The Greatest Show On Earth, The Evidence For Evolution Part 4

Male crab showing narrow, folded-back abdomen To get an idea of some of the wonderful ways in which the crustacean body is modified in detail, while the body plan itself is not modified at all, look at the set of drawings opposite by the famous nineteenth-century zoologist Ernst Haeckel, perhaps Darwin's most devoted disciple in Germany (the devotion was not reciprocated, but even Darwin would surely have admired Haeckel's draughtsmans.h.i.+p). Just as we did with the vertebrate skeleton, look at each body part of these crabs and crayfish, and see how, without fail, you can find its exact opposite number in all the rest. Every bit of the exoskeleton is joined to the 'same' bits, but the shapes of the bits themselves are very different. Once again, the 'skeleton' is invariant, while its parts are anything but. And once again the obvious I would say the only sensible interpretation is that all these crustaceans have inherited the plan of their skeleton from a common ancestor. They have moulded the individual components into a rich variety of shapes. But the plan itself remains, exactly as inherited from the ancestor.

WHAT WOULD D'ARCY THOMPSON HAVE DONE WITH A COMPUTER?

In 1917 the great Scottish zoologist D'Arcy Thompson wrote a book called On Growth and Form On Growth and Form, in the last chapter of which he introduced his famous 'method of transformations'.* He would draw an animal on graph paper, and then he would distort the graph paper in a mathematically specifiable way and show that the form of the original animal had turned into another, related animal. You could think of the original graph paper as a piece of rubber, on which you draw your first animal. Then the transformed graph paper would be equivalent to the same piece of rubber, stretched or pulled out of shape in some mathematically defined way. For example, he took six species of crab and drew one of them, He would draw an animal on graph paper, and then he would distort the graph paper in a mathematically specifiable way and show that the form of the original animal had turned into another, related animal. You could think of the original graph paper as a piece of rubber, on which you draw your first animal. Then the transformed graph paper would be equivalent to the same piece of rubber, stretched or pulled out of shape in some mathematically defined way. For example, he took six species of crab and drew one of them, Geryon Geryon, on ordinary graph paper (the undistorted sheet of rubber). He then distorted his mathematical 'rubber sheet' in five separate ways, to achieve an approximate representation of the other five species of crab. The details of the mathematics don't matter, although they are fascinating. What you can clearly see is that it doesn't take much to transform one crab into another. D'Arcy Thompson himself wasn't very interested in evolution, but it is easy for us to imagine what the genetic mutations would have to do in order to bring about changes like this. That doesn't mean we should think of Geryon Geryon, or any other one of these six crabs, as being ancestral to the others. None of them was, and in any case that is not the point. The point is that whatever the ancestral crab looked like, transformations of this kind kind could change any one of these six species (or a putative ancestor) into any other. could change any one of these six species (or a putative ancestor) into any other.

Haeckel's crustaceans. Ernst Haeckel was a distinguished German zoologist and an excellent zoological artist.

Evolution never happened by taking one adult form and coaxing it into the shape of another. Remember that every adult grows as an embryo. The mutations selected would have worked in the developing embryo by changing the rate of growth of parts of the body relative to other parts. In Chapter 7 we interpreted the evolution of the human skull as a series of changes in the rates of growth of some parts relative to other parts, controlled by genes in the developing embryo. We should expect, therefore, that if we draw a human skull on a sheet of 'mathematical rubber', it should be possible to distort the rubber in some mathematically tidy way and achieve an approximate likeness to the skull of a close cousin, such as a chimpanzee, or perhaps with a bigger distortion a more distant cousin such as a baboon. And this is just what D'Arcy Thompson showed. Note, once again, that it was an arbitrary decision to draw the human skull first, and then transform it into the chimpanzee and the baboon. He could equally well have drawn, say, the chimpanzee first and then worked out the necessary distortions to make the human and the baboon. Or, more interestingly for a book on evolution, which his was not, he might have drawn, say, an Australopithecus Australopithecus skull first on the undistorted rubber, and worked out how to transform it to make a modern human skull. This would surely have worked just as well as the pictures above, and it would have been evolutionarily meaningful in a more direct way. skull first on the undistorted rubber, and worked out how to transform it to make a modern human skull. This would surely have worked just as well as the pictures above, and it would have been evolutionarily meaningful in a more direct way.

D'Arcy Thompson's crab 'transformations'

D'Arcy Thompson's skull 'transformation'

