Science of Discworld III

by Terry Pratchett

  Apologising profusely for lack of space, and time, to write something more comprehensive than his 150,000-word tome, Darwin then moves towards a short summary of his main idea. Writers on science generally appreciate that it is seldom enough to discuss the answer to a question: it is also necessary to explain the question. And that, of course, should be done first. Otherwise your readers will not appreciate the context into which the answer fits. Darwin was clearly aware of this principle, so he begins by pointing out that:

  It is quite conceivable that a naturalist, reflecting on the mutual affinities of organic beings, on their embryological relations, their geographical distribution, geological succession, and other such facts, might come to the conclusion that each species had not been independently created, but had descended, like varieties, from other species. Nevertheless such a conclusion, even if well founded, would be unsatisfactory, until it could be shown how innumerable species inhabiting this world have been modified, so as to acquire those perfections of structure and coadaptation which most justly excites our admiration.

  Already we see a gesture towards Paley – ‘perfections of structure’ is a clear reference to the watch/watchmaker argument, and ‘had not been independently created’ shows that Darwin doesn’t buy Paley’s conclusion. But we also see something that characterises the whole of Origin: Darwin’s willingness to acknowledge difficulties in his theory. Time and again he raises possible objections – not as straw men, to be knocked flat again, but as serious points to be considered. More than once he concludes that there is more to be learned, before the objection can be resolved. Paley, to his credit, did something similar, though he didn’t go as far as admitting ignorance: he knew that he was right. Darwin, a real scientist, not only had his doubts – he shared them with his readers. He would not have arrived at his theory to begin with if he had failed to seek the weaknesses of the hypotheses upon which it was based.

  He also, of course, makes it clear what his own work is adding to the speculations of earlier ‘transmutationists’. Namely: he has come up with a mechanism for species change. There are advantages in being honest about your own limitations: you gain the right to talk about the limitations of others. And now he tells us what that mechanism is. Species, we know, are variable – the domestication of wild species like chickens, cows, and dogs is clear evidence of that. Although that is deliberate selection by humans, it opens the door to selection by nature without human aid:

  I will then pass on to the variability of species in a state of nature … We shall, however, be enabled to discuss what circumstances are most favourable to variation. In the next chapter the Struggle for Existence amongst all organic beings throughout the world, which inevitably follows from their high geometrical powers of increase, will be treated of … The fundamental subject of Natural Selection will be treated at some length in the fourth chapter; and we shall then see how Natural Selection almost inevitably causes much Extinction of the less improved forms of life, and induces what I have called Divergence of Character.

  He then promises four chapters on ‘the most apparent and gravest difficulties of the theory’, prominent among these being to understand how a simple organism or organ can change into a highly complex one – another nod to Paley. The introduction ends with a flourish:

  I can entertain no doubt … that the view which most naturalists entertain, and which I formerly entertained – namely, that each species has been independently created – is erroneous. I am fully convinced that species are not immutable; but that those belonging to what are called the same genera are lineal descendent of some other and generally extinct species … Furthermore, I am convinced that Natural Selection has been the main but not exclusive means of modification.

  In essence, Darwin’s theory of natural selection, which soon became known as evolution,6 is straightforward. Most people think they understand it, but its simplicity is deceptive, and its subtleties are easily underestimated. Many of the standard criticisms of evolutionary theory stem from common misunderstandings, not from what the theory actually proposes. The ongoing scientific debate about details is often misrepresented as disagreement with the general outline, which is an error based on too simple-minded a view of how science develops and what ‘knowledge’ is.

  Briefly, Darwin’s theory goes like this.

  Organisms, even those in the same species, are variable. Some are bigger than others, or bolder than others, or prettier than others.

  This variability is to some extent hereditary, passed on to offspring.

  Unchecked population growth would quickly exhaust the capacity of the planet, so something checks it: competition for limited resources.

  Therefore as time passes, the organisms that do survive long enough to breed will be modified in ways that improve their chance of surviving to breed, a process called natural selection.

  Ongoing slow changes can lead, in the long run, to big differences.

  The long run has been very long indeed – hundreds of millions of years, maybe more. So by now those differences can have become huge.

  It’s relatively simple to put these six ingredients together and deduce that new species can arise without divine intervention – provided we can justify each ingredient.

  Even though different species seem to stay pretty much the same – think lions, tigers, elephants, hippos, whatever – it is actually rather obvious that, in general, species are not fixed for all time. The changes are relatively slow, which is why we don’t notice them. But they do happen. We’ve already seen that in Darwin’s finches, evolutionary changes can be and have been observed on a timescale of years, and in bacteria they occur on a timescale of days.

