THE SELFISH GENE
"I cannot persuade myself that a beneficent and omnipotent God would have designedly created parasitic wasps with the express intention of their feeding within the living bodies of caterpillars" .
Charles Darwin [1]
Introduction[2]
The Darwinian revolution advanced one step further with Richard Dawkins’ 1976 publication The Selfish Gene [3] . According to Dawkins, it is neither the individual nor the species which is the real driving force behind evolution and natural selection. It is the gene and a selfish one at that, made up of chromosomes of DNA. Genes live in bodies which he describes as ‘survival machines’ for the gene. To put it even more starkly and in his own words “the fundamental unit of selection, and therefore of self-interest, is not the species, nor the group, nor even, strictly, the individual. It is the gene, the unit of heredity”.[4] Genes are transmitted from survival machine to survival machine down the generations over millions of years, while our bodies have but a transient existence here on earth, generally lasting only a few decades and ceasing at some period of time after gene transmission to other bodies has taken place. The individual survival machine is entirely perishable but the gene is immortal.
Dawkins goes on to say that the fundamental unit, the prime mover of all life is the replicator (the gene’s pseudonym), which originally came into existence in the first instance by chance in the random jostling of smaller particles. Even before the coming of life on earth, he says, some rudimentary evolution of molecules could have occurred by the ordinary processes of physics and chemistry[5]. Once a replicator has come into existence, it is capable of generating an indefinitely large set of copies of itself. These copies are not, however, perfect, and sometimes mistakes are made. Some even losing the power of self-replication, and their kind ceases to exist when they themselves cease to exist. Others can still replicate, but less effectively. Yet other varieties happen to find themselves in possession of new tricks: they turn out to be even better self-replicators than their predecessors and contemporaries. It is their descendants that come to dominate the population. As time goes by, the world becomes filled with the most powerful and ingenious replicators.
The success that a replicator has in the world will depend on what kind of a world it is - the pre-existing conditions. Among the most important of these conditions will be other replicators and their consequences. Replicators that are mutually beneficial will come to predominate in each other's presence. Replicators survive, not only by virtue of their own intrinsic properties, but by virtue of their consequences on the world: the environment in which we live. These consequences, however tortuous and indirect, “feed back” and affect the success of the replicator at getting itself copied. Replicators need not last forever, only so long as to produce additional replicators (fecundity) and retain their structure largely intact (fidelity).[6]
Metamorphosis to cells and multi-celled bodies
At some point in the evolution of life on our earth, this ganging up of mutually compatible replicators began to be formalised in the creation of discrete vehicles—cells and, later, many-celled bodies. Vehicles that evolved a “bottlenecked life cycle” (from single cell to large vehicle composed of many cells, and back again to single cell[7]) prospered, and became more discrete and vehicle-like.
Originally biologists focused on the individual organisms, the vehicles in which genes were housed while the replicators, now known as genes, were seen as part of the machinery used by these individual organisms. As Dawkins reminds us, it requires a deliberate mental effort to turn biology the right way up again, and remind ourselves that the replicators come first, in importance as well as in history.
Phenotype and extended phenotype[8]
One way to remind ourselves is to reflect that, even today, not all the phenotypic effects of a gene (the sum total of all the bodily parts and functions that our genotype creates to advance its cause) are bound up in the individual body in which it sits. Certainly in principle, and also in fact, the gene reaches out through the individual body wall and manipulates objects in the world outside, some of them inanimate, some of them other living beings, some of them a long way away. With only a little imagination we can see the gene as sitting at the centre of a radiating web of extended phenotypic power. And an object in the world is the centre of a converging web of influences from many genes sitting in many organisms. The long reach of the gene knows no obvious boundaries. The whole world is criss-crossed with causal arrows joining genes to phenotypic effects, far and near.
