by Tim Harding, B.Sc.
(An edited version of this essay was published in The Skeptic magazine, June 2015, Vol 35 No 2, under the title ‘Darwin’s Missing Link’. The essay is based on a talk presented to the Mordi Skeptics on Tuesday 5 May 2015).
Charles Darwin (1809-1882) is best known for his major contributions to evolutionary theory. In 1859, Darwin published his theory of natural selection as the mechanism of evolution in his revolutionary book On the Origin of Species. This book provided compelling evidence overcoming the scientific rejection of earlier concepts of transmutation of species. The basic principles of his theory have been shown to be correct and are now widely accepted as the basis of mainstream zoology, botany and ecology.
On the other hand, in a later book Darwin got it wrong with the mechanisms of inheritance. The empirical rules of genetics, based solely on observational results, were largely understood since Gregor Mendel’s ‘wrinkled pea’ experiments in the 1860s. The postulated units of inheritance were called genes, but in Charles Darwin’s time it was not understood where genes were located in the body or what they physically consisted of. Darwin knew that there must have been a physical mechanism for inheritance, but his speculations about it – called pangenesis – were incorrect. Fortunately for the credibility of his theory of evolution by natural selection, he published these speculations later in a separate 1868 book titled Variation of Animals and Plants Under Domestication.
Darwin’s early career
Charles Robert Darwin was born in Shrewsbury, England, on 12 February 1809 at his family home, The Mount. He was the fifth of six children of wealthy society doctor and financier Robert Darwin, and Susannah Darwin (née Wedgwood).
Darwin went to Edinburgh University in 1825 to study medicine. In his second year he neglected his medical studies for natural history and spent four months assisting Robert Grant’s research into marine invertebrates. Grant revealed his enthusiasm for the concept of transmutation of species (the altering of one species into another), but Darwin initially rejected this concept (probably for religious reasons).
Ideas about the transmutation of species were controversial as they conflicted with theological beliefs that species were unchanging parts of a designed hierarchy and that humans were unique, unrelated to other animals. The political and religious implications were intensely debated, but transmutation was not accepted by the scientific mainstream until Darwin’s theory.
In December 1831, Darwin had joined the Beagle ship voyage as a gentleman naturalist and geologist. In South America, he discovered fossils resembling huge armadillos, and noted the geographical distribution of modern species in hope of finding their ‘centre of creation’. As the Beagle neared England in 1836, he began to think that species might not be immutable after all.
In March 1837, ornithologist John Gould announced that mockingbirds collected on the Galápagos Islands represented three separate species each unique to a particular island, and that several distinct birds from those islands were all classified as finches. Darwin began speculating, in a series of notebooks, on the possibility that ‘one species does change into another’ to explain these findings, and around July of that year sketched a genealogical branching of a single evolutionary tree. Unconventionally, Darwin asked questions of fancy pigeon and animal breeders as well as established scientists.
Charles Darwin in 1860, aged 51
In late September 1838, Darwin started reading Thomas Malthus’s An Essay on the Principle of Population with its statistical argument that human populations, if unrestrained, breed beyond their means and struggle to survive. Darwin related this to the struggle for existence among wildlife and plants, so that the survivors would pass on their form and abilities, and unfavourable variations would be destroyed. By December 1838, he had noted a similarity between the act of breeders selecting traits and a Malthusian nature selecting among variants thrown up by chance.
Darwin now had the framework of his theory of natural selection, but he was fully occupied with his career as a geologist and held off writing a sketch of his theory until his book on The Structure and Distribution of Coral Reefs was completed in May 1842.
Evolution by natural selection
Darwin continued to research and extensively revise his theory of natural selection while focusing on his main work of publishing the scientific results of the Beagle voyage. He tentatively wrote of his ideas to the famous Scottish geologist Charles Lyell in January 1842; then in June he roughed out a 35-page pencil sketch of his theory. Darwin began correspondence about his theorising with the botanist Joseph Dalton Hooker in January 1844, and by July had rounded out his sketch into a 230-page essay, to be expanded with his research results and published if he died prematurely.
