The Greatest Show on Earth: The Evidence for Evolution Read online

  You want high milk yield in cows, orders of magnitude more gallons than could ever be needed by a mother to rear her babies? Selective breeding can give it to you. Cows can be modified to grow vast and ungainly udders, and these continue to yield copious quantities of milk indefinitely, long after the normal weaning period of a calf. As it happens, dairy horses have not been bred in this way, but will anyone contest my bet that we could do it if we tried? And of course, the same would be true of dairy humans, if anyone wanted to try. All too many women, bamboozled by the myth that breasts like melons are attractive, pay surgeons large sums of money to implant silicone, with (for my money) unappealing results. Does anyone doubt that, given enough generations, the same deformity could be achieved by selective breeding, after the manner of Friesian cows?

  About twenty-five years ago I developed a computer simulation to illustrate the power of artificial selection: a kind of computer game equivalent to breeding prize roses or dogs or cattle. The player is faced with an array of nine shapes on the screen – ‘computer biomorphs’ – the middle one of which is the ‘parent’ of the surrounding eight. All the shapes are constructed under the influence of a dozen or so ‘genes’, which are simply numbers handed down from ‘parent’ to ‘offspring’, with the possibility of small ‘mutations’ intervening on the way. A mutation is just a slight increment or decrement in the numerical value of the parent’s gene. Each shape is constructed under the influence of a particular set of numbers, which are its own particular values of the dozen genes. The player looks over the array of nine shapes and sees no genes but chooses the preferred ‘body’ shape she wants to breed from. The other eight biomorphs disappear from the screen, the chosen one glides to the centre, and ‘spawns’ eight new mutant ‘children’. The process repeats for as many ‘generations’ as the player has time for, and the average shape of the ‘organisms’ on the screen gradually ‘evolves’ as the generations go by. Only genes are passed from generation to generation, so, by directly choosing biomorphs by eye, the player is inadvertently choosing genes. That is just what happens when breeders choose dogs or roses to breed from.

  Biomorphs from the ‘Blind Watchmaker’ program

  So much for the genetics. The game starts to get interesting when we consider the ‘embryology’. The embryology of a biomorph on the screen is the process by which its ‘genes’ – those numerical values – influence its shape. Many very different embryologies can be imagined, and I have tried out quite a few of them. My first program, called ‘Blind Watchmaker’, uses a tree-growing embryology. A main ‘trunk’ sprouts two ‘branches’, then each branch sprouts two branches of its own, and so on. The number of branches, and their angles and lengths, are all under genetic control, determined by the numerical values of the genes. An important feature of the branching tree embryology is that it is recursive. I won’t expound that idea here, but it means that a single mutation typically has an effect all over the tree, rather than just in one corner of it.

  Although the Blind Watchmaker program starts off with a simple branching tree, it rapidly wanders off into a wonderland of evolved forms, many with a strange beauty, and some – depending on the intentions of the human player – coming to resemble familiar creatures such as insects, spiders or starfish. On the left is a ‘safari park’ of creatures that just one player of the game (me) found in the byways and backwaters of this strange computer wonderland. In a later version of the program, I expanded the embryology to allow for genes controlling the colour and shape of the ‘branches’ of the tree.

  A more elaborate program, called ‘Arthromorphs’, which I wrote jointly with Ted Kaehler, then working for the Apple Computer Company, embodies an ‘embryology’ with some interesting biological features specifically geared to breeding ‘insects’, ‘spiders’, ‘centipedes’ and other creatures resembling arthropods. I have explained the arthromorphs in detail, along with the biomorphs, ‘conchomorphs’ (computer molluscs) and other programs in this vein, in Climbing Mount Improbable.

  As it happens, the mathematics of shell embryology are well understood, so artificial selection using my ‘conchomorph’ program is capable of generating extremely lifelike forms (see over). I shall refer back to these programs, to make a completely different point, in the final chapter. Here I have introduced them for the purpose of illustrating the power of artificial selection, even in an extremely over-simplified computer environment. In the real world of agriculture and horticulture, the world of the pigeon fancier or dog breeder, artificial selection can achieve so much more. Biomorphs, arthromorphs and conchomorphs just illustrate the principle, in something like the same way that artificial selection itself is going to illustrate the principle behind natural selection – in the next chapter.

  Conchomorphs: computer-generated shells shaped by artificial selection

  Darwin had first-hand experience of the power of artificial selection and he gave it pride of place in Chapter 1 of On the Origin of Species. He was softening his readers up to take delivery of his own great insight, the power of natural selection. If human breeders can transform a wolf into a Pekinese, or a wild cabbage into a cauliflower, in just a few centuries or millennia, why shouldn’t the non-random survival of wild animals and plants do the same thing over millions of years? That will be the conclusion of my next chapter; but my strategy first will be to continue the softening-up process, to ease the passage towards understanding of natural selection.

  * This isn’t Mayr’s phrase, though it expresses his idea.

  * Who could not love dogs, they are such good sports?

