Mathematics of Life (2011)
Chapter 5. The Origin of Species
Revolution number three got off to a bad start.
The year was 1858; the date, 1 July. The Linnaean Society, then and now the world’s oldest society for natural history and taxonomy, had been in existence for seventy years. It was the final meeting of the session, and the members had their minds on summer holidays and outside activities. The president, Thomas Bell, was delivering his annual review of the highlights of the society’s scientific activities. But it had been a bad year, in his opinion, and there hadn’t been any highlights. ‘The year which has passed,’ said Bell, ‘has not, indeed, been marked by any of those striking discoveries which at once revolutionize, so to speak, the department of science on which they bear’.1
At the time, no one objected to his summing up. Even the two scientific papers that had been squeezed into the programme of the meeting at the last minute had made no impact; when the members departed for their homes, no one seems to have been terribly impressed by them. As was the practice at the time, these papers were read out loud to the Society on behalf of the authors. They were on very similar topics, and their titles were ‘On the tendency of species to form varieties’ and ‘On the perpetuation of varieties and species by natural means of selection’. Their authors, respectively, were Charles Darwin and Alfred Russel Wallace.
The two papers, deliberately presented simultaneously to avoid any priority disputes, announced the theory of evolution by means of natural selection.
Long before Darwin was born, biologists were trying to understand how the planet’s innumerable species had come to be. Almost everyone considered them to have been divinely created, the default social assumption of the time. But that is to answer every question with the same facile formula. Where do dogs come from? God created them. Where do dragons come from? God created them. Well, no, he didn’t, but you wouldn’t be able to deduce that from the answer.
In his Physicae Auscultationes (‘Lectures on Nature’), the Greek philosopher Aristotle objected to explanations of nature that invoke purpose, such as ‘rain falls in order to make corn grow’. He argued that if this were the case, then rain would also exist in order to spoil the famer’s corn if he threshed it outdoors. Continuing this line of thought, he asked why animals’ anatomical features are so obviously related to one another’s. His answer is surprisingly modern: if anything didn’t work in reasonable concert with the rest of the body, it would have been impossible for the animal to function, so neither the animal nor that combination of features would survive.
By the later eighteenth century, some scientists were beginning to think that over long periods of time, organisms could change. Among them was Darwin’s grandfather, Erasmus. Professionally he was a physician, but he had the broad interests of a polymath, including natural history, physiology, abolition of the slave trade and inventing things. He was a founder member of the Lunar Society, a scientific society which met in Birmingham once a month on the night of the full moon, to make it easier for members to find their way home in the dark. His main claim to biological fame is the Zoonomia of 1794 – 6, in which he asked whether it would be too bold to imagine that
in the great length of time since the earth began to exist, perhaps millions of ages before the commencement of the history of mankind ... all warm-blooded animals have arisen from one living filament, which the great First Cause endued with animality, with the power of acquiring new parts, attended with new propensities, directed by irritations, sensations, volitions and associations, and thus possessing the faculty of continuing to improve by its own inherent activity, and of delivering down these improvements by generation to its posterity.
Erasmus (let me call him that to distinguish him from his grandson) was convinced that species could ‘transmute’ – that is, change spontaneously – and that the process began with a single primitive organism. Biologists now call this idea ‘universal common ancestry’. But Erasmus offered no specific mechanism that could produce such changes.
Darwin doesn’t mention his grandfather’s work in the Origin, possibly because he found Zoonomia too eccentric, but probably because he did not consider it relevant. (We know he had read it, because he wrote the title on the opening page of his ‘B Notebook’, his first recorded step towards the Origin.) Unlike his grandfather, Darwin wanted to know how species changed. Erasmus seems to have thought that animals could acquire new abilities, and that these would automatically be passed on to their descendants. This belief in the ‘inheritance of acquired characters’ was soon to be advocated more explicitly by a better-known figure, whom history has treated somewhat unfairly.
Jean-Baptiste Lamarck, the eleventh child of an upper-class family down on its luck, trained as a Jesuit but abandoned his studies to join the French army, then at war with Prussia. When illness forced him to retire from the military, he tried his hand at medicine and banking, before settling on botany and becoming keeper of the royal herbarium to King Louis XVI in 1788. He retained the position, but not the royal connection, through the climax of the French Revolution, whereas Louis’s head failed to retain its connection to his body. Lamarck then became curator and professor of invertebrate zoology at the National Museum of Natural History.