At the beginning of this chapter I introduced the idea of 'h.o.m.ology', using the arms of bats and humans as an example. Indulging an idiosyncratic use of language, I said that the skeletons were identical while the bones were different. D'Arcy Thompson's transformations furnish us with a way to make this idea more precise. In this formulation, two organs for example, bat hand and human hand are h.o.m.ologous if it is possible to draw one on a sheet of rubber and then distort the rubber to make the other one. Mathematicians have a word for this: 'homeomorphic'.*Zoologists recognized h.o.m.ology in pre-Darwinian times, and pre-evolutionists would describe, say, bat wings and human hands as h.o.m.ologous. If they had known enough mathematics, they would have been happy to use the word 'homeomorphic'. In post-Darwinian times, when it became generally accepted that bats and humans share a common ancestor, zoologists started to define h.o.m.ology in evolutionary terms. h.o.m.ologous resemblances are those inherited from the shared ancestor. The word 'a.n.a.logous' came to be used for resemblances due to shared function, not ancestry. For example, a bat wing and an insect wing would be described as a.n.a.logous, as opposed to the h.o.m.ologous bat wing and human arm. If we want to use h.o.m.ology as evidence for the fact of evolution, we can't use evolution to define it. For this purpose, therefore, it is convenient to revert to the pre-evolutionary definition of h.o.m.ology. The bat wing and human arm are homeomorphic: you can transform one into the other by distorting the rubber on which it is drawn. You cannot transform a bat wing into an insect wing in this way, because there are no corresponding parts. The widespread existence of homeomorphisms, which are not defined in terms of evolution, can be used as evidence for evolution. It is easy to see how evolution could go to work on any vertebrate arm and transform it into any other vertebrate arm, simply by changing relative rates of growth in the embryo.Ever since becoming acquainted with computers as a graduate student in the 1960s, I have wondered what D'Arcy Thompson might have done with a computer. The question became pressing in the 1980s, when affordable computers with screens (as opposed to just paper printers) became common. Drawing on stretched rubber and then distorting the drawing surface in a mathematical way it was just begging begging for the computer treatment! I suggested that Oxford University should bid for a grant to employ a programmer to put D'Arcy Thompson's transformations on a computer screen, and make them available in a user-friendly manner. We got the money, and employed Will Atkinson, a first-cla.s.s programmer and biologist, who became a friend and an adviser to me on my own programming projects. Once he had solved the difficult problem of programming a rich repertoire of mathematical distortions of the 'rubber', it was then a relatively simple task for him to incorporate this mathematical wizardry into a biomorph-style artificial selection program, similar to my own 'biomorph' programs, here described in Chapter 2. As with my programs, the 'player' was confronted with a screen full of animal forms, and invited to choose one of them for 'breeding', generation after generation. Once again there were 'genes' that persisted through the generations, and once again the genes influenced the form of the 'animals'. But in this case, the way the genes influenced animal form was by controlling the distortion of the 'rubber' on which an animal's form had been drawn. Theoretically, therefore, it should have been possible to start with, say, an for the computer treatment! I suggested that Oxford University should bid for a grant to employ a programmer to put D'Arcy Thompson's transformations on a computer screen, and make them available in a user-friendly manner. We got the money, and employed Will Atkinson, a first-cla.s.s programmer and biologist, who became a friend and an adviser to me on my own programming projects. Once he had solved the difficult problem of programming a rich repertoire of mathematical distortions of the 'rubber', it was then a relatively simple task for him to incorporate this mathematical wizardry into a biomorph-style artificial selection program, similar to my own 'biomorph' programs, here described in Chapter 2. As with my programs, the 'player' was confronted with a screen full of animal forms, and invited to choose one of them for 'breeding', generation after generation. Once again there were 'genes' that persisted through the generations, and once again the genes influenced the form of the 'animals'. But in this case, the way the genes influenced animal form was by controlling the distortion of the 'rubber' on which an animal's form had been drawn. Theoretically, therefore, it should have been possible to start with, say, an Australopithecus Australopithecus skull drawn on the undistorted 'rubber', and breed your way through creatures with progressively larger braincases and progressively shorter muzzles progressively more human-like, in other words. In practice it proved very difficult to do anything like that, and I think the fact is, in itself, interesting. skull drawn on the undistorted 'rubber', and breed your way through creatures with progressively larger braincases and progressively shorter muzzles progressively more human-like, in other words. In practice it proved very difficult to do anything like that, and I think the fact is, in itself, interesting.I think one reason it was difficult is, yet again, that D'Arcy Thompson's transformations change one adult adult form into another adult form. As I emphasized in Chapter 8, that is not how genes in evolution work. Every individual animal has a developmental history. It starts as an embryo and grows, by disproportionate growth of different parts of the body, into an adult. Evolution is not a genetically controlled distortion of one adult form into another; it is a genetically controlled alteration in a developmental program. Julian Huxley (grandson of T.H. and brother of Aldous) recognized this when, soon after publication of the first edition of D'Arcy Thompson's book, he modified the 'method of transformations' to study the way early embryos turn into later embryos or adults. That's all I want to say about D'Arcy Thompson's method of transformations here. I'll return to the topic in the final chapter, to make a related point. form into another adult form. As I emphasized in Chapter 8, that is not how genes in evolution work. Every individual animal has a developmental history. It starts as an embryo and grows, by disproportionate growth of different parts of the body, into an adult. Evolution is not a genetically controlled distortion of one adult form into another; it is a genetically controlled alteration in a developmental program. Julian Huxley (grandson of T.H. and brother of Aldous) recognized this when, soon after publication of the first edition of D'Arcy Thompson's book, he modified the 'method of transformations' to study the way early embryos turn into later embryos or adults. That's all I want to say about D'Arcy Thompson's method of transformations here. I'll return to the topic in the final chapter, to make a related point.Comparative evidence has always, as I suggested at the beginning of this chapter, told even more compellingly than fossil evidence in favour of the fact of evolution. Darwin himself took a similar view, at the end of his chapter in On the Origin of Species On the Origin of Species on the 'Mutual Affinities of Organic Beings': on the 'Mutual Affinities of Organic Beings':Finally, the several cla.s.ses of facts which have been considered in this chapter, seem to me to proclaim so plainly, that the innumerable species, genera, and families of organic beings, with which this world is peopled, have all descended, each within its own cla.s.s or group, from common parents, and have all been modified in the course of descent, that I should without hesitation adopt this view even if it were unsupported by other facts or arguments.

