinoceros which, like the giant capybara, went extinct at the time of the Great American Interchange, when the Isthmus of Panama ended South America's island status. These two groups of giant rodents were not particularly closely related to each other, and seem to have evolved their gigantism independently.

A world without rodents would be a very different world. It is less likely to come to pa.s.s than a world dominated by rodents and free of people. If nuclear war destroys humanity and most of the rest of life, a good bet for survival in the short term, and for evolutionary ancestry in the long term, is rats. I have a post-Armageddon vision. We and all other large animals are gone. Rodents emerge as the ultimate post-human scavengers. They gnaw their way through New York, London and Tokyo, digesting spilled larders, ghost supermarkets and human corpses and turning them into new generations of rats and mice, whose racing populations explode out of the cities and into the countryside. When all the relics of human profligacy are eaten, populations crash again, and the rodents turn on each other, and on the c.o.c.kroaches scavenging with them. In a period of intense compet.i.tion, short generations perhaps with radioactively enhanced mutation-rates boost rapid evolution. With human ships and planes gone, islands become islands again, with local populations isolated save for occasional lucky raftings: ideal conditions for evolutionary divergence. Within 5 million years, a whole range of new species replace the ones we know. Herds of giant grazing rats are stalked by sabretoothed predatory rats.2 Given enough time, will a species of intelligent, cultivated rats emerge? Will rodent historians and scientists eventually organise careful archaeological digs (gnaws?) through the strata of our long-compacted cities, and reconstruct the peculiar and temporarily tragic circ.u.mstances that gave ratkind its big break? Given enough time, will a species of intelligent, cultivated rats emerge? Will rodent historians and scientists eventually organise careful archaeological digs (gnaws?) through the strata of our long-compacted cities, and reconstruct the peculiar and temporarily tragic circ.u.mstances that gave ratkind its big break?

THE MOUSE'S TALE.

Of all the thousands of rodents, the house mouse, Mus musculus Mus musculus, has a special tale to tell because it has become the second most intensively studied mammal species after our own. Much more than the proverbial guinea pig, the mouse is a main staple of medical, physiological and genetic laboratories the world over. In particular, the mouse is one of very few mammals apart from ourselves whose genome has so far been completely sequenced.

Two things about these recently sequenced genomes have sparked unwarranted surprise. The first is that mammal genomes seem rather small: of the order of 30,000 genes or maybe even less. And the second is that they are so similar to each other. Human dignity seemed to demand that our genome should be much larger than that of a tiny mouse. And shouldn't it be absolutely larger than 30,000 genes anyway?

This last expectation has led people, including some who should know better, to deduce that the 'environment' must be more important than we thought, because there aren't enough genes to specify a body. That really is a breathtakingly naive piece of logic. By what standard do we decide decide how many genes you need to specify a body? This kind of thinking is based on a subconscious a.s.sumption which is wrong: the a.s.sumption that the genome is a kind of blueprint, with each gene specifying its own little piece of body. As the Fruit Fly's Tale will tell us, it is not a blueprint, but something more like a recipe, a computer program, or a manual of instructions for a.s.sembly. how many genes you need to specify a body? This kind of thinking is based on a subconscious a.s.sumption which is wrong: the a.s.sumption that the genome is a kind of blueprint, with each gene specifying its own little piece of body. As the Fruit Fly's Tale will tell us, it is not a blueprint, but something more like a recipe, a computer program, or a manual of instructions for a.s.sembly.

If you think of the genome as a blueprint, you might expect a big, complicated animal like yourself to have more genes than a little mouse, with fewer cells and a less sophisticated brain. But, as I said, that isn't the way genes work. Even the recipe or instruction-book model can be misleading unless it is properly understood. My colleague Matt Ridley develops a different a.n.a.logy which I find beautifully clear, in his book Nature via Nurture Nature via Nurture. Most of the genome that we sequence is not the book of instructions, or master computer program, for building a human or a mouse, although parts of it are. If it were, we might indeed expect our program to be larger than the mouse's. But most of the genome is more like the dictionary of words available for writing the book of instructions or, we shall soon see, the set of subroutines that are called by the master program. As Ridley says, the list of words in David Copperfield David Copperfield is almost the same as the list of words in is almost the same as the list of words in The Catcher in the Rye The Catcher in the Rye. Both draw upon the vocabulary of an educated native speaker of English. What is completely different about the two books is the order in which those words are strung together.