His most significant publications include the 1809 Philosophie Zoologique (‘Zoological Philosophy’) and the seven-volume Histoire
Naturelle des Animaux sans Vertèbres (‘Natural History of Invertebrates’) published between 1815 and 1822. In these books, and elsewhere, he developed and elaborated a novel idea: animals can change from generation to generation in response to their environment. To Lamarck, moles were not blind because they had been created that way, but because they lived underground and so did not need the sense of sight. Their ancestors had once been sighted, but the ability to see had been lost because it was not needed. He tried to find a credible mechanism for such changes, among them being two ‘forces’: the tendency of living creatures to become more complex, and their tendency to adapt to their surroundings. He thought that living creatures ascended a ladder of progress, propelled by some inherent force that created ever-increasing order.
Today, Lamarck’s name is most often associated with a discredited view of evolution, the ‘inheritance of acquired characters’: if some organism happens to develop a useful feature, such as a longer neck or stronger muscles, then this feature can and will be inherited by its descendants. Thus a blacksmith whose trade causes him to have very strong arms will have sons with strong arms – which was often true, because sons went into their fathers’ trade. However, Lamarck did not believe that evolving organisms changed in a purposeful way, and he did not think that every acquired character would be passed on to future generations. He believed that all changes in organisms had a purely physical origin.
Lamarck distilled his view of adaptation into two laws:
1. If animals use an organ more often, that organ will become stronger and larger. Conversely, any organ that is not used will weaken and eventually disappear.
2. Any such improvement or loss, if it is related to the animals’ long-term environment, will be passed on to future generations.
The second law is where the notion that Lamarck believed in the inheritance of acquired characters comes from. He did, but only for certain types of character. Darwin pointed to Lamarck’s emphasis on use and disuse, and interpreted those aspects of Lamarck’s work as a form of natural selection. He praised Lamarck for drawing attention to ‘the probability of all change in the organic world being the result of law, not miraculous interposition’. In Darwin’s view, Lamarck came close to a scientifically acceptable mechanism for evolution, but fell short.
As a young man, Darwin was interested in geology, having been enormously impressed by the concept of deep time – the idea that the Earth is enormously old, whose significance was emphasised by Charles Lyell. None of this greatly impressed his father, who wanted Darwin to be a doctor and take his rightful place in Victorian society. But Darwin’s stint as a medical student at the University of Edinburgh didn’t work out, so his father decided that the son would do better to settle down and become a country vicar. That would leave him ample spare time to pursue his geological fancies. So in 1828 Darwin entered the University of Cambridge to study theology.
Unfortunately for his father’s careful plans, Darwin was promptly bitten by a bug. Ironically, it came about because of a country vicar, William Kirby, who collaborated with the businessman William Spence on a four-volume treatise, An Introduction to Entomology. The book sparked a national craze for collecting beetles, and Darwin joined in with a passion, hoping to find a new species. He failed, but did find a rare German one. He also developed a second passion, a young woman named Fanny Owen, who ditched him as soon as she discovered he was more interested in beetles.
Neither interest did much for his examination preparations, and he found himself facing a backlog of two years’ work with only two months to do it. One of the important course books was Evidences of Christianity by the Reverend William Paley. Darwin was captivated by its logic and its leftish politics. He scraped a pass and moved on to his final year. Now he had to read another book by Paley, Principles of Moral and Political Philosophy – not because of its orthodoxy, but because students had to learn how to argue against the book’s assertions, such as the irrelevance of an established Church to Christianity.
Darwin decided to read around the topic, and chanced upon Paley’s Natural Theology, which made the case for the divine creation of living creatures. He was impressed by the book’s clarity, but he was also aware that many leading scientists and philosophers found it naive, and this led him to investigate the process that led to scientific laws, reading Sir John Herschel’s Preliminary Discourse on the Study of Natural Philosophy. For light reading he scanned the 3,754 pages of Alexander von Humboldt’s Personal Narrative, about the exploration of South America. From Herschel he learned how to do science; from Humboldt he learned where to do it. He promptly vowed that he would visit the volcanic Canary Islands to see the famed Great Dragon Tree.
This plan collapsed when his friend Marmaduke Ramsay, who was going to go with him, died unexpectedly. While Darwin was trying to work out what to do next, he was offered the post of gentleman companion to a naval officer, Robert FitzRoy. FitzRoy had been charged with carrying out a chronometric survey of the coast of South America – that is, a survey using a marine chronometer (essentially, a very accurate watch) to determine longitude. The ship was to be the Beagle, and FitzRoy was worried because the previous captain had shot himself. Worse, one of FitzRoy’s uncles had slit his own throat when depressed. So FitzRoy determined to stave off suicide by taking along someone capable of intellectual discussion.