MOLECULAR COMPARISONS What Darwin didn't couldn't know is that the comparative evidence becomes even more convincing when we include molecular genetics, in addition to the anatomical comparisons that were available to him.Just as the vertebrate skeleton is invariant across all vertebrates while the individual bones differ, and just as the crustacean exoskeleton is invariant across all crustaceans while the individual 'tubes' vary, so the DNA code is invariant across all living creatures, while the individual genes themselves vary. This is a truly astounding fact, which shows more clearly than anything else that all living creatures are descended from a single ancestor. Not just the genetic code itself, but the whole gene/protein system for running life, which we dealt with in Chapter 8, is the same in all animals, plants, fungi, bacteria, archaea and viruses. What varies is what is written in the code, not the code itself. And when we look comparatively at what is written in the code the actual genetic sequences in all these different creatures we find the same kind of hierarchical tree of resemblance. We find the same family family tree tree albeit much more thoroughly and convincingly laid out as we did with the vertebrate skeleton, the crustacean skeleton, and indeed the whole pattern of anatomical resemblances through all the living kingdoms. albeit much more thoroughly and convincingly laid out as we did with the vertebrate skeleton, the crustacean skeleton, and indeed the whole pattern of anatomical resemblances through all the living kingdoms.If we want to work out how closely related any pair of species is say, how close a hedgehog is to a monkey the ideal would be to look at the complete molecular texts of every gene of both species, and compare every jot and t.i.ttle, as a biblical scholar might compare two scrolls or fragments of Isaiah. But it is time-consuming and expensive. The Human Genome Project took about ten years, representing many person-centuries. Although it would now be possible to achieve the same result in a fraction of the time, it would still be a large and expensive undertaking, as would the hedgehog genome project. Like the Apollo moon landings, and like the Large Hadron Collider (which has just been switched on in Geneva as I write the gigantic scale of this international endeavour moved me to tears when I visited), the complete deciphering of the human genome is one of those achievements that makes me proud to be human. I am delighted that the chimpanzee genome project has now been successfully accomplished, and the equivalent for various other species. If the present rate of progress continues (see 'Hodgkin's Law' below), it will soon be economically feasible to sequence the genome of every pair of species whose closeness of cousins.h.i.+p we might want to measure. Meanwhile, for the most part we have to resort to sampling particular parts of their genomes, and it works pretty well.We can sample by picking out a few choice genes (or proteins, whose sequences are directly translated from genes) and comparing them across species. I'll come to that in a moment. But there are other ways of doing a kind of crude, automatic sampling, and the technologies to do that have been around for longer. An early method, which works surprisingly well, exploits the immune system of rabbits (you could actually use any animal you like, but rabbits do the job nicely). As part of the body's natural defence against pathogens, the rabbit's immune system manufactures antibodies against any foreign protein that enters the bloodstream. Just as you could tell that I have had whooping cough by looking at the antibodies in my blood, so you can tell what a rabbit has been exposed to in the past by looking at its immune response in the present. The antibodies present in the rabbit const.i.tute a history of the natural shocks to which its flesh has been heir including artificially injected proteins. If you inject, say, a chimpanzee protein into a rabbit, the antibodies that it makes will subsequently attack the same protein if it is injected again. But suppose your second injection is of the equivalent protein, not from a chimpanzee but from a gorilla? The rabbit's prior exposure to the chimpanzee protein will have partially partially forearmed it against the gorilla version, but the reaction will be weaker. And it will also have forearmed it against the kangaroo version of the protein, but the reaction will be weaker still, given that the kangaroo is much less closely related to the chimpanzee that did the priming than the gorilla is. The strength of the rabbit's immune response to a subsequent injection of a protein is a measure of the resemblance of that protein to the original to which the rabbit was first exposed. It was by this method, using rabbits, that Vincent Sarich and Allan Wilson, at the University of California at Berkeley, demonstrated in the 1960s that humans and chimpanzees are much more closely related to each other than anybody had previously realized. forearmed it against the gorilla version, but the reaction will be weaker. And it will also have forearmed it against the kangaroo version of the protein, but the reaction will be weaker still, given that the kangaroo is much less closely related to the chimpanzee that did the priming than the gorilla is. The strength of the rabbit's immune response to a subsequent injection of a protein is a measure of the resemblance of that protein to the original to which the rabbit was first exposed. It was by this method, using rabbits, that Vincent Sarich and Allan Wilson, at the University of California at Berkeley, demonstrated in the 1960s that humans and chimpanzees are much more closely related to each other than anybody had previously realized.There are also methods that use the genes themselves, comparing them across species directly rather than comparing the proteins they encode. One of the oldest and most effective of these methods is called DNA hybridization. DNA hybridization is usually what lies behind those statements one often sees along the lines of: 'Humans and chimpanzees share 98 per cent of their genes.' There is some confusion, by the way, about exactly what is meant by percentage figures such as these. Ninety-eight per cent of what what is identical? The exact figure depends on how large the units are that we are counting. A simple a.n.a.logy makes this clear, and it does so in an interesting way, because the differences between the a.n.a.logy and the real thing are as revealing as the similarities. Suppose we have two versions of the same book and we want to compare them. Perhaps it is the book of Daniel, and we want to compare the canonical version with an ancient scroll that has just been discovered in a cave overlooking the Dead Sea. What percentage of the chapters of the two books are identical? Probably zero, for it takes only one discrepancy, anywhere in the whole chapter, for us to say the two are not identical. What percentage of their is identical? The exact figure depends on how large the units are that we are counting. A simple a.n.a.logy makes this clear, and it does so in an interesting way, because the differences between the a.n.a.logy and the real thing are as revealing as the similarities. Suppose we have two versions of the same book and we want to compare them. Perhaps it is the book of Daniel, and we want to compare the canonical version with an ancient scroll that has just been discovered in a cave overlooking the Dead Sea. What percentage of the chapters of the two books are identical? Probably zero, for it takes only one discrepancy, anywhere in the whole chapter, for us to say the two are not identical. What percentage of their sentences sentences are identical? The percentage will now be much higher. Even higher will be the percentage of words that are identical, because words have fewer letters than sentences fewer opportunities to bust the ident.i.ty. But a word resemblance is still broken if any one letter in the word differs. Therefore, if you line the two texts up side by side and compare them letter by letter, the percentage of identical letters will be even higher than the percentage of identical words. So an estimate like '98 per cent in common' doesn't mean anything unless we specify the size of the unit we are comparing. Are we counting chapters, words, letters or what? And the same is true when we compare DNA from two species. If you are comparing whole chromosomes, the percentage shared is zero, because it only takes one tiny difference, somewhere along the chromosomes, to define the chromosomes as different. are identical? The percentage will now be much higher. Even higher will be the percentage of words that are identical, because words have fewer letters than sentences fewer opportunities to bust the ident.i.ty. But a word resemblance is still broken if any one letter in the word differs. Therefore, if you line the two texts up side by side and compare them letter by letter, the percentage of identical letters will be even higher than the percentage of identical words. So an estimate like '98 per cent in common' doesn't mean anything unless we specify the size of the unit we are comparing. Are we counting chapters, words, letters or what? And the same is true when we compare DNA from two species. If you are comparing whole chromosomes, the percentage shared is zero, because it only takes one tiny difference, somewhere along the chromosomes, to define the chromosomes as different.The often-quoted figure of about 98 per cent for the shared genetic material of humans and chimps actually refers neither to numbers of chromosomes nor to numbers of whole genes, but to numbers of DNA 'letters' (technically, base pairs) that match each other within the respective human and chimp genes. But there is a pitfall. If you do the lining up navely, a missing missing letter (or an added letter), as opposed to a mistaken letter, will cause all subsequent letters to mismatch, because they will then all be staggered, one step out (until there is a mistake in the other direction to bring them back into step again). It is clearly unfair to let the estimate of discrepancies be inflated in this way. A scholar's eye, scanning two scrolls of Daniel, automatically copes with this, in a way that is hard to quantify. How can we do it with DNA? This is where we leave our a.n.a.logy with books and scrolls and go straight to the real thing because, as it happens, the real thing DNA is easier to understand than the a.n.a.logy! letter (or an added letter), as opposed to a mistaken letter, will cause all subsequent letters to mismatch, because they will then all be staggered, one step out (until there is a mistake in the other direction to bring them back into step again). It is clearly unfair to let the estimate of discrepancies be inflated in this way. A scholar's eye, scanning two scrolls of Daniel, automatically copes with this, in a way that is hard to quantify. How can we do it with DNA? This is where we leave our a.n.a.logy with books and scrolls and go straight to the real thing because, as it happens, the real thing DNA is easier to understand than the a.n.a.logy!If you gradually heat DNA, there comes a point somewhere around 85C when the bonding between the two strands of the double helix breaks, and the two helices separate. You can think of 85C, or whatever the temperature turns out to be, as a 'melting point'. If you let it cool again, each single helix spontaneously joins up again with another single helix, or fragment of single helix, wherever it finds one with which it can pair, using the ordinary base-pairing rules of the double helix. You might think that this would always be the partner from which it lately separated, and with which, of course, it is perfectly matched. Indeed it could be, but it usually isn't as tidy as that. Fragments of DNA will find other fragments with which they can pair, and they will usually not be exactly their original partners. And indeed, if you add separated fragments of DNA from another species, fragments of the single strands are quite capable of joining up with fragments of single strands from the wrong species, in just the same way as they will join up with single strands from the right species. Why should they not? It is the remarkable conclusion of the WatsonCrick molecular biology revolution that DNA is just DNA. It doesn't 'care' whether it is human DNA, chimp DNA or apple DNA. Fragments will happily pair off with complementary fragments