When a person is made, or when a mouse is made, both embryologies draw upon the same dictionary of genes: the normal vocabulary of mammal embryologies. The difference between a person and a mouse comes out of the different orders with which the genes, drawn from that shared mammalian vocabulary, are deployed, the different places in the body where this happens, and its timing. All this is under the control of particular genes whose business it is to turn other genes on, in complicated and exquisitely timed cascades. But such controlling genes const.i.tute only a minority of the genes in the genome.

Don't misunderstand 'order' as meaning the order in which the genes are strung out along the chromosomes. With notable exceptions, which we shall meet in the Fruit Fly's Tale, the order of genes along a chromosome is as arbitrary as the order in which words are listed in a vocabulary usually alphabetical but, especially in phrase books for foreign travel, sometimes an order of convenience: words useful in airports; words useful when visiting the doctor; words useful for shopping, and so on. The order in which genes are stored on chromosomes is unimportant. What matters is that the cellular machinery finds the right gene when it needs it, and it does this using methods that are becoming increasingly understood. In the Fruit Fly's Tale, we'll return to those few cases, very interesting ones, where the order of genes arranged on the chromosome is non-arbitrary in something like the foreign phrase-book sense. For now, the important point is that what distinguishes a mouse from a man is mostly not the genes themselves, nor the order in which they are stored in the chromosomal 'phrase-book', but the order in which they are turned on: the equivalent of d.i.c.kens or Salinger choosing words from the vocabulary of English and arranging them in sentences.

In one respect the a.n.a.logy of words is misleading. Words are shorter than genes, and some writers have likened each gene to a sentence. But sentences aren't a good a.n.a.logy, for a different reason. Different books are not put together by permuting a fixed repertoire of sentences. Most sentences are unique. Genes, like words but unlike sentences, are used over and over again in different contexts. A better a.n.a.logy for a gene than either a word or a sentence is a toolbox subroutine in a computer.

The computer I happen to be familiar with is the Macintosh, and it is some years since I did any programming so I am certainly out of date with the details. Never mind the principle remains, and it is true of other computers too. The Mac has a toolbox of routines stored in ROM (Read Only Memory) or in System files permanently loaded at start-up time. There are thousands of these toolbox routines, each one doing a particular operation, which is likely to be needed, over and over again, in slightly different ways, in different programs. For example the toolbox routine called ObscureCursor hides the cursor from the screen until the next time the mouse is moved. Unseen to you, the ObscureCursor 'gene' is called every time you start typing and the mouse cursor vanishes. Toolbox routines lie behind the familiar features shared by all programs on the Mac (and their imitated equivalents on Windows machines): pulldown menus, scrollbars, shrinkable windows that you can drag around the screen with the mouse, and many others.

The reason all Mac programs have the same 'look and feel' (that very similarity famously became the subject of litigation) is precisely that all Mac programs, whether written by Apple, or by Microsoft, or by anybody else, call the same toolbox routines. If you are a programmer who wishes to move a whole region of the screen in some direction, say following a mouse drag, you would be wasting your time if you didn't invoke the ScrollRect toolbox routine. Or if you want to place a check mark by a pulldown menu item, you would be mad to write your own code to do it. Just write a call of CheckItem into your program, and the job is done for you. If you look at the text of a Mac program, whoever wrote it, in whatever programming language and for whatever purpose, the main thing you'll notice is that it consists largely of invocations of familiar, built-in toolbox routines. The same repertoire of routines is available to all programmers. Different programs string calls of these routines together in different combinations and sequences.

The genome, sitting in the nucleus of every cell, is the toolbox of DNA routines available for performing standard biochemical functions. The nucleus of a cell is like the ROM of a Mac. Different cells, for example liver cells, bone cells and muscle cells, string 'calls' of these routines together in different orders and combinations when performing particular cell functions including growing, dividing, or secreting hormones. Mouse bone cells are more similar to human bone cells than they are to mouse liver cells they perform very similar operations and need to call the same repertoire of toolbox routines in order to do so. This is the kind of reason why all mammal genomes are approximately the same size as each other they all need the same toolbox.

Nevertheless, mouse bone cells do behave differently from human bone cells; and this too will be reflected in different calls to the toolbox in the nucleus. The toolbox itself is not identical in mouse and man, but it might as well be identical without in principle jeopardising the main differences between the two species. For the purpose of building mice differently from humans, what matters is differences in the calling of toolbox routines, more than differences in the toolbox routines themselves.

THE BEAVER'S TALE.