His famous 1859 book On the Origin of Species was written for non-specialist readers and attracted widespread interest upon its publication. As Darwin was already an eminent scientist, his findings were taken seriously. The evidence he presented generated scientific, philosophical, and religious discussion. The debate over the book contributed to the campaign by Thomas Huxley and his fellow members of the X Club to secularise science by promoting scientific naturalism. Within two decades there was widespread scientific agreement that evolution, with a branching pattern of common descent, had occurred, but scientists were slow to give the mechanism of natural selection the significance that it deserved.
Diagram representing the divergence of species, from Darwin’s Origin of Species
Darwin’s theory of evolution is based on some key facts (based on wild populations without human interference), which biologist Ernst Mayr has summarised as follows:
- Every species is fertile enough that if all offspring survived to reproduce the population would grow.
- Despite periodic fluctuations, populations remain roughly the same size.
- Resources such as food are limited and are relatively stable over time.
- Individuals in a population vary significantly from one another.
- Much of this variation is heritable.
From these key facts, the following important inferences may be made, once again summarised by Ernst May:
- A struggle for survival ensues.
- Individuals less suited to the environment are less likely to survive and less likely to reproduce.
- Individuals more suited to the environment are more likely to survive and more likely to reproduce and leave their heritable traits to future generations, which produces the process of natural selection.
- This slow process gradually results in populations changing to adapt to their environments, and ultimately, these variations accumulate over time to form new species.
Natural selection provided a mechanism for variation and eventual speciation, but it did not explain the inheritance of variation. Without some way to explain the inheritance of characteristics acted on by natural selection, his theory would be incomplete.
Mechanisms of inheritance
Before the advent of genetics, Hippokratic theories attempted to explain inheritance in terms of a blending of fluids extracted from all parts of both male and female bodies during intercourse. It was thought that the characteristics of the offspring are determined by the relative amounts and strength of fluids from each part of the body of each parent.
On the other hand, ‘preformationist’ theories held that the new mammalian offspring is already preformed in miniature, either within the egg of its mother or in the semen of its father. Both of these types of theories incorporated ‘encasement’, which was the thesis that God created all future organisms in miniature, and that reproduction was just the growth and development of these miniatures.
Hippokratic theories were very good at explaining inheritance but very bad at explaining growth and development; whilst preformationist theories were the opposite – very good at explaining growth and development but very bad at explaining inheritance. To give some examples, Hippokratic theories were unable to adequately explain phenomena such as the regeneration of freshwater polyps; while preformationist theories were unable to adequately explain how the mating of a mare with a donkey produces a mule.
Darwin came to his hypothesis of pangenesis, from a different direction – to fill a gap left in his theory of evolution. Darwin’s breeding experiments on domestic animals (mainly pigeons) in the 1850s and 60s were part of his attempts to complete his evolution theory. He was attempting in these experiments to show just how quickly varying characteristics can be amplified by domestic breeding, and therefore how natural selection can operate.
Darwin called his explanation of inheritance ‘the hypothesis of Pangenesis’, which he published in 1868. However, he provides a more succinct description of this hypothesis in an earlier unpublished manuscript on pangenesis sent to Thomas Huxley in 1865:
“Furthermore, I am led to believe from analogies immediately to be given that protoplasm or formative matter which is throughout the whole organisation, is generated by each different tissue and cell or aggregate of similar cells; – that as each tissue or cell becomes developed, a superabundant atom or gemmule as may be called of the formative matter is thrown off; – that these almost infinitely numerous and infinitely minute gemmules unite together in due proportion to form the true germ; – that they have the power of self-increase or propagation; and that they here run through the same course of development, as that which the true germ, of which they are to constitute elements, has to run through, before they can be developed into their parent tissues or cells. This may be called the hypothesis of Pangenesis”.
The Laws of Inheritance & Pangenesis
Darwin further proposed that his hypothesis would not only account for inheritance, but also for development:
“The development of each being, including all the forms of metamorphosis and metagenesis, as well as the so-called growth of the higher animals, in which structure changes, though not in a striking manner, depends on the presence of gemmules thrown off at each period of life, and on their development, at a corresponding period, in union with the preceding cells”.