wherever they find them. Nevertheless, the strength of bonding is not always equal. Single-stranded lengths of DNA bond more tightly with matching single strands than they do with less similar single strands. This is because more of the 'letters' of the DNA (Watson and Crick's 'bases') find themselves opposite partners with which they cannot pair. The bonding of the strands is therefore weakened like a zip fastener with some teeth missing.How shall we measure this strength of bonding, after fragments from different species have found each other and united? By an almost ludicrously simple method. We measure the 'melting point' of the bonds. You remember I said that the melting point of double-stranded DNA is about 85C. This is true of normal, properly matched double-stranded DNA, as when a strand of human DNA is 'melted' away from a complementary strand of human DNA. But when the bonding is weaker as when a human strand has bonded with a chimpanzee strand a slightly lower temperature is sufficient to break the bond. And when human DNA has bonded with DNA from a more distant cousin like a fish or a toad, an even lower temperature suffices to separate them. The difference between the melting point when a strand is bonded to one of its own kind, and the melting point when it is bonded to a strand from another species, is our measure of the genetic distance between the two species. As a rule of thumb, each decrease by 1 Celsius in the 'melting point' is approximately equivalent to a drop of 1 per cent in the number of DNA letters matched (or an increase of 1 per cent in the number of missing teeth in the zip fastener).There are complications in the method, which I haven't gone into, and tricky problems, which have ingenious solutions. For instance, if you mix human with chimp DNA, much of the fragmented human DNA will bond with other human DNA fragments, and much of the chimp DNA will bond with its own kind. How do you separate off the hybrid DNA, whose 'melting point' is what you really want to measure, from the 'same-kind' DNA? The answer is by a clever trick involving previous radioactive labelling. But the details would take us too far off our path. The main point here is that DNA hybridization is the technique that leads scientists to figures like 98 per cent for the genetic similarity between humans and chimpanzees, and it yields predictably lower percentages as you move to more distantly related pairs of animals.The newest method of measuring the similarity between a pair of matching genes from different species is the most direct, and the most expensive: actually read the sequence of letters in the genes themselves, using the same methods as were used for the Human Genome Project. Although it is still expensive to compare the entire genome, you can get a good approximation by comparing a sample of genes, and this is now increasingly done.Whichever technique we use for measuring similarity between two species, whether it is rabbit antibodies, or melting points, or direct sequencing, the next step is pretty much the same. Having obtained a single number representing the similarity between each pair of species, we then place the figures in a table. Take a set of species and write their names, in the same order, as both the column headings and the row headings. Then place the percentage similarities in the appropriate cells. The table will be triangular (half of a square) because, for example, the percentage similarity between human and dog will be the same as the similarity between dog and human. So if you filled in all of a square table each of the two halves either side of the diagonal would mirror the other.Now, what sort of results should we expect? On the evolution model we should predict that you'll find yourself putting a high score in the cell connecting human and chimpanzee; a lower score in the cell connecting human and dog. The human/dog cell should theoretically have an identical resemblance score to the chimpanzee/ dog cell because humans and chimpanzees have exactly the same degree of relation to dogs. It should be identical, too, to the monkey/ dog cell and the lemur/dog cell. This is because humans, chimpanzees, monkeys and lemurs are all connected to the dog via their common ancestor, an early primate (which probably looked a bit like a lemur). The same score should show up in the human/cat, chimpanzee/cat, monkey/cat and lemur/cat cells, because cats and dogs are related to all primates via the shared ancestor of all carnivores. There should be a much lower score ideally equally low in all the cells uniting, say, a squid with any mammal. And it shouldn't matter which mammal you choose, since all are equally distant from a squid.These are strong theoretical expectations, but there is no reason why, in practice, they should not be violated. If they were violated, it would be evidence against evolution. What actually happens turns out to be within statistical margins of error just what we should expect on the a.s.sumption that evolution has happened. This is another way of saying that, if you put the genetic distances between pairs of species on the limbs of a tree, everything adds up in a satisfying way. Of course the adding up is not quite perfect. Numerical expectations in biology are seldom realized with better than approximate accuracy.Comparative DNA (or protein) evidence can be used to decide on the evolutionary a.s.sumption which pairs of animals are closer cousins than which others. What turns this into extremely powerful evidence for evolution is that you can construct a tree of genetic resemblances separately for each gene in turn. And the important result is that every gene delivers approximately the same tree of life. Once again, this is exactly what you would expect if you were dealing with a true family tree. It is not what you would expect if a designer had surveyed the whole animal kingdom and picked and chosen or 'borrowed' the best proteins for the job, wherever in the animal kingdom they might be found.The earliest large-scale study along these lines was done by a group of geneticists in New Zealand led by Professor David Penny. Penny's group took five genes which, although not identical across all mammals, were similar enough to have earned the same name in all. The details don't matter, but for the record the five genes were those for haemoglobin A, haemoglobin B (haemoglobins give blood its red colour), fibrinopeptide A, fibrinopeptide B (fibrinopeptides are used in clotting blood) and cytochrome C (which plays an important role in cellular biochemistry). They chose eleven mammals to compare: rhesus monkey, sheep, horse, kangaroo, rat, rabbit, dog, pig, human, cow and chimpanzee.Penny and his colleagues thought statistically. They wanted to calculate the probability that, purely by chance, two molecules would yield the same family tree, if evolution wasn't true. So they tried to imagine all possible trees that could terminate in eleven descendants. It's a surprisingly large number. Even if you limit yourself to 'binary trees' (that is, trees with branches that only bifurcate no tri-furcating or higher-furcating), the total number of possible trees is more than 34 million. The scientists patiently looked at every one of the 34 million trees and compared each one with the other 33,999,999 trees. No, of course they didn't! It would take too much computer time. They did, however, devise a clever statistical approximation, a shortcut equivalent to that mammoth calculation.This is how the method of approximation worked. They took the first of the five genes, say haemoglobin-A (in all cases I use the name of the protein to stand for the gene that codes for that protein). Of all those millions of trees, they wanted to find which was the most 'parsimonious' where haemoglobin-A was concerned. Parsimonious here means 'needing to postulate the minimum amount of evolutionary change'. For example, all those thousands of trees that a.s.sumed that the closest cousin to a human was a kangaroo while humans and chimpanzees are more distantly related, proved to be very unparsimonious trees: they needed to a.s.sume a lot of evolutionary change, in order to yield the result that kangaroos and humans had a recent common ancestor. Haemoglobin-A's verdict would be along these lines:This is a terribly unparsimonious tree. Not only do I have to put in lots of mutational work in order to end up so different in humans and kangaroos, despite our close cousins.h.i.+p according to this tree, I also have to put in lots of mutational work in the other direction, in order to ensure that, despite their great separation on this particular tree, humans and chimps somehow ended up with such similar haemoglobin-A. I vote against this tree.Haemoglobin-A delivers a verdict of this kind, some verdicts more favourable than others, on each of the 34 million trees, and finally ends up choosing a few dozen top-ranking trees. Of each of these top-ranking trees, haemoglobin-A would say something like this:This tree puts humans and chimpanzees as close cousins, and it puts sheep and cows as close cousins, and it puts kangaroos out on a limb. This turns out to be a very good tree, because it makes me do hardly any mutational work at all to explain the evolutionary changes. This is an excellently parsimonious tree. It gets the haemoglobin-A vote!Of course, it would have been nice if haemoglobin-A, and every other gene, could have come up with a single most parsimonious tree, but that is too much to ask. Among the 34 million trees, it is only to be expected that several slightly different trees should tie for haemoglobin-A's top-ranking slot.Now, how about haemoglobin-B? How about cytochrome-C? Each one of the five proteins is ent.i.tled to its own separate vote, to find its own preferred (that is, most parsimonious) trees from among the 34 million trees. It would be perfectly possible for cytochrome-C to come up with a completely different vote on which is the most parsimonious tree. It could turn out that the cytochrome-C of humans really is very similar to that of kangaroos, and very different from that of chimpanzees. Far from saluting the close pairing of sheep and cow discerned by haemoglobin-A, cytochrome-C might find that it hardly needs to mutate at all in order to place sheep very close to, say, monkeys, and in order to place cows very close to rabbits. On the creation hypothesis there is no reason why that shouldn't happen. But what Penny and his colleagues actually found was that there was astonis.h.i.+ngly high agreement among all five proteins (and they used yet more clever statistics to show how unlikely such concordance would be by chance). All five proteins 'voted' for pretty much the same subset of trees from among the 34 million possible trees. This is, of course, exactly what we would expect on the a.s.sumption that there really is only one true tree relating all eleven animals, and it is the family family tree: the tree of evolutionary relations.h.i.+ps. What is more, the consensus tree that the five molecules all voted for turned out to be the same as zoologists had already worked out on anatomic and palaeontological, not molecular, grounds. tree: the tree of evolutionary relations.h.i.+ps. What is more, the consensus tree that the five molecules all voted for turned out to be the same as zoologists had already worked out on anatomic and palaeontological, not molecular, grounds.The Penny study was published in 1982, quite a while ago now. The intervening years have seen a prolific multiplication of detailed evidence on the exact sequences of genes of lots and lots of species of animals and plants. Agreement on the most parsimonious trees now extends far beyond the eleven species and five molecules that Penny and his colleagues studied. Theirs was just a nice example, overwhelming as their statistical evidence proved. The sum total of genetic sequence data now available puts the matter beyond all conceivable doubt. Far more convincingly even than the (also highly convincing) fossil evidence, the evidence from comparisons among genes is converging, rapidly and decisively, on a single great tree of life. Above is a tree for the eleven species of the Penny study, which represents a modern consensus vote from many different parts of the mammalian genome. It is the consistency of agreement among all the different genes in the genome that gives us confidence, not only in the historical accuracy of the consensus tree itself, but also in the fact that evolution has occurred.