This suited Darwin down to the ground, but his father refused permission, until he received a letter from Darwin’s uncle Josiah, saying that the trip would be the making of the young man. So off Darwin went, on what eventually became a five-year voyage round the world. First landfall was St Jago, a rugged volcanic outcrop of the Cape Verde Islands, with impressive volcanoes where Darwin could pursue his geological interests, and fertile valleys where he could do natural history. He found flatworms in Brazil, fossils in Argentina and naked savages in Tierra del Fuego. Similarities between shells on the beach and fossils high in the Chilean Cordillera convinced him that the Andes must have been pushed up high above sea level by vast geological forces. But the climax of the voyage, scientifically speaking, came when the Beagle arrived at the volcanic Galápagos Islands, which basically were a nest of volcanoes.
Darwin’s stay there was brief, but it allowed him to collect specimens from what he quickly realised was very newly formed land. Many of its native creatures were bizarre: a species of penguin living on the equator; the only known marine iguanas, which foraged for algae beneath the ocean’s turbulent waves; the only known species of cormorant that could not fly; and giant tortoises up to two metres in circumference. He was amazed by the spectacular blue-footed boobies, which dived into the sea from a great height, like avian arrows in search of fishy targets, and he was intrigued by the mockingbirds, which differed subtly from one island to the next. Looking more closely, he discovered that they formed at least three distinct species: one on Charles Island (now Santa Maria), another on Chatham Island (San Cristóbal), and a third on James Island (San Salvador).
He collected a few dull-looking finches and warblers, but found them rather boring.
The voyage continued to Tahiti, New Zealand and Australia. Five years and three days after the Beagle departed from Plymouth, a travel-weary Darwin arrived home. When he walked in through the door, he found his father eating breakfast, who greeted him without enthusiasm, remarking, ‘Why, the shape of his head is quite altered.’
Only after his return from the Beagle voyage did Darwin start to think seriously about what he had seen.
Near the end of the voyage, in Australia, he had made one major discovery: the origin of coral reefs. Lyell had suggested that reefs must be built on top of submerged volcanoes, but Darwin came up with a different idea: coral reefs start out in regions where the seas are shallow, but then the sea floor slowly falls. The corals grow faster than the seabed can drop, so the living tip of the reef remains near the surface. On the basis of this, and his Andes observations, he was made a Fellow of the Royal Geological Society. His scientific reputation was made not by evolution, but by geology.
However, Darwin’s training as a geologist made him aware of some puzzling aspects of what he had observed. Lyell, who was a firm believer in divine creation, explained the diversity of living creatures, and their adaptation to their environments, in terms of local geological conditions. Darwin was sceptical. The Galápagos finches, which he had dismissed as uninteresting, were coming back to haunt him. He had misunderstood them. In fact, he had misunderstood them so badly that he hadn’t even realised they were all finches – he thought some were wrens, and others blackbirds. On his return to England he immediately gave all the relevant specimens to John Gould, a finch expert at the Zoological Society. It took Gould only ten days to convince himself that they were all finches, very closely related, but constituting an astonishing twelve distinct species (now considered to be thirteen). Why so many species on such a small group of tiny islands?
Initially Darwin hadn’t been interested in the question, but now he began to take it seriously – and it bothered him. For once, his skills as an observer had deserted him, perhaps for lack of time. He hadn’t recorded which specimens came from which island. And he’d assumed that the finches formed huge flocks, which all fed on the same things. But when he examined their beaks more closely, he realised this must be wrong. There were many different forms of beak, suited to different foods.
The Darwin family were Unitarians, which predisposed Darwin to the belief that God worked only on vast scales of space and time. In support of this view he wrote
It is derogatory that the Creator of countless systems of worlds should have created each of the myriads of creeping parasites and slimy worms which have swarmed each day of life on land and water on this one globe. We cease being astonished, however much we may deplore, that a group of animals should have been directly created to lay their eggs in bowels and flesh of others – that some organisms should delight in cruelty . . . From death, famine, rapine, and the concealed war of nature we can see that the highest good, which we can conceive, the creation of the higher animals has directly come.
Mainstream Victorian theologians were no longer in favour of Paley’s arguments, for similar reasons. If God continually intervened in His creation, that seemed to suggest that He had bungled it. Why else keep tinkering? The theist view was being replaced by a deist one: yes, the Creator set up the universe, along with its laws, but then He stood back and left it running, to work out its own destiny according to those laws. And one consequence of those laws seemed to be that species could change. Darwin had been keeping a notebook, his Red Notebook. Now he started a new, secret one, the B Notebook, on the transmutation of species. Slowly he assembled a long list of puzzles that made much more sense if species could change. But even if they did change, he still didn’t know how.