Through these mechanisms, Darwin proposed that inheritance and development were tied together – not only in the generation of offspring and early stages of embryonic life, but throughout the life of the organism. By giving ‘gemmules’ the power to be modified throughout the life of an organism and then be transferred to the next generation, he argued that inheritance should be viewed as a form of growth.
By means of this single hypothesis, Darwin attempted to not only fill a gap in his theory of evolution, but whether he meant to or not, he created an apparent synthesis between the then competing paradigms relating to inheritance and development.
After reading Variation Under Domestication, Francis Galton (a cousin of Darwin’s) arranged for a series of experiments to be conducted on rabbits initially housed in the Zoological Gardens of London and later at his Kensington home. His intention was to demonstrate the transmission of ‘gemmules’ to succeeding generations via blood injected from one rabbit to another, using coat colour as a marker. Galton ultimately found that not a single instance of induced variation of coat colour occurred in a total of 88 offspring from blood transfused parents, and in 1871 published his results in Nature.
In later editions of Variation Under Domestication, Darwin admitted in a footnote that he would have expected to find ‘gemmules’ in the blood, although their presence was not absolutely necessary to his hypothesis. Darwin’s response is unconvincing, as he provides no alternative explanation as to how the ‘gemmules’ are transmitted from the parents’ somatic cells to the germ cells. He made no real attempt to modify his hypothesis in response to Galton’s falsification of it, indicating a possible abandonment of commitment to his hypothesis.
After the rediscovery of Mendel’s work in the 1890s, scientists tried to determine which molecules in the cell were responsible for inheritance. In 1910, Thomas Hunt Morgan argued that genes are on chromosomes, based on observations of a sex-linked white eye mutation in fruit flies. In 1913, his student Alfred Sturtevant used the phenomenon of genetic linkage to show that genes are arranged linearly on the chromosome. It was soon discovered that chromosomes consisted of DNA and proteins, but DNA was not identified as the gene carrier until 1944. Watson and Crick’s breakthrough discovery of the chemical structure of DNA in 1953 finally revealed how genetic instructions are stored inside organisms and passed from generation to generation.
In view of the fact that it took another 85 years after Darwin’s book Variation Under Domestication before the molecular mechanisms of inheritance to be discovered, Darwin can hardly be blamed for getting it wrong way back in 1868. This was before even chromosomes had been discovered, let alone DNA.
On the plus side, Darwin’s theory of evolution by natural selection, with its tree-like model of branching common descent, has become the unifying theory of the life sciences. The theory explains the diversity of living organisms and their adaptation to the environment. It makes sense of the geologic record, biogeography, parallels in embryonic development, biological homologies, vestigiality, cladistics, phylogenetics and other fields, with unrivalled explanatory power; it has also become essential to applied sciences such as medicine, agriculture, conservation and environmental sciences.
Darwin, Charles (1859) The Origin Of Species. 6th ed. 1873. London: John Murray.
Darwin, Charles (1875) The Variation of Animals and Plants Under Domestication, Vol II London: John Murray.
Mayr, Ernst (1982) The Growth of Biological Thought: Diversity, Evolution, and Inheritance Harvard University Press.
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The story, still sometimes repeated in creationist circles, goes like this: it is the 1960s, at Nasa's Goddard Space Flight Centre in Maryland, and a team of astronomers is using cutting-edge computers to recreate the orbits of the planets, thousands of years in the past. Suddenly, an error message flashes up. There's a problem: way back in history, one whole day appears to be missing.
The scientists are baffled, until a Christian member of the team dimly recalls something and rushes to fetch a Bible. He thumbs through it until he reaches the Book of Joshua, chapter 10, in which Joshua asks God to stop the world for . . . "about a full day!" Uproar in the computer lab. The astronomers have happened upon proof that God controls the universe on a day-to-day basis, that the Bible is literally true, and that by extension the "myth" of creation is, in fact, a reality. Darwin was wrong – according to another creationist rumour, he'd recanted on his deathbed, anyway – and here, at last, is scientific evidence!