Family tree for Penny's eleven species If molecular genetic technology continues to expand at its present exponential rate, by the year 2050 deriving the complete sequence of an animal's genome will be cheap and quick, scarcely any more trouble than taking its temperature or its blood pressure. Why do I say that genetic technology is expanding exponentially? Could we even measure it? There is a parallel in computer technology called Moore's Law. Named after Gordon Moore, one of the founders of the Intel computer chip company, it can be expressed in various ways because several measures of computer power are linked to each other. One version of the law states that the number of units that can be packed into an integrated circuit of a given size doubles every eighteen months to two years or so. It is an empirical law, meaning that, rather than deriving from some piece of theory, it just turns out to be true when you measure the data. It has held good over a period of about fifty years so far, and many experts think it will do so for at least a few more decades. Other exponential trends, with a similar doubling time, which can be regarded as versions of Moore's Law, include the increase in speed of computation, and size of memory, per unit cost. Exponential trends always lead to startling results, as Darwin demonstrated when, with the aid of his mathematician son George, he took the elephant as an example of a slow-breeding animal and showed that, in just a few centuries of unrestricted exponential growth, the descendants of just one pair of elephants would carpet the earth. Needless to say, population growth of elephants is not, in practice, exponential. It is limited by compet.i.tion for food and s.p.a.ce, by disease, and by many other things. That, indeed, was Darwin's whole point, for that is where natural selection steps in.But Moore's Law really has remained in force, at least approximately, for fifty years. Although n.o.body has a very clear idea why, various measures of computer power actually have increased exponentially in practice, where Darwin's elephant trend is exponential only in theory. It occurred to me that there might be a similar law in force for genetic technology and the sequencing of DNA. I suggested it to Jonathan Hodgkin, Oxford's Professor of Genetics (who had once been an undergraduate pupil of mine). To my delight, it turned out that he had already thought of it and measured it, in preparation for a lecture at his old school. He estimated the cost of sequencing a standard length of DNA at four dates in history, 1965, 1975, 1995 and 2000. I inverted his figures to 'bangs for the buck', or 'How much DNA could you sequence for 1,000?' I plotted the figures on a logarithmic scale, chosen because an exponential trend will always show up as a straight line when plotted logarithmically. Sure enough, Hodgkin's four points fall pretty well on a straight line. I fitted a line to the points (for the technique of linear regression, see note on p. 112 p. 112) and then took the liberty of projecting it on into the future. More recently, just as this book was going to press, I showed this section to Professor Hodgkin, and he told me the most recent data of which he was aware: the duckbilled platypus genome, which was sequenced in 2008 (the platypus was a good choice, because of its strategic position in the tree of life: the ancestor that it shares with us lived 180 million years ago, which is nearly three times as long ago as the extinction of the dinosaurs). I've drawn the platypus's point as a star on the graph, and you can see that it fits pretty well near the projected line that was calculated from the earlier data.The slope of the line for what I am now calling (without permission) Hodgkin's Law is only slightly shallower than that for Moore's Law. The doubling time is a bit more than two years, where the Moore's Law doubling time is a bit less than two years. DNA technology is intensely dependent on computers, so it's a good guess that Hodgkin's Law is at least partly dependent on Moore's Law. The arrows on the right indicate the genome sizes of various creatures. If you follow the arrow towards the left until it hits the sloping line of Hodgkin's Law, you can read off an estimate of when it will be possible to sequence a genome the same size as the creature concerned for only 1,000 (of today's money). For a genome the size of yeast's, we need wait only till about 2020. For a new mammal genome (as far as this kind of back-of-envelope calculation is concerned, all mammals are equally expensive), the estimated date is just this side of 2040. It's an exhilarating prospect: a ma.s.sive database of DNA sequences, cheaply and easily obtained from all corners of the animal and plant kingdoms. Detailed DNA comparisons will fill in all the gaps in our knowledge about the actual evolutionary relatedness of every species to every other: we shall know, with complete certainty, the entire family tree of all living creatures.* Goodness knows how we'll plot it; it won't fit on any practical-sized sheet of paper. Goodness knows how we'll plot it; it won't fit on any practical-sized sheet of paper.