Back in England, Darwin wrote a series of books – two on the Beagle voyage, one on coral reefs, one on the geology of South America and a massive series of four huge tomes on barnacles. (He had been advised to become an expert on some limited range of organisms to cement his reputation as a naturalist, and settled on barnacles.) The barnacles added further weight to the argument against special creation – there were hundreds of different species, most of them very similar to many of the others, all minor variations on the same underlying theme. God’s inordinate fondness for beetles now seemed to extend to an equally inordinate fondness for barnacles. Creation of each species, one by one, seemed absurd. So much neater to create just one, and then let it . . . change.
As Darwin’s ideas on the transmutation of species began to crystallise, he realised that a simple mechanism might explain how it happened. The idea was triggered when he read Thomas Malthus’s 1826 Essay on the Principle of Population. Malthus’s argument relied on some simple mathematics. He asserted that populations of living creatures, if their growth is not restrained by lack of food or predation, grow ‘geometrically’: the population size at successive instants of time is multiplied by the same fixed amount. For example, if every pair of finches produces four surviving adults, then the finch population repeatedly doubles. The numbers grow very rapidly – the modern term is ‘exponentially’:
and so on. But Malthus reckoned that the available resources, such as food supply, grew more slowly: ‘arithmetically’, increasing by the same fixed amount after each instant of time. For example,
and so on, where each number exceeds the previous one by 2. The modern term is ‘linearly’.
Linear growth, even if the number added at each step is very large, will always be beaten in the long run by exponential growth, even if the multiplying factor is only slightly larger than 1. So Malthus deduced that unrestrained growth will always outstrip resources, and concluded that growth always has limits. The argument as presented is simplistic, with its reliance on tidy numerical sequences, but its conclusion is robust. Anything remotely like exponential growth will eventually beat anything that is roughly linear – including growing like some fixed power, squares, cubes, and so on.
Malthus was interested mainly in the human population, but Darwin noticed that the same reasoning must apply to animal populations. Such populations are roughly constant. The number of blackbirds in a given part of the country may fluctuate from year to year, but it doesn’t explode. On average, it stays much the same. Yet each pair of blackbirds produces dozens of offspring. What keeps population sizes stable? Clearly, as Malthus said, competition for resources: food, a place to live, mates and, of course, the effects of predators, another kind of competition. The inevitable result would be ‘natural selection’. The only creatures that could pass their characters on to the next generation would be those that bred . . . and in order to breed, they had to survive to adulthood.
Just as a human breeder might deliberately choose to breed from a faster horse or a thinner dog, so nature would – must – unconsciously ‘choose’ whichever adult organisms won the competition to survive and breed. And that wasn’t a lottery, it wasn’t a totally random process. Healthy animals would tend to beat sick ones; not always, but often. Strong animals would tend to beat weak ones. It might not always be obvious to a human scientist which strategy was best (a small animal can hide where a big one can’t, for instance), but nature would carry out the experiment automatically and find out what worked.
Here was the missing mechanism. It was well known that different organisms in a given species are not always identical. This process of natural selection would favour certain differences, but suppress others. The result would be gradual changes. How far might such changes lead? Enough small changes, piled on top of one another, can amount to a big one. A very big one. Big enough, Darwin thought, to generate entirely new species – given enough time.
Was there enough time? Darwin’s geological background left him in little doubt.
How old is the Earth? This may seem a silly question in a book about biology, but even the stoutest supporter of evolution accepts that the process must take a lot of time. Ten thousand years would be woefully inadequate for humans to evolve from an ape-like ancestor – let alone for the ancestor to evolve from fish, and fish to evolve from microbes.
Convince people that the Earth is a mere ten thousand years old, and the battle against what some inhabitants of the US Bible Belt call ‘evilution’ is won. Evolution must be nonsense. So Creationists now routinely dispute the scientifically established figure for the age of the Earth, which is about 4.6 billion years.
Until about 150 years ago, the Bible was one of the main sources of information about the past, so it made sense to try to deduce the date of creation from its contents. James Ussher, Archbishop of Armagh, was an intelligent and capable man with a gift for languages. In 1650 he published the first of two works devoted to Biblical chronology: Annals of the Old Testament. A sequel appeared in 1654. In these works he employed Biblical scholarship to work out the date of creation, and the answer he came up with was around nightfall on the day before 23 October 4004 BC.3
He was not alone in his endeavours. A decade earlier, John Lightfoot, using similar methods, deduced that the creation occurred near the autumnal equinox in 3929 BC. Isaac Newton, venerated today as one of the principal architects of the Age of Reason, arrived at a date of 4000 BC. Johannes Kepler, famed for his discovery of the laws of planetary motion, proposed 3992 BC. There was a strong consensus that the Earth was about 6,000 years old, so after 1700 the King James Bible included Ussher’s chronology among its annotations. It is therefore only to be expected that for several centuries, well-informed Christians knew that the Bible stated the Earth’s age as 6,000 years.