Replicators are no longer peppered freely through the sea; they are packaged in huge colonies—individual bodies. And phenotypic consequences, instead of being evenly distributed throughout the world, have in many cases congealed into those same bodies. But the individual body, so familiar to us on our planet, did not have to exist. The only kind of entity that has to exist in order for life to arise, anywhere in the universe, is the immortal replicator.
So, all life on earth – from bacteria to elephants to plant life – are survival machines for the same kind of replicator (DNA molecules), but “there are many different ways of making a living in the world”, and the replicators have built a vast range of machines to exploit them”[9].
Why is the gene so ‘selfish’?
This is only to be expected in any entity that deserves the title of a basic unit of natural selection, says Dawkins. Some people regard the species as the unit of natural selection, others the population or group within the species, and yet others the individual, but Dawkins prefers to think of the gene as the fundamental unit of natural selection, and therefore the fundamental unit of self-interest.
It should be remembered that the selfish gene is not just one single physical bit of DNA. Just as in the primeval soup, it is all replicas of a particular bit of DNA, distributed throughout the world. Endowing it with conscious aims for the moment, a single selfish gene is trying to get more numerous in the gene pool, by helping to programme the bodies in which it finds itself to survive and to reproduce. It may even be able to assist replicas of itself that are sitting in other bodies, thus affording an example of individual altruism brought about by gene selfishness[10].
It is they who are the immortals
Natural selection in its most general form means the differential survival of entities. Some entities live and others die but, in order for this selective death to have any impact on the world, an additional condition must be met. Each entity must exist in the form of lots of copies, and at least some of the entities must be potentially capable of surviving in the form of copies for a significant period of evolutionary time. A gene is not indivisible, but it is seldom divided. It is either definitely present or definitely absent in the body of any given individual.
A gene travels intact from grandparent to grandchild, passing straight through the intermediate generation without being merged with other genes. If genes continually blended with each other, natural selection as we now understand it would be impossible. Another aspect of the “particulateness” of the gene is that it does not grow senile; it is no more likely to die when it is a million years old than when it is only a hundred. It leaps from body to body down the generations, manipulating body after body in its own way and for its own ends, abandoning a succession of mortal bodies before they sink into senility and death.
The genes are the immortals, or rather: they are defined as genetic entities that come close to deserving the title. We, the individual survival machines in the world, can expect to live a few more decades. But the genes in the world have an expectation of life that must be measured not in decades but in thousands and millions of years.
Sexual reproduction
In sexually reproducing species, the individual is too large and too temporary a genetic unit to qualify as a significant unit of natural selection. A population is not a discrete enough entity to be a unit of natural selection. It is not stable and unitary enough to be 'selected' in preference to another population. An individual body seems discrete enough while it lasts, but how long is that? Each individual is unique. You cannot get evolution by selecting between entities when there is only one copy of each entity!
Sexual reproduction is not replication. Just as a population is contaminated by other populations, so an individual's posterity is contaminated by that of his sexual partner. Your children are only half you, your grandchildren only a quarter you. In a few generations the most you can hope for is a large number of descendants, each of whom bears only a tiny portion of you, a few genes, even if a few do bear your surname as well.
Dawkins elaborates upon this theme later in the text with his comment that our role as gene machines created to pass on our genes is destined to be forgotten in three generations:
“Your child, even your grandchild, may bear a resemblance to you, perhaps in facial features, in a talent for music, in the colour of her hair. But as each generation passes, the contribution of your genes is halved. It does not take long to reach negligible proportions. Our genes may be immortal but the collection of genes which is any one of us is bound to crumble away. Elizabeth II is a direct descendant of William the Conqueror. Yet it is quite probable that she bears not a single one of the old king's genes. We should not seek immortality in reproduction". [11]
Individuals are not stable things, they are fleeting. Chromosomes too are shuffled into oblivion, like hands of cards soon after they are dealt. But the cards themselves survive the shuffling. The cards are the genes. The genes are not destroyed by crossing-over[12], they merely change partners or rather habitats and march on. They are the replicators and we are their survival machines or habitats. When we have served our purpose we are cast aside. Genes are forever.