'Hodgkin's Law'

The largest-scale attempt in that direction so far has been made by a group a.s.sociated with David Hillis, brother of Danny Hillis who pioneered one of the first supercomputers. The Hillis plot makes the tree diagram more compact by wrapping it around in a circle. You can't see the gap, where the two ends almost meet, but it lies between the 'bacteria' and the 'archaea'. To see how the circular plot works, look at the greatly stripped-down version tattooed on the back of Dr Clare D'Alberto of the University of Melbourne, whose enthusiasm for zoology is more than skin deep. Clare has graciously allowed me to reproduce the photograph in this book (see colour page 25 page 25). Her tattoo includes a small sample of eighty-six species (the number of terminal twigs). You can see the gap in the circular plot, and imagine the circle opened out. The smaller number of ill.u.s.trations around the edge are strategically chosen from bacteria, protozoa, plants, fungi, and four animal phyla. The vertebrates are represented by the weedy sea dragon on the right, a surprising fish, protected by its resemblance to seaweed. The Hillis circular plot is the same, except that it has three thousand species. Their names appear around the outside edge of the circle above, far too small to read though h.o.m.o sapiens h.o.m.o sapiens is helpfully marked 'You are here'. You can get an idea of how spa.r.s.e a sampling of the tree even this huge plot is when I tell you that the closest relatives of humans that it can fit in the circle are rats and mice. The mammals had to be stripped down drastically, in order to fit in all the other branches of the tree to the same depth. Just imagine trying to plot a similar tree with ten million species instead of the three thousand included here. And ten million is not the most extravagant estimate of the number of surviving species. It's well worth downloading the Hillis tree from his website (see endnotes), and then printing it as a wall hanging, on a piece of paper which, they recommend, should be at least 54 inches wide (even bigger would be an advantage). is helpfully marked 'You are here'. You can get an idea of how spa.r.s.e a sampling of the tree even this huge plot is when I tell you that the closest relatives of humans that it can fit in the circle are rats and mice. The mammals had to be stripped down drastically, in order to fit in all the other branches of the tree to the same depth. Just imagine trying to plot a similar tree with ten million species instead of the three thousand included here. And ten million is not the most extravagant estimate of the number of surviving species. It's well worth downloading the Hillis tree from his website (see endnotes), and then printing it as a wall hanging, on a piece of paper which, they recommend, should be at least 54 inches wide (even bigger would be an advantage).