As the centuries passed, a series of scientific advances provided viable alternatives to Biblical scholarship, and made it possible to date the Earth’s rocks objectively, and with increasing accuracy, over increasing periods of time. These dates flatly contradicted Ussher’s chronology.
It took a while for the magnitude of deep time to sink in. Initially, ten million years was daring, but soon estimates of the order of a hundred million years became commonplace. There is now a very strong consensus among virtually all scientists that the Earth is close to 4.6 billion years old – three-quarters of a million times as old as proposed by the theological chronologists. That makes evolution far more plausible.
Prove that the Earth is young, and evolution is a dead duck. One consequence is Young Earth Creationism, which maintains, contrary to a truly gigantic body of scientific evidence, that the Earth is somewhere between 5,700 and 10,000 years old. Surveys indicate that about 45% of modern American adults accept this figure. They also show that most of those who do are low-paid and poorly educated.4 Bearing in mind that in the United States it is close to social suicide to admit to agnosticism, let alone atheism, these figures should be neither surprising nor especially discouraging to those of us who accept the science.
Armed with deep time and natural selection, Darwin now had the answer to the puzzles in his B Notebook. But as a solid Christian (albeit a Unitarian, a sect often characterised as believing in ‘at most one God’) he was uncomfortably aware that what he held in his hands was theological dynamite. His wife, Emma, was very religious; he knew she would find his new theories offensive, and he had no wish to upset her. He had no wish to be seen as attacking the Church, either. So he did what most of us would do in such circumstances: he dithered.
He amassed ever more extensive evidence for natural selection. He listed its weaknesses too: Darwin’s greatest strength was intellectual honesty. He discussed his ideas with a few trusted colleagues, among them Lyell and Joseph Hooker. In his head, Darwin conceived of a huge multi-volume work, so perfect and so well argued that no sensible person could disagree with it. He might have tinkered and polished and amended and dithered forever, were it not for events taking place half a world away, of which he yet knew nothing.
The agents of those events were a tropical typhoon and a Victorian explorer named Alfred Russel Wallace. Darwin came from a wealthy family. Wallace didn’t, and made a living by travelling to distant and exotic parts, collecting butterflies and beetles and other exotic creatures, and selling them. These items were popular with the middle and upper classes in Victorian times, and there were dealers who specialised in them. Wallace went to the Amazon in 1848, and by 1854 he was in Borneo hunting orang-utans. He was beginning to think they might represent human ancestors.
Stuck indoors when a typhoon was dumping rain all over Borneo, Wallace started playing around with a few ideas that had suddenly occurred to him about what he called ‘the introduction of species’. He wrote them up as a scientific article and sent it to the Annals and Magazine of Natural History. This was a rather unprestigious publication, but Lyell noticed when it published Wallace’s paper and told Darwin, since there seemed to be some similarities with the ideas that Darwin was explaining to his friends. Another friend, Edward Blyth, also drew it to Darwin’s attention, writing to tell him that he thought the article was ‘Good! Upon the whole.’ A worried Darwin got hold of a copy, and wrote back in relief to say that it was ‘nothing very new . . . it seems all creation with him.’
So Darwin relaxed. He had a generous nature, and encouraged Wallace to continue with the work, not realising where that might lead. Wallace followed the well-meant advice, and soon came up with a better idea, essentially identical to Darwin’s concept of natural selection. In June 1858 he sent Darwin a twenty-page letter outlining the argument, from which it was immediately obvious that Wallace had come up with a very similar theory, so similar that Darwin declared that his life’s work was ‘smashed’, adding: ‘If Wallace had my sketch of 1842, he could not have made a better short abstract!’
Trying to salvage something from the wreckage, Lyell suggested that the two men should publish their discoveries simultaneously, and Wallace agreed. He had no wish to steal Darwin’s thunder; he hadn’t realised that the great man had been working on anything remotely related. Since he now knew that Darwin had far better evidence, and much more material, Wallace did not wish to steal the limelight.
Spurred on by the worry of being beaten to the punch, Darwin quickly put together a short version of his own work. Hooker and Lyell got the two papers inserted into the schedule of the Linnaean Society. The Society was about to shut up shop for the summer, but its council fitted in an extra meeting at the last minute. The two papers were duly read to an audience of about thirty fellows . . . and we’ve already seen how they were received.