The life of any one physical DNA molecule may be quite short, perhaps a matter of months, certainly not more than one lifetime. But a DNA molecule could theoretically live on in the form of copies of itself for a hundred million years, and just like the ancient replicators in the primeval soup, copies of a particular gene may be distributed all over the world. The difference is that the modern versions are all neatly packaged inside the chromosomes the bodies of survival machines. It is the copying and the copies which make the genes virtually immortal.
The properties that a successful unit of natural selection must have are longevity, fecundity, and copying-fidelity. The gene (a piece of chromosome which is sufficiently short for it to last, potentially, for long enough for it to function as a significant unit of natural selection) is the largest entity which, at least potentially, has these properties. It is a long-lived replicator, existing in the form of many duplicate copies, but it is not infinitely long-lived. How long will depend on how much more likely a 'bad' genetic unit is to die than its 'good' allele (the gene that is successful in getting itself included in the instructions for building a new body on a chromosome).
A gene can live for a million years, but many new genes do not even make it past their first generation. The few new ones that succeed do so partly because they are lucky, but mainly because they have what it takes, and that means they are good at making survival machines – machines which develop to maturity and reproduce themselves by transmitting the genes to their off-spring.
At the gene level, altruism must be bad and selfishness good. Genes are competing directly with their alleles for survival – remember that their alleles in the gene pool are rivals for their slot on the chromosomes of future generations. Any gene that behaves in such a way as to increase its own survival chances in the gene pool at the expense of its alleles will, by definition, tautologously, tend to survive.
The gene is the basic unit of selfishness.
Before embarking on an analysis of the significance of the phenotype and its extended counterpart, it is as well to consider the story thus far and a few related aspects:
The phenotype and its extended counterpart
Dawkins begins the last chapter of his book with the observation that an uneasy tension disturbs the heart of the selfish gene theory. It is the tension between gene and the individual body as the fundamental agent of life:
A body doesn't look like the product of a loose and temporary federation of warring genetic agents who hardly have time to get acquainted before embarking in sperm or egg for the next leg of “the great genetic diaspora”[15]. It has one single-minded brain which coordinates a cooperative of limbs and sense organs to achieve one end.
So, how do we resolve this paradox?
On any sensible view of the matter Darwinian selection does not work on genes directly. DNA is “cocooned in protein, swaddled in membranes, shielded from the world and invisible to natural selection”[16].
The bodily manifestation of the gene
If selection tried to choose DNA molecules directly it would hardly find any criterion by which to do so. All genes look alike. The important differences between genes emerge only in their effects. This usually means effects on the processes of embryonic development and hence on bodily form and behaviour. Successful genes are genes that, in the environment influenced by all the other genes in a shared embryo, have beneficial effects on that embryo. Beneficial means that they make the embryo likely to develop into a successful adult, an adult likely to reproduce and pass those very same genes on to future generations.
Our bodies may be important to us, but from our genes’ point of view, they are nothing more than vehicles to get themselves intact into the next generation. The entire collection of the genes that make up you and me is called our genotype. The sum total of all the bodily parts and functions that our genotype creates to advance its cause – you and me – is called our phenotype.
Phenotypes need not be limited to the boundaries of our bodies. They also include the behaviours brought about by our genes, for example a beaver’s brain circuits that lead it to gnaw trees and build dams to ensure that its habitat is free from predators. From an evolutionary perspective, the dam – and even the pond it creates – is as much an extension of the beaver’s genes as its own body is. This is an example of what is known as an extended phenotype[17].
Although genetic in origin, it is often convenient to see these phonotypic effects as grouped together in distinct vehicles such as individual organisms.[18] In other words, the technical word phenotype is used for the bodily manifestation of a gene, the effect that a gene, in comparison with its alleles (rivals for a spot on a chromosome), has on the body, via development. The phenotypic effect of some particular gene might be, say, green eye colour. In practice most genes have more than one phenotypic effect, say green eye colour and curly hair. Natural selection favours some genes rather than others not because of the nature of the genes themselves, but because of their consequences—their phenotypic effects[19].