A 'phenotype' is that which is influenced by genes. That pretty much means everything about a body. But there is a subtlety of emphasis which flows from the word's etymology. Phaino Phaino is Greek for 'show', 'bring to light', 'make appear', 'exhibit', 'uncover', 'disclose', 'manifest'. The phenotype is the external and visible manifestation of the hidden genotype. The is Greek for 'show', 'bring to light', 'make appear', 'exhibit', 'uncover', 'disclose', 'manifest'. The phenotype is the external and visible manifestation of the hidden genotype. The Oxford English Dictionary Oxford English Dictionary defines it as 'the sum total of the observable features of an individual, regarded as the consequence of the interaction of its genotype with its environment' but it precedes this definition by a subtler one: 'A type of organism distinguishable from others by observable features.' defines it as 'the sum total of the observable features of an individual, regarded as the consequence of the interaction of its genotype with its environment' but it precedes this definition by a subtler one: 'A type of organism distinguishable from others by observable features.'

Darwin saw natural selection as the survival and reproduction of certain types of organism at the expense of rival types of organism. 'Types' here doesn't mean groups or races or species. In the subt.i.tle of The Origin of Species The Origin of Species, the much misunderstood phrase 'preservation of favoured races' most emphatically does not mean races in the normal sense. Darwin was writing before genes were named or properly understood, but in modern terms what he meant by 'favoured races' was 'possessors of favoured genes'.

Selection drives evolution only to the extent that the alternative types owe their differences to genes: if the differences are not inherited, differential survival has no impact on future generations. For a Darwinian, phenotypes are the manifestations by which genes are judged by selection. When we say that a beaver's tail is flattened to serve as a paddle, we mean that genes whose phenotypic expression included a flattening of the tail survived by virtue of that phenotype. Individual beavers with the flat-tailed phenotype survived as a consequence of being better swimmers; the responsible genes survived inside them, and were pa.s.sed on to new generations of flat-tailed beavers.

At the same time, genes that expressed themselves in huge, sharp incisor teeth capable of gnawing through wood also survived. Individual beavers are built by permutations of genes in the beaver gene pool. Genes have survived through generations of ancestral beavers because they have proved good at collaborating with other genes in the beaver gene pool, to produce phenotypes that flourish in the beaver way of life.

At the same time again, alternative co-operatives of genes are surviving in other gene pools, making bodies that survive by prosecuting other life trades: the tiger co-operative, the camel co-operative, the c.o.c.kroach co-operative, the carrot co-operative. My first book, The Selfish Gene The Selfish Gene, could equally have been called The Cooperative Gene The Cooperative Gene without a word of the book itself needing to be changed. Indeed, this might have saved some misunderstanding (some of a book's most vocal critics are content to read the book by t.i.tle only). Selfishness and co-operation are two sides of a Darwinian coin. Each gene promotes its own selfish welfare, by co-operating with the other genes in the s.e.xually stirred gene pool which is that gene's environment, to build shared bodies. without a word of the book itself needing to be changed. Indeed, this might have saved some misunderstanding (some of a book's most vocal critics are content to read the book by t.i.tle only). Selfishness and co-operation are two sides of a Darwinian coin. Each gene promotes its own selfish welfare, by co-operating with the other genes in the s.e.xually stirred gene pool which is that gene's environment, to build shared bodies.

But beaver genes have special phenotypes quite unlike those of tigers, camels or carrots. Beavers have lake phenotypes, caused by dam phenotypes. A lake is an extended phenotype extended phenotype. The extended phenotype is a special kind of phenotype, and it is the subject of the rest of this tale, which is a brief summary of my book of that t.i.tle. It is interesting not only in its own right but because it helps us to understand how conventional phenotypes develop. It will turn out that there is no great difference of principle between an extended phenotype like a beaver lake, and a conventional phenotype like a flattened beaver tail.

How can it possibly be right to use the same word, phenotype, on the one hand for a tail of flesh, bone and blood, and on the other hand for a body of still water, stemmed in a valley by a dam? The answer is that both are manifestations of beaver genes; both have evolved to become better and better at preserving those genes; both are linked to the genes they express by a similar chain of embryological causal links. Let me explain.

The embryological processes by which beaver genes shape beaver tails are not known in detail, but we know the kind of thing that goes on. Genes in every cell of a beaver behave as if they 'know' what kind of cell they are in. Skin cells have the same genes as bone cells, but different genes are switched on in the two tissues. We saw this in the Mouse's Tale. Genes, in each of the different kinds of cells in a beaver's tail, behave as if they 'know' where they are. They cause their respective cells to interact with each other in such a way that the whole tail a.s.sumes its characteristically hairless flattened form. There are formidable difficulties in working out how they 'know' which part of the tail they are in, but we understand in principle how these difficulties are overcome; and the solutions, like the difficulties themselves, will be of the same general kind when we turn to the development of tiger feet, camel humps and carrot leaves.