Darwin polished up his essay and changed its title to On the Origin of Species and Varieties by Means of Natural Selection. On the advice of his publisher, John Murray, he cut out the ‘and Varieties’. The first print run of 1,250 copies went on sale in November 1859 and sold out immediately.
The Wallace – Darwin theory of natural selection is simple – deceptively so, as will become apparent. The main points are:
1. living creatures differ from one another, even within a given species;
2. many of these differences can be passed on to offspring;
3. the environment cannot sustain all offspring produced, so there is competition for survival;
4. the survivors tend to be ‘better’ at surviving than the previous generation.
The deduction is that species can gradually change; and given enough time, small changes can combine into large ones.
This, said Darwin, is how new species arise from existing ones.
Natural selection is deceptively simple: most attempts to explain the ideas in non-technical terms, including my own attempt here, are forced to simplify a very complex process. Often they oversimplify it. A classic instance is Herbert Spencer’s characterisation of natural selection as ‘survival of the fittest’. Another is ‘nature red in tooth and claw’, from Tennyson’s poem ‘In Memoriam A.H.H.’,5 which actually referred to humanity, but was quickly seized upon by both proponents and opponents of evolution. A third is the idea, often stated by biologists as a flat fact, that evolution is random.
Spencer coined his memorable phrase in 1864, in his Principles of Biology. Biologically, ‘fitness’ is about how well an organism is equipped to ‘fit into’ its environment, but many people assumed it was about being healthy and in good physical shape. In addition, Spencer’s vision was closer to Lamarckism, so the book muddied already turbid waters. Critics often cite the phrase ‘survival of the fittest’ as proof that evolution is a ‘tautology’: survival is used to demonstrate fitness, and then fitness is proposed as the reason for survival.
There are two mistakes here. First, a tautology is a logical statement that is unconditionally true; the correct objection should be to circular reasoning. For example, it is fallacious to argue that the existence of life implies that there must be some supernatural ‘living essence’, and then use that essence to explain life. The more serious mistake is the assumption that Spencer’s dumbed-down phrase accurately describes the technical concept of natural selection. This misleadingly suggests that biologists think that creatures have an innate fitness, and the fitter creature wins. But evolutionary biologists do not use survival as a demonstration of fitness and then use fitness to justify survival. What matters is selection: since creatures compete for limited resources, some survive and some don’t. The process is a kind of filter, and what matters is that some organisms pass through it successfully, while others don’t. And filters do not just allow things to pass through, or block them, at random: there is a degree of systematic bias. If we don’t understand in great detail how the filter works, we may not be able to predict what will pass through and what won’t, but we can still predict that the distinction will be systematic.6 Organisms don’t compare their fitnesses to determine a winner. They just compete, and find out which one wins.
The word ‘compete’ also bears examination. Think of a wood, populated by foxes, rabbits and owls. Which competitions are the most significant for evolution? ‘Nature red in tooth and claw’ leads us to home in on red-toothed foxes, or predatory owls hunting down innocent rabbits: the obvious competition is between predator and prey. But which organisms constitute a rabbit’s most serious competition?
Other rabbits.
Rabbits are in competition with one another for the same resources, and in the struggle to survive the same dangers. They are competing for the same food, and trying to escape from the same foxes and owls.7 The rabbits that win these ongoing competitions are the ones that will survive to breed, and the abilities that enable them to do so (if they are of a kind that can be inherited) may pass to their descendants and similarly improve their survival prospects. The same goes for foxes, whose main competitors are other foxes, and for owls, whose main competitors are other owls.
Competition between foxes and rabbits happens as well, and it also has an evolutionary effect, but on a slower timescale. It leads to an ‘arms race’ in which, say, rabbits acquire the ability to run faster, but foxes evolve to do the same, and the whole cycle goes round and round, driving both creatures to ever greater efforts.
However, there is a third competition going on here: it is between foxes and owls. Foxes don’t usually eat owls, and owls don’t eat foxes – well, maybe baby ones. But they both eat rabbits, and if the rabbit supply gets too low, owls and foxes can’t avoid coming up against each other. This competition is indirect; the participants may not even be aware that their competitor exists. All they know is that there aren’t enough rabbits around and they’re going hungry.
Natural selection is not merely a matter of pitting one organism against another. It happens in the context of the surrounding environment – the entire local ecosystem. The rabbit population depends on the plants they eat, the availability of suitable soil to dig burrows, the amount of low cover to hide in. These things depend on other, less obvious features of the ecology, such as insects to fertilise the plants, and fungi and bacteria to condition the soil. So what evolves is really the entire ecosystem. Yes, the evolution is driven by what happens to individual organisms, but the ecosystem determines the context in which they compete. Mathematical models discussed in Chapter 14 underline this point.