Darwinians have usually chosen to discuss genes whose phenotypic effects benefit, or penalize, the survival and reproduction of whole bodies. They have tended not to consider benefits to the gene itself. This is partly why the paradox at the heart of the theory doesn’t normally make itself felt. For instance, a gene may be successful through improving the running speed of a predator. The whole predator's body, including all its genes, is more successful because it runs faster. Its speed helps it survive to have children; and therefore more copies of all its genes, including the gene for fast running, are passed on. Here the paradox conveniently disappears because what is good for one gene is good for all.
But what if a gene exerted a phenotypic effect that was good for itself but bad for the rest of the genes in the body? And what if a mutant gene arose that just happened to have an effect, not upon something obvious like eye colour or curliness of hair, but upon meiosis itself[20]?
The extended phenotype and its effects on the world outside the body
The phenotypic effects of a gene are normally seen as all the effects that it has on the body in which it sits. This is the conventional definition. But the phenotypic effects of a gene need to be thought of as all the effects that it has on the world. The phenotypic effects of a gene are the tools by which it levers itself into the next generation, and these tools may reach outside the individual body wall. What might it mean in practice to speak of a gene as having an extended phenotypic effect on the world outside the body in which it sits? Examples that spring to mind are artefacts like beaver dams, bird nests and caddis houses[21]. So genes in one organism can have extended phenotypic effects on the body of another organism such as in the case of snail shells which are secreted by the snail's own cells.
In all cases in which natural selection has favoured genes for manipulation, it is legitimate to speak of those same genes as having (extended phenotypic) effects on the body of the manipulated organism. It doesn't matter in which body a gene physically sits. The target of its manipulation may be the same body or a different one.
Natural selection favours those genes that manipulate the world to ensure their own propagation, and an animal's behaviour tends to maximize the survival of the genes 'for' that behaviour, whether or not those genes happen to be in the body of the particular animal performing it. The theorem could apply, of course, to animal behaviour, colour, size, shape, anything.
To state it in bland terms, the doctrine of the extended phenotype is that the phenotypic effect of a gene (genetic replicator) is best seen as an effect upon the world at large and only incidentally upon the individual organism – or any other vehicle – in which it happens to be. [22]
The organism and the gene as rival candidates for the central role in natural selection
Remember that the fundamental units of natural selection, the basic things that survive or fail to survive, that form lineages of identical copies with occasional random mutations, are called replicators. DNA molecules are replicators. They generally, and for reasons that we shall come to, gang together into large communal survival machines or 'vehicles'. The vehicles that we know best are individual bodies like our own. A body is not a replicator. It is a vehicle. Vehicles don't replicate themselves; they work to propagate their replicators. Replicators don't behave, don't perceive the world, don't catch prey or run away from predators; they make vehicles that do all those things[23].
For many purposes it is convenient for biologists to focus their attention at the level of the vehicle. For other purposes it is convenient for them to focus their attention at the level of the replicator. Gene and individual organism are not rivals for the same starring role in the Darwinian drama. They are cast in different, complementary and in many respects equally important roles: the role of replicator and the role of vehicle.
In The Extended Phenotype, Dawkins divides the question up into three.
The answer to the first two questions derives from the benefits to be achieved from co-operation. Part of the answer comes by looking at how modern DNA molecules cooperate in the chemical factories that are living cells. DNA molecules make proteins. Proteins work as enzymes, catalysing particular chemical reactions. Often a single chemical reaction is not sufficient to synthesise a useful end-product. In a human pharmaceutical factory the synthesis of a useful chemical needs a production line. The starting chemical cannot be transformed directly into the desired end-product. A series of intermediates must be synthesized in strict sequence. Each gene flourishes only in the presence of the right set of other genes. In other words, cells came together (or failed to separate after cell division) to form many-celled bodies.