Inevitably, those of us who aren't professional scientists have to take a lot of science on trust. And one of the things that makes it so easy to trust the standard view of evolution, in particular, is amply illustrated by the legend of the Nasa astronomers: the doubters are so deluded or dishonest that one needn't waste time with them. Unfortunately, that also makes it embarrassingly awkward to ask a question that seems, in the light of recent studies and several popular books, to be growing ever more pertinent. What if Darwin's theory of evolution – or, at least, Darwin's theory of evolution as most of us learned it at school and believe we understand it – is, in crucial respects, not entirely accurate?
Such talk, naturally, is liable to drive evolutionary biologists into a rage, or, in the case of Richard Dawkins, into even more of a rage than usual. They have a point: nobody wants to provide ammunition to the proponents of creationism or "intelligent design", and it's true that few of the studies now coming to public prominence are all that revolutionary to the experts. But in the culture at large, we may be on the brink of a major shift in perspective, with enormous implications for how most of us think about how life came to be the way it is. As the science writer David Shenk puts it in his new book, The Genius in All of Us, "This is big, big stuff – perhaps the most important [discoveries] in the science of heredity since the gene."
Take, to begin with, the Swedish chickens. Three years ago, researchers led by a professor at the university of Linköping in Sweden created a henhouse that was specially designed to make its chicken occupants feel stressed. The lighting was manipulated to make the rhythms of night and day unpredictable, so the chickens lost track of when to eat or roost. Unsurprisingly, perhaps, they showed a significant decrease in their ability to learn how to find food hidden in a maze.
The surprising part is what happened next: the chickens were moved back to a non-stressful environment, where they conceived and hatched chicks who were raised without stress – and yet these chicks, too, demonstrated unexpectedly poor skills at finding food in a maze. They appeared to have inherited a problem that had been induced in their mothers through the environment. Further research established that the inherited change had altered the chicks' "gene expression" – the way certain genes are turned "on" or "off", bestowing any given animal with specific traits. The stress had affected the mother hens on a genetic level, and they had passed it on to their offspring.
The Swedish chicken study was one of several recent breakthroughs in the youthful field of epigenetics, which primarily studies the epigenome, the protective package of proteins around which genetic material – strands of DNA – is wrapped. The epigenome plays a crucial role in determining which genes actually express themselves in a creature's traits: in effect, it switches certain genes on or off, or turns them up or down in intensity. It isn't news that the environment can alter the epigenome; what's news is that those changes can be inherited. And this doesn't, of course, apply only to chickens: some of the most striking findings come from research involving humans.
One study, again from Sweden, looked at lifespans in Norrbotten, the country's northernmost province, where harvests are usually sparse but occasionally overflowing, meaning that, historically, children sometimes grew up with wildly varying food intake from one year to the next. A single period of extreme overeating in the midst of the usual short supply, researchers found, could cause a man's grandsons to die an average of 32 years earlier than if his childhood food intake had been steadier. Your own eating patterns, this implies, may affect your grandchildren's lifespans, years before your grandchildren – or even your children – are a twinkle in anybody's eye.
It might not be immediately obvious why this has such profound implications for evolution. In the way it's generally understood, the whole point of natural selection – the so-called "modern synthesis" of Darwin's theories with subsequent discoveries about genes – is its beautiful, breathtaking, devastating simplicity. In each generation, genes undergo random mutations, making offspring subtly different from their parents; those mutations that enhance an organism's abilities to thrive and reproduce in its own particular environment will tend to spread through populations, while those that make successful breeding less likely will eventually peter out.
As years of bestselling books by Dawkins, Daniel Dennett and others have seeped into the culture, we've come to understand that the awesome power of natural selection – frequently referred to as the best idea in the history of science – lies in the sheer elegance of the way such simple principles have generated the unbelievable complexities of life. From two elementary notions – random mutation, and the filtering power of the environment – have emerged, over millennia, such marvels as eyes, the wings of birds and the human brain.