  The most obvious evidence for the variability of species, in Darwin’s day and ours, was the domestication of animals – sheep, cows, pigs, chickens, dogs, cats …

  … and pigeons. Darwin was rather knowledgeable about pigeons, he belonged to two London pigeon clubs. Every pigeon-fancier knows that by selectively breeding particular combinations of male and female pigeons, it is possible to produce ‘varieties’ of pigeons with particular characteristics. ‘The diversity of the breeds is something astonishing,’ says Darwin in the first chapter of Origin. The English carrier pigeon has a wide mouth, large nostrils, elongated eyelids, a long beak. The short-faced tumbler has a short stubby beak like a finch. The common tumbler flies high up in a tightly knit flock, and has an odd habit of falling about in the sky, whence its name. The runt (despite its name) is huge, with a long beak and large feet. The barb is like the carrier but with a short, broad beak. The pouter has an inflatable crop and can puff out its chest. The turbit has a short beak and a line of reversed feathers on its chest. The Jacobin has so many reversed feathers on its neck that they form a hood. Then there are the trumpeter, laugher, fantail … These are not separate species: they can interbreed, to produce viable ‘hybrids’ – cross-breeds.

  The enormous variety of dogs is so well known that we don’t even need to mention examples. It’s not that the dog species is exceptionally malleable, just that dog-breeders have been unusually active and imaginative. There is a dog for every purpose that a dog can carry out. Again, they’re all dogs, not new (albeit related) species. They can mostly (barring really big size differences) interbreed, and artificial insemination can take care of mere size. Dog sperm plus dog egg makes fertile dog zygote, and, eventually, dog – independently of breed. That’s why pedigree pooches need a pedigree, to guarantee that their parentage is pure. If the different varieties of dog were different species, that wouldn’t be necessary.

  In modern times, it has become clear that cats are just as malleable, but the cat-breeders have only just got going on exotic cats. The same goes for cows, pigs, goats, sheep … and what about flowers? The number of varieties of garden flowers is immense.

  By avoiding the creation of hybrids, the breeder can maintain the individual varieties over many generations. Pouter pigeons breed with pouters to produce (a substantial p
roportion of) pouters. Carriers mated with carriers produce (mostly) carriers. The underlying genetics, about which Darwin and his contemporaries knew nothing, is complicated enough that apparent hybrids can sometimes arise from what seems to be pure stock, just as two brown-eyed parents can nonetheless have a blue-eyed child. So pigeon-breeders have to eliminate the hybrids.

  The existence of these cross-bred varieties does not, of itself, explain how new species can arise of their own accord. Varieties are not species; moreover, the guiding hand of the breeder is evident. But varieties do make it clear that there must be plenty of variability within a species. In fact, the variability is so great that one can readily imagine selective breeding leading to entirely new species, given enough time. And the avoidance of hybrids can maintain varieties from one generation to the next, so their characters (biologese for the features that distinguish them) are heritable (biologese for ‘able to be passed from one generation to the next’). So Darwin has his first ingredient: heritable variability.

  The next ingredient was easier (though still controversial in some quarters). It was time. Oodles and oodles of time, the Deep Time of geologists. Not a few thousand years, but millions, tens of millions … billions, in fact, though that was further than the Victorians were willing to go. Deep Time, as we’ve previously observed, is contrary to the biblical chronology of Bishop Ussher, which is why the idea remains controversial among certain Christian fundamentalists, who have bizarrely chosen to fight their corner on the weakest of grounds, completely needlessly. Deep Time is supported by so much evidence that a truly committed fundamentalist has to believe that his God is deliberately trying to fool him. Worse, if we can’t trust the evidence of our own eyes, then we can’t trust the apparent element of ‘design’ in living creatures either. We can’t trust anything.

  Lyell realised that the age of the Earth must be many millions of years, when he looked at sedimentary rocks. These are rocks like limestone or sandstone which form in layers, and have been deposited either underwater, as muddy sediments, or in deserts, as accumulating sand. (Independent evidence for these processes comes from the fossils found in such rocks.) By studying the rate at which modern sediments accumulate, and comparing that with the thickness of known beds of sedimentary rock, Lyell could estimate the time it had taken for the layers of rock to be deposited. Something in the range 1000–10,000 years would produce a layer about a metre thick. But the chalk cliffs of the south coast, around Dover, are hundreds of metres thick. So that’s several hundred thousand years of deposition, and we’ve only dealt with one of the numerous layers of rock that make up the geological column – the historical sequence of different rocks.

  We now have many other kinds of evidence for the great age of our planet. The rate of decay of radioactive elements, which we can measure today and extrapolate backwards, is in general agreement with the evidence of the rock layers. The rate of movement of the continents, when combined with the distances they have moved, is again consistent with other estimates. We’ve seen that India was once attached to Africa, but about 200 million years ago it broke off, and by 40 million years ago it had moved all the way to its current position, butting up against Asia and pushing up the Himalayas.

  When continents move apart – as Africa and South America, or Europe and North America, are doing now – new material forms on the ocean floor, flowing out from the mantle beneath to form huge mid-ocean ridges. The rocks in the ridges contain a record of the changes in the Earth’s magnetic field, ‘frozen in’ as the rock cooled. They show a long series of repeated reversals of the field polarity. Sometimes the ‘north’ magnetic pole is at the northern end of the Earth, as now, but every so often the polarity flips, so that the magnetic pole near the northern end is the ‘south’ one. Mathematical models of the Earth’s magnetic field predict that such reversals occur roughly once every five million years. Count the number of reversals in the ocean-ridge rocks, multiply by five million … again, the numbers fit reasonably well, and careful comparisons and a lot of disputation by experts lead to revised numbers that fit even better.