And why did cells gang together? Why the “lumbering robots” (the bodies or vehicles)? This is another question about cooperation. But the domain has shifted from the world of molecules to a larger scale. Many-celled bodies outgrow the microscope. They can even become elephants or whales. Being big is not necessarily a good thing: most organisms are bacteria and very few are elephants. But when the ways of making a living that are open to small organisms have all been filled, there are still prosperous livings to be made by larger organisms: large organisms can eat smaller ones, for instance, and can avoid being eaten by them.
And the advantages of being in a club of cells don’t stop with size. The cells are a clone. All contain the same genes, although different genes will be turned on in the different specialist cells. Genes in each cell type are directly benefiting their own copies in the minority of cells specialized for reproduction, the cells of the immortal germ line.
Thirdly, why do bodies participate in a 'bottlenecked' life cycle? [25]
Dawkins defines bottlenecked in this way: no matter how many cells there may be in the body of an elephant, the elephant began life as a single cell, a fertilised egg. The fertilised egg is a narrow bottleneck which, during embryonic development, widens out into the trillions of cells of an adult elephant. And no matter how many cells, of no matter how many specialised types, cooperate to perform the unimaginably complicated task of running an adult elephant, the efforts of all those cells converge on the final goal of producing single cells again—sperms or eggs. The elephant not only has its beginning in a single cell, a fertilised egg. Its end, meaning its goal or end-product, is the production of single cells, fertilised eggs of the next generation. The life cycle of the broad and bulky elephant both begins and ends with a narrow bottleneck. This bottlenecking is characteristic of the life cycles of all many-celled animals and most plants. Why? What is its significance?
If life cycles become ‘bottlenecked’, living material seems bound to become boxed into discrete, unitary organisms, and the more that living material is boxed into discrete survival machines, the more will the cells of those survival machines concentrate their efforts on that special class of cells that are destined to ferry their shared genes through the bottleneck into the next generation. The two phenomena, bottlenecked life cycles and discrete organisms, go hand in hand. As each evolves, it reinforces the other. The two are mutually enhancing[26].
Dawkins returns to these themes in his 2009 book The Greatest Show on Earth, when he opines that natural selection is the means by which those breeds or species which survive in the natural world of competition between themselves and each other reproduce and pass on the genes that helped them to survive. The individuals thus “chosen” to survive by their superior equipment are the most likely to reproduce, and therefore most likely to pass on the genes (lengths of DNA code) for possessing superior equipment. In this way, every gene pool, or mixture of genes in every breed or species tends to become filled with genes for making superior equipment for survival and reproduction.
The result is “a great evolutionary armament race, whose results for defence, are manifested in such devices as speed, alertness, armour, spinescence, burrowing habits, nocturnal habits, poisonous secretions, nauseous taste, and procryptic, aposematic, and mimetic coloration; and for offence, in such counter-attributes as speed, surprise, ambush, allurement, visual acuity, claws, teeth, poison fangs and anticryptic and alluring coloration. Just as greater speed in the pursued has developed in relation to greater speed in the pursuer; or defensive armour in relation to aggressive weapons; so the perfection of concealing devices has evolved in response to increased powers of perception[27].
The race in evolutionary time is a race to build up equipment for races run in real time. And what that actually means is that genes for making the equipment to outsmart or outrun the other side build up in gene pools, on the two sides. Secondly, the equipment for running fast is used to outrun rivals of the same species who are fleeing from the same predator. “It is all about gene survival. Natural selection cares only for the survival and reproduction of individual genes, that is, the survival of self-replicating instructions for self-replication, and along the way nature is neither kind nor unkind, neither against suffering nor for it, unless it affects the survival of DNA”[28].