Is evolution random? It’s clearly not predetermined – you can no more forecast which rabbit will make it through the day than you can predict whether France will beat Mexico in the World Cup. But there are clear trends, some events are more likely than others, and so on. Down at the molecular level, genetic changes can perhaps be characterised as random. Mutations, genetic changes, can be caused by chemicals and cosmic rays, which are effectively random.8 But that doesn’t imply that evolution itself is equally likely to change in one direction as it is in another, and the reason, yet again, is natural selection. Depending on the appropriate context, selection introduces a degree of preference. Some mutations improve the creature’s survival chances, some decrease them, and most are neutral.
Let me suggest an analogy. The motion of individual molecules in water is random, but that doesn’t mean that water is just as likely to flow uphill as down. The selective effect of gravity leads to a strong preference for down. But what the water does, in bulk, is not merely determined by moving downwards. Where it ends up depends on the landscape in which it flows. Natural selection is rather like the force of gravity (though not as strongly selective and not as easy to characterise independently of its effects). Mutations are like the random excursions of water molecules. And the environment is like the landscape.
In 1973 Theodosius Dobzhansky, a prominent evolutionary biologist, wrote an essay with the title ‘Nothing in biology makes sense except in the light of evolution’.9 Everything that has been discovered in biology since then – which is an awful lot – has reinforced that statement. The word ‘theory’ has two very different meanings, and there is an overwhelming scientific consensus that in ‘theory of evolution’ this word has made the transition from the sense of ‘tentative hypothesis’ to the sense of ‘coherent explanation confirmed by a substantial body of evidence from diverse sources which has survived innumerable attempts to disprove it’. As Richard Dawkins has remarked, the everyday term for this sense of ‘theory’ is ‘fact’, and it is mainly a wish to avoid appearing dogmatic that prevents scientists from employing the same term.
It would hardly be necessary to point this out, were it not for vocal opposition to evolution by a few fundamentalist religious groups. If the world was indeed created by God ten thousand years ago, then the Deity has gone to enormous lengths to fabricate a massive, interlocking network of natural features, specifically designed to mislead any intelligent observer into the mistaken belief that life on Earth has diversified, over billions of years, from simple beginnings. This view of God as liar seems theologically improper, and that is the conclusion that Victorian clergy came to once they had absorbed the scientific discoveries of their age. So did Dobzhansky, a Russian Orthodox Christian.
The evidence for evolution comes from many different sources. The variety of these sources, and their independence from one another, greatly strengthens the scientific case in favour of evolution, because each new source provides a large number of potential ways for the theory to be disproved. So far the basic principle has survived unscathed, but the details of the evolutionary process have been clarified, and sometimes changed, as new evidence comes in. The evidence available today is far more extensive than what was available in Darwin’s time; it is also more quantitative and more precise. The main sources are:
• the pliability of the form and behaviour of organisms, evident in human-induced breeding programmes for dogs, pigeons, horses and other domesticated animals;
• similarities between existing creatures, suggesting a common origin;
• the occurrence of the same biochemical components and systems in many different organisms;
• the fossil record, which reveals coherent sequences of changes over time;
• the geological record, which confirms the dating of fossil species;
• genetic features of organisms, especially DNA sequences, which confirm both lines of descent and the timing of changes;
• relations between the distribution of species and current or historical geographical features;
• observed natural selection in the laboratory and the real world;
• mathematical studies of the effect of selection principles on changes in complex systems.
Evolution’s critics often claim that because we can’t observe the past, the theory is not scientifically testable. But science is about inference as well as direct observation. When Haldane was asked what evidence could possibly disprove evolution, his immediate response was: ‘Fossil rabbits in the Cretaceous’. Fossils are relics of living creatures from the past, transformed into and preserved in the rocks of our planet. Because rocks are often datable, many fossils can be reliably assigned to specific periods of history.
Fossils provide a rather sparse record of the life forms of the past, because it is very rare for any individual organism to become a fossil. However, there have been an awful lot of organisms over the past few hundred million years, and more than 250,000 different fossil species have been discovered. The number of individual known fossils is large (more than three million just from the La Brea tar pits at Los Angeles, for example) and is increasing rapidly with the discovery of new sites, all over the world, and with improved techniques for locating fossils and analysing them.