The Hillis plot THE MOLECULAR CLOCK Now, while we are talking molecules, we have some unfinished business left over from the chapter on evolutionary clocks. There, we looked at tree rings, and at various kinds of radioactive clocks, but we deferred consideration of the so-called molecular clock until we had learned about some other aspects of molecular genetics. The time has now come. Think of this section as an appendix to the chapter on clocks.The molecular clock a.s.sumes that evolution is true, and that it proceeds at a sufficiently constant rate through geological time to be used as a clock in its own right, provided that it can be calibrated using fossils, which are in turn calibrated with radioactive clocks. Just as a candle clock a.s.sumes that candles burn at a fixed and known rate, and a water clock a.s.sumes that water drains from a bucket at a rate that can be calibrated, and a grandfather clock a.s.sumes that a pendulum swings at a fixed rate, so the molecular clock a.s.sumes that there are certain aspects of evolution itself itself that proceed at a fixed rate. That fixed rate can be calibrated against those parts of the evolutionary record that are well doc.u.mented with (radioactively datable) fossils. Once calibrated, the molecular clock can then be used for other parts of evolution that are not well doc.u.mented by fossils. For example, it can be used for animals that don't have hard skeletons and seldom fossilize. that proceed at a fixed rate. That fixed rate can be calibrated against those parts of the evolutionary record that are well doc.u.mented with (radioactively datable) fossils. Once calibrated, the molecular clock can then be used for other parts of evolution that are not well doc.u.mented by fossils. For example, it can be used for animals that don't have hard skeletons and seldom fossilize.Nice idea, but what gives us the right to hope that we can find evolutionary processes that go at a fixed rate? Indeed, much evidence suggests that evolutionary rates are highly variable. Long before the modern era of molecular biology, J. B. S. Haldane proposed the darwin darwin as a measure of evolutionary rates. Suppose that, over evolutionary time, some measured characteristic of an animal is changing in a consistent direction. For example, suppose the mean leg length is increasing. If, over a period of a million years, leg length increases by a factor of as a measure of evolutionary rates. Suppose that, over evolutionary time, some measured characteristic of an animal is changing in a consistent direction. For example, suppose the mean leg length is increasing. If, over a period of a million years, leg length increases by a factor of e e (2.718 . . ., a number chosen for reasons of mathematical convenience, which we needn't go into), (2.718 . . ., a number chosen for reasons of mathematical convenience, which we needn't go into),* the rate of evolutionary change is said to be one darwin. Haldane himself a.s.sessed the rate of evolution of the horse as approximately 40 millidarwins, while it has been suggested that the evolution of domestic animals under artificial selection should be measured in kilodarwins. The rate of evolution of guppies transplanted to a predator-free stream, as described in Chapter 5, has been estimated as 45 kilodarwins. The evolution of 'living fossils' such as the rate of evolutionary change is said to be one darwin. Haldane himself a.s.sessed the rate of evolution of the horse as approximately 40 millidarwins, while it has been suggested that the evolution of domestic animals under artificial selection should be measured in kilodarwins. The rate of evolution of guppies transplanted to a predator-free stream, as described in Chapter 5, has been estimated as 45 kilodarwins. The evolution of 'living fossils' such as Lingula Lingula ( (page 140) is probably to be measured in microdarwins. You get the point: rates of evolution of things that you can see and measure, like legs and beaks, are hugely variable.If rates of evolution are so variable, how can we hope to use them as a clock? This is where molecular genetics comes to the rescue. At first sight, it will not be clear how this can be so. When measurable characteristics like leg length evolve, what we are seeing is the outward and visible manifestation of an underlying genetic change. How, then, can it be the case that rates of change at the molecular level provide a good clock while rates of leg or wing evolution don't? If legs and beaks undergo change at rates ranging from microdarwins to kilodarwins, why should molecules be any more reliable as clocks? The answer is that the genetic changes that manifest themselves in outward and visible evolution of things like legs and arms are a very small tip of the iceberg, and they are the tip that is heavily influenced by varying natural selection. The majority of genetic change at the molecular level is neutral neutral, and can therefore be expected to proceed at a rate that is independent of usefulness and might even be approximately constant within any one gene. A neutral genetic change has no effect on the survival of the animal, and this is a helpful credential for a clock. This is because genes that affect survival, positively or negatively, would be expected to evolve at a changed rate, reflecting this.When the neutral theory of molecular evolution was first proposed by, among others, the great j.a.panese geneticist Motoo Kimura, it was controversial. Some version of it is now widely accepted and, without going into the detailed evidence here, I am going to accept it in this book. Since I have a reputation as an arch-'adaptationist' (allegedly obsessed with natural selection as the major or even only driving force of evolution) you can have some confidence that if even I support the neutral theory it is unlikely that many other biologists will oppose it!*A neutral mutation is one that, although easily measurable by molecular genetic techniques, is not subject to natural selection, either positive or negative. 'Pseudogenes' are neutral for one kind of reason. They are genes that once did something useful but have now been sidelined and are never transcribed or translated. They might as well not exist, as far as the animal's welfare is concerned. But as far as the scientist is concerned they very much exist, and they are exactly what we need for an evolutionary clock. Pseudogenes are only one cla.s.s of those genes that are never translated in embryology. There are other cla.s.ses which are preferred by scientists for molecular clocks, but I won't go into detail. What pseudogenes are useful for is embarra.s.sing creationists. It stretches even their creative ingenuity to make up a convincing reason why an intelligent designer should have created a pseudogene a gene that does absolutely nothing and gives every appearance of being a superannuated version of a gene that used to do something unless he was deliberately setting out to fool us.Leaving pseudogenes aside, it is a remarkable fact that the greater part (95 per cent in the case of humans) of the genome might as well not be there, for all the difference it makes. The neutral theory applies even to many of the genes in the remaining 5 per cent the genes that are read and used. It applies even to genes that are totally vital for survival. I must be clear here. We are not saying that a gene to which the neutral theory applies has no effect on the body. What we are saying is that a mutant version of the gene has exactly the same effect as the unmutated version. However important or unimportant the gene itself may be, the mutated version has the same effect as the unmutated version. Unlike pseudogenes, where the gene itself can properly be described as neutral, we are now talking about cases where it is only mutations mutations (i.e. changes in genes) that can strictly be described as neutral, not genes themselves. (i.e. changes in genes) that can strictly be described as neutral, not genes themselves.Mutations can be neutral for various reasons. The DNA code is a 'degenerate code'. This is a technical term meaning that some code 'words' are exact synonyms of each other.* When a gene mutates into one of its synonyms, you might as well not bother to call it a mutation at all. Indeed, it isn't a mutation, as far as consequences on the body are concerned. And for the same reason it isn't a mutation at all as far as natural selection is concerned. But it is a mutation as far as molecular geneticists are concerned, for they can see it using their methods. It is as though I were to change the font in which I write a word, say kangaroo to kangaroo. You can still read the word, and it still means the same Australian hopping animal. The change of typeface from Minion to Helvetica is detectable but irrelevant to the meaning. When a gene mutates into one of its synonyms, you might as well not bother to call it a mutation at all. Indeed, it isn't a mutation, as far as consequences on the body are concerned. And for the same reason it isn't a mutation at all as far as natural selection is concerned. But it is a mutation as far as molecular geneticists are concerned, for they can see it using their methods. It is as though I were to change the font in which I write a word, say kangaroo to kangaroo. You can still read the word, and it still means the same Australian hopping animal. The change of typeface from Minion to Helvetica is detectable but irrelevant to the meaning.Not all neutral mutations are quite so neutral as that. Sometimes the new gene translates into a different protein, but the 'active site' (remember the carefully shaped 'dents' that we met in Chapter 8) of the new protein remains the same as the old one. Consequently, there is literally no effect on the embryonic development of the body. The unmutated and the mutated form of the gene are still synonyms as far as their effects on bodies are concerned. It is also possible (although 'ultra-Darwinists' like me incline against the idea) that some mutations really do change the body, but in such a way as to have no effect on survival, one way or the other.So, to sum up on the neutral theory, to say that a gene, or a mutation, is 'neutral' doesn't necessarily mean that the gene itself is useless. It could be vitally important to the animal's survival. What it means is that the mutated form of a gene which might or might not be important for survival is no different different from the unmutated form with respect to its effects (which might be very important) on survival. As it happens, it is probably true to say that most mutations are neutral. They are undetectable by natural selection, but detectable by molecular geneticists; and that is an ideal combination for an evolutionary clock. from the unmutated form with respect to its effects (which might be very important) on survival. As it happens, it is probably true to say that most mutations are neutral. They are undetectable by natural selection, but detectable by molecular geneticists; and that is an ideal combination for an evolutionary clock.None of this is to downgrade the all-important tip of the iceberg the minority of mutations that are not neutral. It is they that are selected, positively or negatively, in the evolution of improvements. They are the ones whose effects we actually see and natural selection 'sees' too. They are the ones whose selection gives living things their breathtaking illusion of design. But it is the rest of the iceberg the neutral mutations, which are in the majority that concern us when we are talking about the molecular clock.As geological time goes by, the genome is subjected to a rain of attrition in the form of mutations. In that small portion of the genome where the mutations really matter for survival, natural selection soon gets rid of the bad ones and favours the good ones. The neutral mutations, on the other hand, simply pile up, unpunished and unnoticed except by molecular geneticists. And now we need a new technical term: fixation fixation. A new mutation, if it is genuinely new, will have a low frequency in the gene pool. If you revisit the gene pool a million years later, it is possible that the mutation will have increased in frequency to 100 per cent or something close to it. If that happens, the mutation is said to have 'gone to fixation'. We shall no longer think of it as a mutation. It has become the norm. The obvious way for a mutation to go to fixation is for natural selection to favour it. But there is another way. It can go to fixation by chance. Just as a once proud surname can die out for lack of male heirs, so the alternatives to the mutation we are talking about can just happen to disappear from the gene pool. The mutation itself can become frequent in the gene pool, by the same luck as has led 'Smith' to emerge as the commonest surname in England. Of course it is much more interesting if the gene goes to fixation for a good reason that's natural selection but it can also happen by chance, given a large enough number of generations. And geological time is vast enough for neutral mutations to go to fixation at a predictable rate. The rate at which they do so varies, but it is characteristic of particular genes, and, given that most mutations are neutral, this is precisely what makes the molecular clock possible.It's fixation that matters for the molecular clock, because 'fixed' genes are the ones that we look at when we compare two modern animals to try to estimate how long ago their ancestors split apart. Fixed genes are the genes that characterize a species. They are the ones that are all but universal in the gene pool. And we can compare the genes that have become fixed in one species with the genes that have become fixed in another, in order to estimate how recently the two species split apart. There are complications, which I won't go into because Yan Wong and I discussed them fully in 'The Epilogue to the Velvet Worm's Tale'. With reservations, and with various important correction factors, the molecular clock works.Just as radioactive clocks tick at hugely variable speeds, with half-lives ranging from fractions of a second through to tens of billions of years, so different genes provide a marvellous spread of molecular clocks, suitable for timing evolutionary change on scales ranging from a million to a billion years, and all stages in between. Just as each radioactive isotope has its characteristic half-life, so each gene has a characteristic turnover rate the rate at which new mutations typically go to fixation by random chance. Histone genes characteristically turn over at a rate of one mutation per billion years. Fibrinopeptide genes are a thousand times faster, with a turnover of one new mutation fixed per million years. Cytochrome-C and the suite of haemoglobin genes have intermediate turnovers, with times to fixation measured in millions to tens of millions of years.Neither radioactive clocks nor molecular clocks tick in a regular fas.h.i.+on like a pendulum clock or a watch. If you could hear them ticking, they'd sound like a Geiger counter, the radioactive clocks literally so since a Geiger counter is precisely what you would use to listen to them. A Geiger counter doesn't tick regularly, like a watch; it ticks at random, the ticks coming in strange, stuttering bursts. That's how mutations, and fixations, would sound, if we could hear them on the immensely long timescale of geology. But, whether stuttering like a Geiger counter or ticking metronomically like a watch, the important thing about a timekeeper is that it should tick at a known average average rate. That's what radioactive clocks do, and that's what molecular clocks do. rate. That's what radioactive clocks do, and that's what molecular clocks do.I introduced the molecular clock by saying that it a.s.sumes the fact of evolution and therefore can't be used in evidence of it. But now, having understood how the clock works, we can see that I was too pessimistic. The very existence of pseudogenes useless, untranscribed genes that bear a marked resemblance to useful genes is a perfect example of the way animals and plants have their history written all over them. But that is a topic that must wait for the next chapter.