In other words, genes evolve to make copies of themselves more successfully[29]. But how do mutations and natural selection operate in the context of a parasites’ power to manipulate their hosts? If the power of a gene can extend to manipulation of the physical world, Dawkins wondered, could it not extend as well to the manipulation of another living creature? Dawkins argues that it could, and he pointed to parasites as the prime example. The ability of a parasite to control the behavior of a host is encoded in its genes. If one of those gene’s mutated, the host’s behavior would change: if a wasp acquires a mutation that compels its ladybug host to begin to act as a bodyguard, for example, its offspring carrying that trait will thrive, because fewer of them will be killed by predators[30].
A twist in the tail of the gene: the meme
Daniel Dennett, a philosopher begins the last paragraph of his Afterword to the 1999 edition of Dawkins’ Extended Phenotype (first published 1982) with the comment that “(t)he logic of Darwinian thinking is not just about genes”[31]. Dawkins himself had touched upon this theme in a somewhat surprising fashion in The Selfish Gene when he said that Darwinism is too big a theory to be confined to the narrow context of the gene, and that “for an understanding of the evolution of modern man, we must begin by throwing out the gene as the sole basis of our ideas on evolution”. [32]
Dawkins named his “new replicator” the meme, a concept which involves the mimicry of ideas such as tunes, ideas, catch-phrases, clothes fashions, ways of making pots or of building arches”. One example of a propagated meme is the idea of God and “blind faith”. Like their genetic counterpart, to be a successful replicator, memes also possess longevity, fecundity and copying-fidelity[33]. A “good meme” spreads because brains are receptive to it, and the receptiveness of brains is ultimately shaped by (genetic) natural selection. “The very fact that animals imitate other animals at all must ultimately be explicable in terms of their Darwinian fitness” [34].
But whether gene or meme, Dawkins manages to finish up on a note of optimism: “We are built as gene machines and cultured as meme machines, but we have the power to turn against our creators. We, alone on earth, can rebel against the tyranny of the selfish replicators…. by deliberately cultivating an nurturing pure, disinterested altruism - something that has no place in nature, something that has never happened before in the whole history of the world” [35].
[1] From a letter to the pioneering botanist, Asa Gray, a minister, 22 May 1860 in The Life and Letters of Charles Darwin. Nowadays Darwin might have added, the loa loa eye burrowing worm, a nematode transmitted to humans through fly bites, which burrows through the eyeballs of children in east Africa, blinding them. "The worm cannot live in any other way, except by burrowing through eyeballs": David Attenborough at https://www.newscientist.com/blogs/shortsharpscience/2009/01/eye-burrowing-worms-national-t.html ;orthe Ommatokoita elongata crustacean which permanently attaches itself to the front of the eyes of the Greenland Shark, permanently damaging their corneas and rendering 90% of the species blind: http://www.bbc.com/earth/story/20141028-the-mystery-shark-of-the-arctic
[2] This overview is drawn from Chapters 2, 3 and 13 of The Selfish Gene (SG).
[3] My references are to the 1989 New version, Oxford University Press, Oxford.
[4] Ibid, 11.
[5] Ibid, 13, 264.
[6] EP 84.
[7] This is explained in more detail in Chapter 13 of SG and is the subject of later elaboration here.
[8] These terms are also the subject of later elaboration.
[9] Ibid 21.
[10] SG 89.
[11] Ibid 199.
[12] The process of taking bits and pieces of matching paternal and maternal genes, and cutting and exchanging matching portions, regardless of what may be written on them. This is the subject of elaboration in the sub-page on "DNA, genes and their ilk".
[13] SG 88.
[14] Ibid 234.
[15] Ibid.
[16] Ibid 235.
[17] These definitions are drawn from Dawkins 2009 work The Greatest Show on Earth.
[18] EP 133.
[19] SG 235.
[20] These interesting questions are explored at 235 ff.
[21] The larvae of the caddis fly are especially interesting: “Using cement of their own manufacture, they skilfully build tubular houses for themselves out of materials that they pick up from the bed of the stream. The house is a mobile home, carried about as the caddis walks, like the shell of a snail or hermit crab except that the animal builds it instead of growing it or finding it”: Ibid 238.