  * This would be strictly true on the model of genetics that Mendel offered us, and the model of genetics that all biologists followed until the Watson–Crick revolution of the 1950s. It is nearly but not quite true, given what we now know about genes as long stretches of DNA. For all practical purposes we can take it as true.

  † On the farm where I spent my childhood, we had one especially obstreperous and aggressive cow called Arusha. Arusha was ‘a character’ and a problem. One day the herdsman, Mr Evans, ruefully remarked: ‘Seems to me, Arusha is more like a cross between a bull and a cow.’

  * There is a persistent, but false, rumour that Darwin possessed a bound copy of the German journal in which Mendel published his results but that the relevant pages were found uncut on Darwin’s death. The meme probably originates from the fact that he possessed a book called Die Pflanzen-mischlinge by W. O. Focke. Focke did briefly refer to Mendel, and the page where he did so was indeed uncut in Darwin’s copy. But Focke laid no special emphasis on Mendel’s work and showed no evidence of understanding its profound significance, so it is not obvious that Darwin would have picked it out even if he had cut the relevant page. In any case, Darwin’s German was not great. If he had read Mendel’s paper, the history of biology would have been very different. It is arguable that even Mendel himself did not understand the full importance of his findings. If he had, he might have written to Darwin. In the library of Mendel’s monastery in Brno, I have held in my hand Mendel’s own copy (in German) of On the Origin of Species and seen his marginalia, which indicate that he read it.

  † Beginning in 1908 with the endearingly eccentric, cricket-loving mathematician G. H. Hardy and, independently, the German doctor Wilhelm Weinberg, the theory culminated in the work of the great geneticist and statistician Ronald Fisher, and, again largely independently, his co-founders of population genetics, J. B. S. Haldane and Sewall Wright.

  * Not suckle: mothers suckle, babies suck.

  CHAPTER 3

  THE PRIMROSE PATH TO MACRO-EVOLUTION

  CHAPTER 2 showed how the human eye, working by selective breeding over many generations, sculpted and kneaded dog flesh to assume a bewildering variety of forms, colours, sizes and behaviour patterns. But we are humans, accustomed to making choices that are deliberate and planned. Are there other animals that do the same thing as human breeders, perhaps without deliberation or intention but with similar results? Y
es, and they carry this book’s softening-up program steadily forward. This chapter embarks on a step-by-step seduction of the mind as we pass from the familiar territory of dog breeding and artificial selection to Darwin’s giant discovery of natural selection, via some colourful intermediate stages. The first of these intermediate steps along the path of seduction (is it over the top to call it a primrose path?) takes us into the honeyed world of flowers.

  Wild roses are agreeable little flowers, pretty enough, but nothing to write home about in the terms one might lavish on, say, ‘Peace’ or ‘Lovely Lady’ or ‘Ophelia’. Wild roses have a delicate aroma, unmistakable, but not to-swoon-for like ‘Memorial Day’ or ‘Elizabeth Harkness’ or ‘Fragrant Cloud’. The human eye and the human nose went to work on wild roses, enlarging them, shaping them, doubling up the petals, tinting them, refining the bloom, boosting natural fragrances to heady extremes, adjusting habits of growth, eventually entering them in sophisticated hybridization programs until, today, after decades of skilful selective breeding, there are hundreds of prized varieties, each with its own evocative or commemorative name. Who would not like to have a rose named after her?

  INSECTS WERE THE FIRST DOMESTICATORS

  Roses tell the same story as dogs, but with one difference, which is relevant to our softening-up strategy. The flower of the rose, even before human eyes and noses embarked on their work of genetic chiselling, owed its very existence to millions of years of very similar sculpting by insect eyes and noses (well, antennae, which is what insects smell with). And the same is true of all the flowers that beautify our gardens.

  The sunflower, Helianthus annuus, is a North American plant whose wild form looks like an aster or large daisy. Cultivated sunflowers today have been domesticated to the point where their flowers are the size of a dinner plate.* ‘Mammoth’ sunflowers, originally bred in Russia, are 12 to 17 feet high, the head diameter is close to one foot, which is more than ten times the size of a wild sunflower’s disc, and there is normally only one head per plant, instead of the many, much smaller, flowers of the wild plant. The Russians started breeding this American flower, by the way, for religious reasons. During Lent and Advent, the use of oil in cooking was banned by the Orthodox Church. Conveniently, and for a reason that I – untutored in the profundities of theology – shall not presume to fathom, sunflower seed oil was deemed to be exempt from this prohibition.† This provided one of the economic pressures that drove the recent selective breeding of the sunflower. Long before the modern era, however, native Americans had been cultivating these nutritious and spectacular flowers for food, for dyes and for decoration, and they achieved results intermediate between the wild sunflower and the extravagant extremes of modern cultivars. But before that again, sunflowers, like all brightly coloured flowers, owed their very existence to selective breeding by insects.