They are also of the same general kind in the development of the neuronal and neurochemical mechanisms that drive behaviour. Copulatory behaviour in beavers is instinctive. A male beaver's brain orchestrates, via hormonal secretions into the blood, and via nerves controlling muscles tugging on artfully hinged bones, a symphony of movements. The result is precise co-ordination with a female, who herself is moving harmoniously in her own symphony of movements, equally carefully orchestrated to facilitate the union. You may be sure that such exquisite neuromuscular music has been honed and perfected by generations of natural selection. And that means selection of genes. In beaver gene pools, genes survived whose phenotypic effects on the brains, the nerves, the muscles, the glands, the bones, and the sense organs of generations of ancestral beavers improved the chances of those very genes pa.s.sing through those very generations to arrive in the present.

Genes 'for' behaviour survive in the same kind of way as genes 'for' bones, and skin. Do you protest that there aren't 'really' any genes for behaviour; only genes for the nerves and muscles that make the behaviour? You are still wrecked among heathen dreams. Anatomical structures have no special status over behavioural ones, where 'direct' effects of genes are concerned. Genes are 'really' or 'directly' responsible only for proteins or other immediate biochemical effects. All other effects, whether on anatomical or behavioural phenotypes, are indirect. But the distinction between direct and indirect is vacuous. What matters in the Darwinian sense is that differences differences between genes are rendered as between genes are rendered as differences differences in phenotypes. It is only differences that natural selection cares about. And, in very much the same way, it is differences that geneticists care about. in phenotypes. It is only differences that natural selection cares about. And, in very much the same way, it is differences that geneticists care about.

Remember the 'subtler' definition of phenotype in the Oxford English Dictionary: Oxford English Dictionary: 'A type of organism distinguishable from others by observable features.' The key word is distinguishable. A gene 'for' brown eyes is not a gene that directly codes the synthesis of a brown pigment. Well, it might happen to be, but that is not the point. The point about a gene 'for' brown eyes is that its possession makes a 'A type of organism distinguishable from others by observable features.' The key word is distinguishable. A gene 'for' brown eyes is not a gene that directly codes the synthesis of a brown pigment. Well, it might happen to be, but that is not the point. The point about a gene 'for' brown eyes is that its possession makes a difference difference to eye colour to eye colour when compared when compared with some alternative version of the gene an 'allele'. The chains of causation that culminate in the difference between one phenotype and another, say between brown and blue eyes, are usually long and tortuous. The gene makes a protein which is different from the protein made by the alternative gene. The protein has an enzymatic effect on cellular chemistry, which affects X which affects Y which affects Z which affects ... a long chain of intermediate causes which affects ... the phenotype of interest. The allele makes the with some alternative version of the gene an 'allele'. The chains of causation that culminate in the difference between one phenotype and another, say between brown and blue eyes, are usually long and tortuous. The gene makes a protein which is different from the protein made by the alternative gene. The protein has an enzymatic effect on cellular chemistry, which affects X which affects Y which affects Z which affects ... a long chain of intermediate causes which affects ... the phenotype of interest. The allele makes the difference difference when its phenotype is compared with the corresponding phenotype, at the end of the correspondingly long chain of causation that proceeds from the alternative allele. Gene differences cause phenotypic differences. Gene changes cause phenotypic changes. In Darwinian evolution alleles are selected, vis a vis alternative alleles, by virtue of the differences in their effects on phenotypes. when its phenotype is compared with the corresponding phenotype, at the end of the correspondingly long chain of causation that proceeds from the alternative allele. Gene differences cause phenotypic differences. Gene changes cause phenotypic changes. In Darwinian evolution alleles are selected, vis a vis alternative alleles, by virtue of the differences in their effects on phenotypes.

The beaver's point is that this comparison between phenotypes can happen anywhere along the chain of causation. All intermediate links along the chain are true phenotypes, and any one of them could const.i.tute the phenotypic effect by which a gene is selected: it only has to be 'visible' to natural selection, n.o.body cares whether it is visible to us. There is no such thing as the 'ultimate' link in the chain: no final, definitive phenotype. Any consequence of a change in alleles, anywhere in the world, however indirect and however long the chain of causation, is fair game for natural selection, so long as it impinges on the survival of the responsible allele, relative to its rivals.