Yet epigenetics suggests this isn't the whole story. If what happens to you during your lifetime – living in a stress-inducing henhouse, say, or overeating in northern Sweden – can affect how your genes express themselves in future generations, the absolutely simple version of natural selection begins to look questionable. Rather than genes simply "offering up" a random smorgasbord of traits in each new generation, which then either prove suited or unsuited to the environment, it seems that the environment plays a role in creating those traits in future generations, if only in a short-term and reversible way. You begin to feel slightly sorry for the much-mocked pre-Darwinian zoologist Jean-Baptiste Lamarck, whose own version of evolution held, most famously, that giraffes have long necks because their ancestors were "obliged to browse on the leaves of trees and to make constant efforts to reach them". As a matter of natural history, he probably wasn't right about how giraffes' necks came to be so long. But Lamarck was scorned for a much more general apparent mistake: the idea that lifestyle might be able to influence heredity. "Today," notes David Shenk, "any high school student knows that genes are passed on unchanged from parent to child, and to the next generation and the next. Lifestyle cannot alter heredity. Except now it turns out that it can . . ."
Epigenetics is the most vivid reason why the popular understanding of evolution might need revising, but it's not the only one. We've learned that huge proportions of the human genome consist of viruses, or virus-like materials, raising the notion that they got there through infection – meaning that natural selection acts not just on random mutations, but on new stuff that's introduced from elsewhere. Relatedly, there is growing evidence, at the level of microbes, of genes being transferred not just vertically, from ancestors to parents to offspring, but also horizontally, between organisms. The researchers Carl Woese and Nigel Goldenfield conclude that, on average, a bacterium may have obtained 10% of its genes from other organisms in its environment.
To an outsider, this is mind-blowing: since most of the history of life on earth has been the history of micro-organisms, the evidence for horizontal transfer suggests that a mainly Darwinian account of evolution may be only the latest version, applicable to the most recent, much more complex forms of life. Perhaps, before that, most evolution was based on horizontal exchange. Which gives rise to a compelling philosophical puzzle: if a genome is what defines an organism, yet those organisms can swap genes freely, what does it even mean to draw a clear line between one organism and another? "It's natural to wonder," Goldenfield told New Scientist recently, "if the very concept of an organism in isolation is still valid at this level." In natural selection, we all know, the fittest win out over their rivals. But what if you can't establish clear boundaries between rivals in the first place?
It is a decade since the biologist Randy Thornhill and the anthropologist Craig Palmer published The Natural History of Rape. In the book, they made an argument that – however obnoxious at first glance – seemed, to many, to follow straightforwardly from the logic of natural selection. Evolution tells us that the traits that flourish down the generations are the ones that help organisms reproduce. Evolutionary psychology argues that there's no reason to exclude psychological traits. And since rape is indeed a trait that occurs all too frequently in human society, it follows that a desire to commit rape must be adaptive. There must be a genetic basis for it – a "rape gene", in the words of some media stories following the book's publication – because, in prehistoric times, those men who possessed the tendency would reproduce more successfully than those who didn't. Therefore, the authors concluded, rape was – to use a loaded term that has been getting Darwinians in trouble since Darwin – "natural".
Understandably, the book was hugely controversial. But by the time it was published, there was nothing all that radical about the idea that natural selection might be able to illuminate any and every aspect of human behaviour. Evolutionary psychology, in the hands of various practitioners, sought to explain why militarism is so prevalent in human societies, or why men tend to dominate women in so many hierarchical organisations. If the field seems less politically charged these days, that is only because it has permeated our consciousness so deeply that it has become less questioned.
For much of the late Noughties, a week never seemed to pass without one new book or news story attributing some facet of modern-day life to the evolutionary past: men were more prone to sexual jealousy than women because a woman who conceives becomes unavailable for imminent future acts of reproduction; men preferred women with waist-to-hip ratios of 0.7 because of natural selection. It explained music and art and why we reward senior executives with top-floor corner offices (because we evolved to want a clear view of our enemies approaching across the savannah). Leftwing and feminist critics did frequently misinterpret evolutionary psychology, imagining that when scholars described some trait as adaptive, they meant it was morally justifiable. But that was how many such findings – often better described as speculations – came to be believed. We're not exactly saying it's right for, say, men to sleep around, evolutionary psychologists would observe with a knowing sigh, but . . . well, good luck trying to change millennia of evolved behaviour.