  The Grand Canyon is a deep gash through layers of rock one mile (1.6km) thick. You have a choice. You can understand what the record of the rocks is telling you here: it took a very long time to lay down those rocks, and quite a long time – though less – for flash-flooding in the Colorado river to erode them again. Or you can follow one book that until recently was displayed in the ‘science’ section of the Grand Canyon bookstore, until a lot of scientists complained, and assert that the Grand Canyon is evidence for Noah’s flood. The first choice fits huge amounts of evidence and geological understanding. The second is an excellent test of faith, because it fits absolutely nothing. A flood that lasted only 40 days could never have produced that kind of geological formation. A miracle? In that case, the Sahara desert could equally well be hailed as evidence for Noah’s flood, miraculously not forming a deep canyon. Once you admit miracles, you can’t pursue a logical thread.

  Anyway, that’s the second ingredient – Deep Time. It takes huge amounts of time to change organisms into entirely new species, if all you can do – as Darwin believed – is make very gradual changes. But even Deep Time, when combined with heritable variation, is not enough to lead to the kind of organised, coherent changes that are needed to create new species. There has to be a reason for such changes to occur, as well as opportunity and time. Darwin, as we’ve seen, found his reason in Malthus’s contention that the unchecked growth of organisms is exponential, whereas that of resources is linear. In the long run, exponential growth always wins.

  The first assertion is pretty much correct, the second highly debatable. The qualifier ‘unchecked’ is crucial, and real populations only grow exponentially if there are plenty of resources available. Typically, the growth starts exponentially with a small population and then levels off as the population size increases. But in most species, two parents (let’s think sexual species here) produce some larger number of offspring. A breeding female starling lays about 16 eggs in her life, and with ‘unchecked’ growth, the starling population would multiply by 8 every lifetime. It would not be long before the planet was knee-deep in starlings. So, of necessity, 14 of those 16 offspring (on average) fail to breed – usually because something eats them. Just two become parents in their turn. A female frog may lay 10,000 eggs in her life, and nearly all die in various grotesque ways to achieve each two parents; a female cod contributes forty million or thereabouts of her offspring to planktonic food chains, for each two that breed. Here the multiplier, with ‘unchecked’ growth, would be 20 million per cod-lifetime. Unchecked growth simply doesn’t bear thinking about as a realistic prospect.

  We suspect that Malthus plumped for linear growth of resources for a slightly silly reason. Victorian school-textbook mathematics distinguished two main types of sequence: geometric (exponential) and arithmetic (linear). There were plenty of other possibilities, but they didn’t get into the textbooks. Having already assigned geometric growth to organisms, Malthus was left with arithmetic growth for resources. His main point doesn’t depend on the actual growth rate, in any case, as long as it is less than exponential. As the starling example shows, most offspring die before breeding, and that’s the main point here.

  Given that most young starlings cannot possibly become parents, the question arises: which ones will? Darwin felt that the ones that survived to breed would be the ones best suited for survival, which makes sense. If one starling is better at finding food, or hanging on to it, than another one, then it’s clear which one is more likely to do best if food supplies become limited. The better one might be unlucky and get eaten by a hawk; but across the population, starlings that are better equipped to survive are generally the ones that do survive.

  This process of ‘natural selection’ in effect plays the role of an external breeder. It chooses certain organisms and eliminates the rest. The choice is not conscious – there is no consciousness to do the choosing, and
no preconceived purpose – but the end result is very similar. The main difference is that natural selection makes sensible choices, whereas human selection can make ridiculous ones (like dogs with faces so flattened they can hardly breathe). Sensible choices lead to sensible animals and plants, ones that are beautifully adapted for survival in whatever environment they happened to be in when natural selection was moulding them.

  It is just like breeding new varieties of pigeon, but without a human breeder. Natural selection exploits the same variability of organisms that pigeon-breeding does. It makes choices based on survival value (in some environment) rather than whim. It is typically much slower than human intervention, but the timescale is so vast that this slowness doesn’t matter much. Heritable variation plus natural selection inevitably lead, over Deep Time, to the origination of species.

  Nature does it all on her own. There is no need for a series of acts of special creation. That doesn’t imply that special creation has not occurred. It just removes any logical imperative for it.

  Paley was wrong.

  The watches don’t need a watchmaker.

  They can make themselves.

  1 Literally, ‘jointed acorn’

  2 Yes, we know it sounds unlikely, but apparently there are such things.

  3 It was a good job that God had such a fondness for beetles.

  4 Its potential for Librarianship was not widely recognised at that time.

  5 No, not long-haired South American beasts of burden, but Latin for ‘gaps’.

  6 The term was around in Victorian times, as a phenomenon but not a specific mechanism. Darwin didn’t use it in Origin, nor in the later The Descent of Man. However, the final word in Origin is ‘evolved’.