Now, let's look at the embryological chain of causation leading to dam-building in beavers. Dam-building behaviour is a complicated stereotypy, built into the brain like a fine-tuned clockwork mechanism. Or, as if to follow the history of clocks into the electronic age, dam-building is hard wired in the brain. I have seen a remarkable film of captive beavers imprisoned in a bare, unfurnished cage, with no water and no wood. The beavers enacted, 'in a vacuum', all the stereotyped movements normally seen in natural building behaviour when there is real wood and real water. They seem to be placing virtual wood into a virtual dam wall, pathetically trying to build a ghost wall with ghost sticks, all on the hard, dry, flat floor of their prison. One feels sorry for them: it is as if they are desperate to exercise their frustrated dam-building clockwork.

Only beavers have this kind of brain clockwork. Other species have clockwork for copulation, scratching and fighting, and so do beavers. But only beavers have brain clockwork for dam-building, and it must have evolved by slow degrees in ancestral beavers. It evolved because the lakes produced by dams are useful. It is not totally clear what they are useful for, but they must have been useful for the beavers who built them, not just any old beavers. The best guess seems to be that a lake provides a beaver with a safe place to build its lodge, out of reach for most predators, and a safe conduit for transporting food. Whatever the advantage it must be a substantial one, or beavers would not devote so much time and effort to building dams. Once again, note that natural selection is a predictive theory. The Darwinian can make the confident prediction that, if dams were a useless waste of time, rival beavers who refrained from building them would survive better and pa.s.s on genetic tendencies not to build. The fact that beavers are so anxious to build dams is very strong evidence that it benefited their ancestors to do so.

Like any other useful adaptation, the dam-building clockwork in the brain must have evolved by Darwinian selection of genes. There must have been genetic variations in the wiring of the brain which affected dam-building. Those genetic variants that resulted in improved dams were more likely to survive in beaver gene pools. It is the same story as for all Darwinian adaptations. But which is the phenotype? At which link in the chain of causal links shall we say the genetic difference exerts its effect? The answer, to repeat it, is all links where a difference is seen. In the wiring diagram of the brain? Yes, almost certainly. In the cellular chemistry that, in embryonic development, leads to that wiring? Of course. But also behaviour behaviour the symphony of muscular contractions that is behaviour this too is a perfectly respectable phenotype. Differences in building behaviour are without doubt manifestations of differences in genes. And, by the same token, the the symphony of muscular contractions that is behaviour this too is a perfectly respectable phenotype. Differences in building behaviour are without doubt manifestations of differences in genes. And, by the same token, the consequences consequences of that behaviour are also entirely allowable as phenotypes of genes. What consequences? Dams, of course. And lakes, for these are consequences of dams. Differences between lakes are influenced by differences between dams, just as differences between dams are influenced by differences between behaviour patterns, which in turn are consequences of differences between genes. We may say that the characteristics of a dam, or of a lake, are true phenotypic effects of genes, using exactly the logic we use to say that the characteristics of a tail are phenotypic effects of genes. of that behaviour are also entirely allowable as phenotypes of genes. What consequences? Dams, of course. And lakes, for these are consequences of dams. Differences between lakes are influenced by differences between dams, just as differences between dams are influenced by differences between behaviour patterns, which in turn are consequences of differences between genes. We may say that the characteristics of a dam, or of a lake, are true phenotypic effects of genes, using exactly the logic we use to say that the characteristics of a tail are phenotypic effects of genes.

Conventionally, biologists see the phenotypic effects of a gene as confined within the skin of the individual bearing that gene. The Beaver's Tale shows that this is unnecessary. The phenotype of a gene, in the true sense of the word, may extend outside the skin of the individual. Birds' nests are extended phenotypes. Their shape and size, their complicated funnels and tubes where these exist, all are Darwinian adaptations, and so must have evolved by the differential survival of alternative genes. Genes for building behaviour? Yes. Genes for wiring up the brain so it is good at building nests of the right shape and size? Yes. Genes for nests of the right shape and size? Yes, by the same token, yes. Nests are made of gra.s.s or sticks or mud, not bird cells. But the point is irrelevant to the question of whether differences between nests are influenced by differences between genes. If they are, nests are proper phenotypes of genes. And nest differences surely must be influenced by gene differences, for how else could they have been improved by natural selection?

Artefacts like nests and dams (and lakes) are easily understood examples of extended phenotypes (see plate 7) (see plate 7). There are others where the logic is a little more ... well, extended. For example, parasite genes can be said to have phenotypic expression in the bodies of their hosts. This can be true even where, as in the case of cuckoos, they don't live inside their hosts. And many examples of animal communication as when a male canary sings to a female and her ovaries grow can be rewritten in the language of the extended phenotype. But that would take us too far from the beaver, whose tale will conclude with one final observation. Under favourable conditions the lake of a beaver can span several miles, which may make it the largest phenotype of any gene in the world.