Far more than biologists, evolutionary psychologists bought in to the ultra-simple version of natural selection, and so they stand to lose far more from advances in our understanding of what's really been going on. They were always prone to telling "just-so stories" – spinning plausible tales about why some trait might be adaptive, instead of demonstrating that it was – and numerous recent studies have begun to chip away at what evidence there was. (That waist-to-hip ratio finding, for example, doesn't seem to hold up in the face of international and historical research.) And now, if epigenetics and other developments are coming to suggest that environment can alter heredity, the very terms of the debate – of nature versus nurture – suddenly become shaky. It's not even a matter of settling on a compromise, a "mixture" of nature and nurture. Rather, the concepts of "nature" and "nurture" seem to be growing meaningless. What does "nature" even mean if you can nurture the nature of your descendants?
This is one central argument of Shenk's new book, subheaded Why Everything You've Been Told About Genetics, Talent and IQ is Wrong. All our popular notions about talent and "genetic gifts", he points out, start to collapse if the eating habits of Tiger Woods's ancestors, for example, might have played a role in Woods's golfing abilities. (Woods always crops up in discussions on the origins of genius; more recently, he has started cropping up in evolutionary psychology discussions about whether promiscuity is inevitable.)
"What all this evidence shows is that we need a much more subtle and nuanced understanding of Darwinism and natural selection," Shenk says. "I think that's inevitably going to happen among scientists. The question is how much nuance will carry over into the public sphere . . . it's really funny how difficult it is to have this conversation, even with a lot of people who understand the science. We're stuck with a pretty limited way of viewing all this, and I think part of that comes from the terms" – such as nature and nurture – "that we have."
Among the arsenal of studies at Shenk's disposal is one published last year in the Journal of Neuroscience, involving mice bred to possess genetically inherited memory problems. As small recompense for having been bred to be scatterbrained, they were kept in an environment full of stimulating mouse fun: plenty of toys, exercise and attention. Key aspects of their memory skills were shown to improve, and crucially so did those of their offspring, even though the offspring had never experienced the stimulating environment, even as foetuses.
"If a geneticist had suggested as recently as the 1990s that a 12-year-old kid could improve the intellectual nimbleness of his or her future children by studying harder now," writes Shenk, "that scientist would have been laughed right out of the hall." Not so now.
And then there is Jerry Fodor, the American philosopher. I started reading What Darwin Got Wrong, the new book he has co-authored with the cognitive scientist Massimo Piattelli-Palmarini, one morning, along with that day's first coffee. A few pages later, as the coffee kicked in, I grasped with astonishment what Fodor had done. He hadn't just identified evidence that natural selection was more complicated than previously thought – he'd uncovered a glaring flaw in the whole notion! Natural selection, he explains, simply "cannot be the primary engine of evolution". I got up and refilled my cup. But by the time I returned, his argument had slipped from my grasp. Suddenly, he seemed obviously wrong, tied up in philosophical knots of his own creation. I alternated between these two convictions. Was Fodor's critique so devastatingly correct that his critics – Dawkins, Dennett, the Cambridge philosopher Simon Blackburn, and many others – simply couldn't see it? Had he actually managed to . . . but then it slipped away again, vanishing into mental fog.
I called Fodor and asked him to explain his point in language an infant school pupil could understand. "Can't be done," he shot back. "These issues really are complicated. If we're right that Darwin and Darwinists have missed the point we've been making for 150 years, that's not because it's a simple point and Darwin was stupid. It's a really complicated issue."
Fodor's objection is a distant cousin of one that rears its head every few years: doesn't "survival of the fittest" just mean "survival of those that survive", since the only criterion of fitness is that a creature does, indeed, survive and reproduce? The American rightwing noisemaker Ann Coulter makes the point in her 2006 pro-creationist tirade Godless: The Church of American Liberalism. "Through the process of natural selection, the 'fittest' survive, [but] who are the 'fittest'? The ones who survive!" she sneers. "Why, look – it happens every time! The 'survival of the fittest' would be a joke if it weren't part of the belief system of a fanatical cult infesting the Scientific Community."