  The same is true of most of the flowers we are aware of – probably all the flowers that are coloured anything other than green and whose smell is anything more than just vaguely plant-like. Not all the work was done by insects – for some flowers the pollinators that did the initial selective breeding were hummingbirds, bats, even frogs – but the principle is the same. Garden flowers have been further enhanced by us, but the wild flowers with which we started only caught our attention in the first place because insects and other selective agents had been there before us. Generations of ancestral flowers were chosen by generations of ancestral insects or hummingbirds or other natural pollinators. It is a perfectly good example of selective breeding, with the minor difference that the breeders were insects and hummingbirds, not humans. At least, I think the difference is minor. You may not, in which case I still have some softening up to do.

  What might tempt us to think it a major difference? For one thing, humans consciously set out to breed, say, the darkest, most blackish purple rose they can, and they do it to satisfy an aesthetic whim, or because they think other people will pay money for it. Insects do it not for aesthetic reasons but for reasons of . . . well, here we need to back up and look at the whole matter of flowers and their relationship with their pollinators. Here’s the background. For reasons I won’t go into now, it is of the essence of sexual reproduction that you shouldn’t fertilize yourself. If you did that, after all, there’d be little point in bothering with sexual reproduction in the first place. Pollen must somehow be transported from one plant to another. Hermaphroditic plants that have male and female parts within one flower often go to elaborate lengths to stop the male half from fertilizing the female half. Darwin himself studied the ingenious way this is achieved in primroses.

  Taking the need for cross-fertilization as a given, how do flowers achieve the feat of moving pollen across the physical gap that separates them from other flowers of the same species? The obvious way is by the wind, and plenty of plants use it. Pollen is a fine, light powder. If you release enough of it on a breezy day, one or two grains may have the luck to land on the right spot in a flower of the right species. But wind pollination is wasteful. A huge surplus of pollen needs to be manufactured, as hay fever sufferers know. The vast majority of pollen grains land somewhere other than where they should, and all that energy and costly matériel is wasted. There is a more directed way for pollen to be targeted.

  Why don’t plants choose the animal option, and walk around looking for another plant of the same species, then copulate with it? That’s a harder question to deal with than you might think. It’s circular simply to assert that plants don’t walk, but I’m afraid that will have to do for now.* The fact is, plants don’t walk. But animals walk. And animals fly, and they have nervous systems capable of directing them towards particular targets, with sought-for shapes and colours. So if only there were some way to persuade an animal to dust itself with pollen and then walk or preferably fly to another plant of the right species . . .

  Well, the answer’s no secret: that’s exactly what happens. The story is in some cases highly complex and in all cases fascinating. Many flowers use a bribe of food, usually nectar. Maybe bribe is too loaded a word. Would you prefer ‘payment for services rendered’? I’m happy with both, so long as we don’t misunderstand them in a human way. Nectar is sugary syrup, and it is manufactured by plants specifically and only for paying, and fuelling, bees, butterflies, hummingbirds, bats and other hired transport. It is costly to make, funnelling off a proportion of the sunshine energy trapped by the leaves, the solar panels of the plant. From the point of view of the bees and hummingbirds, it is high-energy aviation fuel. The energy locked up in the sugars of nectar could have been used elsewhere in the economy of the plant, perhaps to make roots, or to fill the underground storage magazines that we call tubers, bulbs and corms, or even to make huge quantities of pollen for broadcasting to the four winds. Evidently, for a large number of plant species, the trade-off works out in favour of paying insects and birds for their wings, and fuelling their flight muscles with sugar. It’s not a totally overwhelming advantage, however, because some plants do use wind pollination, presumably because details of their economic circumstances tip their balance that way. Plants have an energy economy and, as with any economy, trade-offs may favour different options under different circumstances. That’s an important lesson in evolution, by the way. Different species do things in different ways, and we often won’t understand the differences until we have examined the whole economy of the species.

  If wind pollination is at one end of a continuum of cross-fertilization techniques – shall we call it the profligate end? – what is at the other end, the ‘magic bullet’ end? Very few insects can be relied upon to fly like a magic bullet straight from the flower where they have picked up pollen to another flower of exactly the right species. Some just go to any old flower, or possibly any flower of the right colour, and it is still a matter of luck whether it happens to be the same species as the flower that has just paid it in nectar. Nevertheless, there are some lovely examples of flowers that lie far out towards the magic bullet e
nd of the continuum. High on the list are orchids, and it’s no wonder that Darwin devoted a whole book to them.

  Both Darwin and his co-discoverer of natural selection, Wallace, called attention to an amazing orchid from Madagascar, Angraecum sesquipedale (see colour page 4), and both men made the same remarkable prediction, which was later triumphantly vindicated. This orchid has tubular nectaries that reach down more than 11 inches by Darwin’s own ruler. That’s nearly 30 centimetres. A related species, Angraecum longicalcar, has nectar-bearing spurs that are even longer, up to 40 centimetres (more than 15 inches). Darwin, purely on the strength of A. sesquipedale’s existence in Madagascar, predicted in his orchid book of 1862 that there must be ‘moths capable of extension to a length of between ten and eleven inches’. Wallace, five years later (it isn’t clear whether he had read Darwin’s book) mentioned several moths whose probosces were nearly long enough to meet the case.