Despite the relative rarity of fossils, the record is sometimes very extensive, with few significant gaps, and it then offers clear evidence of systematic long-term evolutionary changes. The classic example is the evolution of the horse, between 54 million years ago and a million years ago. The sequence begins with a horse-like mammal a mere 0.4 metres long. This genus was originally given the poetic name Eohippus (‘dawn horse’), but has since been renamed Hyracotherium because of the rules of taxonomy, which in this case managed to deliver a silly result.10 The sequence continues with Mesohippus, 35 million years old and 0.6 metres long; then Merychippus, 15 million years old and 1 metre long; then Pliohippus, 8 million years old and 1.3 metres long; and finally (so far) Equus, essentially the same as the modern horse, 1 million years old and 1.6 metres long.
Taxonomists can track, in great detail, the sequence of changes that occurred in this lineage of ancient horse ancestors, for example in the animal’s teeth and hooves. They can also track the timing of these changes, because rocks can be dated. So now evidence from geology can be thrown into the mix. In principle it would take only one fossil species in the wrong stratum of rock to cast doubt on the evolutionary story – and that is Haldane’s point. In practice it would take several independent instances, because there might exist sensible explanations for a few isolated exceptions. The plain fact is that the succession of rocks, their ages as determined by a variety of different methods, and the evolutionary sequences of fossils all agree to a remarkable extent.
A standard objection to the use of fossils to support evolution is the absence of transitional forms, popularly known as ‘missing links’. Neither term is terribly satisfactory: in evolutionary biology there is a sense in which allspecies are in transition (from their ancestral species to their descendants), and transitional forms are not modern ‘links’ but ancient common ancestors. However, the important question is whether these forms are absent because they never existed, or because they did exist but we haven’t yet found any fossils. Evolution predicts the latter – yet another way in which evolution has predictive power – while its detractors assert the former.
As more and more fossils have been discovered, more and more transitional forms have been found. A major example is the transition from fish to land animals – tetrapods, creatures having four limbs instead of fins. The only fossils available to Victorian palaeontologists were either fish or amphibians, leaving a gap of at least 50 million years with no transitional fossils in between. But from 1881 onwards, new fossil discoveries have inserted a whole series of intermediates between fish and amphibians: Osteolepis, Eusthenopteron, Panderichthys, Tiktaalik, Elginerpeton, Obruchevichthys, Ventastega, Acanthostega, Ichthyostega, Hynerpeton, Tulerpeton, Pederpes, Eryops. Dozens of other comparable gaps have been filled in the past twenty years, and every year now sees the discovery of more transitional forms, ever more finely spaced.
One evolutionary critic has remarked that whenever a gap in the fossil record is filled, it creates two more gaps on either side of it. This rather desperate excuse is true in a trivial sense, but it constitutes a serious misunderstanding of the nature of scientific inference. Each discovery of another transitional form represents a successful prediction for evolution and a failure for its detractors. Moreover, the two gaps that are created are significantly smaller than the one that was there before. A new transitional form is one more nail in the coffin of special creation. Enough nails will fix the lid on firmly; it is not necessary to have an unbroken continuum of them all round the edge.
It wasn’t his wish to create controversy, but Darwin had opened Pandora’s box. As the implications of the Origin struck home, and even more so those of its successor, The Descent of Man, which argued that humans and apes are descended from a common ancestor, hackles were raised. Sensitivities were trampled. Social convention was outraged.
If your default worldview places humankind at the pinnacle of creation, with the rest of the universe as a resource for us to exploit, then the suggestion that people and animals have a lot in common is hard to accept, and the idea that humans and today’s animals evolved from common ancestors is anathema. It inevitably led to snide remarks like, ‘Exactly which ape was your grandfather, Mr Darwin?’, and unflattering cartoons, many of which would now be classed as racist.
From a more dispassionate viewpoint, however, the close relationship between humans and animals is obvious. We eat like animals, reproduce like animals, excrete like animals. Our anatomical features correspond closely to those of many other members of the animal kingdom. Our skeletons match those of most mammals virtually bone for bone; only the shapes and sizes are a bit different. Our brains have a lot in common with those of mammals, amphibians and reptiles. Our hands are very similar to those of the great apes, and not so different from those of monkeys and lemurs.
When a difference of opinion on such matters was just that, the outcome was pre-empted by social convention. In the predominantly Christian tradition of the Western world, it was taken for granted that people are completely different from animals. The differences, such as our ability to talk, write, compose music or paint portraits, were emphasised; the similarities (especially those related to embarrassing bodily functions) were ignored, minimised or, as a last resort, denied. But as scientific evidence accumulated, this stance became difficult to maintain. In Victorian England, and most of Europe, religious people slowly came round to the idea that the creation story in Genesis is a metaphor, and they accepted the discoveries of science as insights into God’s creation. Atheists had no problem anyway. But those who believed in the literal truth of the Bible painted themselves into an intellectual corner, by tying their entire belief system to an unconvincing denial of a huge and ever-growing body of scientific evidence.