This argument, perhaps uniquely among all arguments ever made by Coulter, feels persuasive, not least because it is a reasonable criticism of some pop-Darwinism. In fact, though, it's entirely possible for scientists to measure fitness using criteria other than survival, and thus to avoid circular logic. For example, you might hypothesise that speed is a helpful thing to have if you're an antelope, then hypothesise the kind of leg structure you'd want to have, as an antelope, in order to run fast; then you'd examine antelopes to see if they do indeed have something approximating this kind of leg structure, and you'd examine the fossil record, to see if other kinds of leg died out.
Fodor's point is more complex than this, although it's also possible that it is not really a point at all: several reviews of the book by professional evolutionary theorists and philosophers have concluded that it is, indeed, nonsense. As far as I can make out, it can be summarised in three steps. Step one: Fodor notes – undeniably correctly – that not every trait a creature possesses is necessarily adaptive. Some just come along for the ride: for example, genes that express as tameness in domesticated foxes and dogs also seem to express as floppy ears, for no evident reason. Other traits are, as logicians say, "coextensive": a polar bear, for example, has the trait of "whiteness" and also the trait of "being the same colour as its environment". (Yes, that's a brain-stretching, possibly insanity-inducing statement. Take a deep breath.) Step two: natural selection, according to its theorists, is a force that "selects for" certain traits. (Floppy ears appear to serve no purpose, so while they may have been "selected", as a matter of fact, they weren't "selected for". And polar bears, we'd surely all agree, were "selected for" being the same colour as their environment, not for being white per se: being white is no use as camouflage if snow is, say, orange.)
Step three is Fodor's coup de grace: how, he says, can that possibly be? The whole point of Darwinian evolution is that it has no mind, no intelligence. But to "select for" certain traits – as opposed to just "selecting" them by not having them die out – wouldn't natural selection have to have some kind of mind? It might be obvious to you that being the same colour as your environment is more important than being white, if you're a polar bear, but that's because you just ran a thought-experiment about a hypothetical situation involving orange snow. Evolution can't run thought experiments, because it can't think. "Darwin has a theory that centrally turns on the notion of 'selection-for'," says Fodor. "And yet he can't give an account – nobody could give an account – of how natural selection could distinguish between correlated traits. He waffles."
Those of us baffled by this argument can take solace in the fact that we're not alone. The general response to Fodor among evolutionary thinkers has been a mixture of derision and awkwardness, as if one of their previously esteemed colleagues had entered the senior common room naked. Says Dennett, via email: "Jerry Fodor's book is a stunning demonstration of how abhorrence of an idea (Jerry's visceral dislike of evolutionary thinking) can derange an otherwise clever thinker . . . a responsible academic is supposed to be able to control irrational impulses, [but] Fodor has simply collapsed in the face of his dread and composed some dreadfully bad arguments." What Darwin Got Wrong, Dennett concludes, is "a book that so transparently misconstrues its target that it would be laughable were it not such dangerous mischief".
It would be jawdroppingly surprising, to say the least, were Fodor to be right. A safer, if mealy-mouthed, conclusion to draw is that his work acts as an important warning to those of us who think we understand natural selection. It's probably not a bankrupt concept, as Fodor claims. But nor should laypeople assume that it's self-evidently simple and exhaustively true.
The irony in all this is that Darwin himself never claimed that it was. He went to his deathbed protesting that he'd been misinterpreted: there was no reason, he said, to assume that natural selection was the only imaginable mechanism of evolution. Darwin, writing before the discovery of DNA, knew very well that his work heralded the beginning of a journey to understand the origins and development of life. All we may be discovering now is that we remain closer to the beginning of that journey than we've come to think.
• From Time magazine, an excellent piece on epigenetics: http://bit.ly/5Kyj5q
• The Genius in All of Us: Why Everything You've Been Told about Genetics, Talent and IQ is Wrong, by David Shenk, is published by Doubleday. What Darwin Got Wrong by Jerry Fodor and Massimo Piattelli-Palmarini is published by Profile, price £20
• For more on "horizontal evolution" see New Scientist: http://bit.ly/4zzAsr
• Also from New Scientist, more on the role of viruses in evolution: http://bit.ly/bD4NLC
This article was amended on 19 March 2010. Genes undergo random mutations, rather than cause them (ninth paragraph). This has been corrected in the online version.