* You may be surprised to hear that horses evolved in North America, because it is widely known that when the European invaders first came to the Americas, the sight of them on horseback amazed the natives. The bulk of horse evolution did indeed take place in America. Then horses spread to the rest of the world, shortly (by geological standards) before going extinct in America. They are American animals that have been re-introduced to America by man. You may be surprised to hear that horses evolved in North America, because it is widely known that when the European invaders first came to the Americas, the sight of them on horseback amazed the natives. The bulk of horse evolution did indeed take place in America. Then horses spread to the rest of the world, shortly (by geological standards) before going extinct in America. They are American animals that have been re-introduced to America by man.

* A single bone in mammals. The reptilian lower jaw is more complicated and thereby hangs a fascinating tale that I reluctantly omitted from this book (you can't have everything). In an amazing feat of evolutionary legerdemain, the smaller bones of the reptilian lower jaw were coopted into the mammalian ear, where they const.i.tute an exquisitely delicate bridge, to transport sound from the eardrum to the inner ear. A single bone in mammals. The reptilian lower jaw is more complicated and thereby hangs a fascinating tale that I reluctantly omitted from this book (you can't have everything). In an amazing feat of evolutionary legerdemain, the smaller bones of the reptilian lower jaw were coopted into the mammalian ear, where they const.i.tute an exquisitely delicate bridge, to transport sound from the eardrum to the inner ear.

* The Dutch 'wildebeest' is increasingly used in preference to 'gnu'. I am trying to save 'gnu' because, if it dies out altogether, the witty song by Flanders and Swann won't make sense any more. ('Gnor am I in the least / Like that dreadful hartebeest / Oh gno gno gno, I'm a gnu!') The Dutch 'wildebeest' is increasingly used in preference to 'gnu'. I am trying to save 'gnu' because, if it dies out altogether, the witty song by Flanders and Swann won't make sense any more. ('Gnor am I in the least / Like that dreadful hartebeest / Oh gno gno gno, I'm a gnu!')

* I presume my readers know better than the author(s) of Leviticus, w