[22] EP 117.
[23] SG 254.
[24] Ibid at 256, 257 (Dawkins’ summary appears in the The Selfish Gene, Chapter 13). .
[25] Ibid at 258.
[26] Ibid at 264
[27] Hugh Cott, Adaptive Coloration in Animals (1940), cited in Greatest Show, 382.
[28] Source: Greatest Show 383, 384, 390, 391. On the suffering point, Dawkins uses the example of the Ichneumon wasps, which paralyse their prey, say a caterpillar, so that they can lay their eggs in it, but keep it alive so that the newly hatched larvae will have fresh meat upon which to feed as hey gnaw it hollow from within. For their part, the larvae leave the vital organs until last to prolong their supply of fresh meat. Darwin himself used the same example in the Origin of Species – we should not wonder at such cruelty; it is simply part of nature.
[29] Source: drawing upon and explaining Richard Dawkins’ The Selfish Gene (1976) and The Extended Phenotype (1982), “In many respects, a book far ahead of its time”. : Carl Zimmer, “Meet nature’s nightmare: Mindsuckers”, National Geographic, November 2014, 36-55.
[30] A list of this and other similar examples may is reproduced in Appendix 8.
[31] EP 268.
[32] SG 191.
[33] SG 193-4.
[34] EP 110.
[35] SG 201.
Charles Darwin [1]
Introduction[2]
The Darwinian revolution advanced one step further with Richard Dawkins’ 1976 publication The Selfish Gene [3] . According to Dawkins, it is neither the individual nor the species which is the real driving force behind evolution and natural selection. It is the gene and a selfish one at that, made up of chromosomes of DNA. Genes live in bodies which he describes as ‘survival machines’ for the gene. To put it even more starkly and in his own words “the fundamental unit of selection, and therefore of self-interest, is not the species, nor the group, nor even, strictly, the individual. It is the gene, the unit of heredity”.[4] Genes are transmitted from survival machine to survival machine down the generations over millions of years, while our bodies have but a transient existence here on earth, generally lasting only a few decades and ceasing at some period of time after gene transmission to other bodies has taken place. The individual survival machine is entirely perishable but the gene is immortal.
Dawkins goes on to say that the fundamental unit, the prime mover of all life is the replicator (the gene’s pseudonym), which originally came into existence in the first instance by chance in the random jostling of smaller particles. Even before the coming of life on earth, he says, some rudimentary evolution of molecules could have occurred by the ordinary processes of physics and chemistry[5]. Once a replicator has come into existence, it is capable of generating an indefinitely large set of copies of itself. These copies are not, however, perfect, and sometimes mistakes are made. Some even losing the power of self-replication, and their kind ceases to exist when they themselves cease to exist. Others can still replicate, but less effectively. Yet other varieties happen to find themselves in possession of new tricks: they turn out to be even better self-replicators than their predecessors and contemporaries. It is their descendants that come to dominate the population. As time goes by, the world becomes filled with the most powerful and ingenious replicators.
The success that a replicator has in the world will depend on what kind of a world it is - the pre-existing conditions. Among the most important of these conditions will be other replicators and their consequences. Replicators that are mutually beneficial will come to predominate in each other's presence. Replicators survive, not only by virtue of their own intrinsic properties, but by virtue of their consequences on the world: the environment in which we live. These consequences, however tortuous and indirect, “feed back” and affect the success of the replicator at getting itself copied. Replicators need not last forever, only so long as to produce additional replicators (fecundity) and retain their structure largely intact (fidelity).[6]
Metamorphosis to cells and multi-celled bodies
At some point in the evolution of life on our earth, this ganging up of mutually compatible replicators began to be formalised in the creation of discrete vehicles—cells and, later, many-celled bodies. Vehicles that evolved a “bottlenecked life cycle” (from single cell to large vehicle composed of many cells, and back again to single cell[7]) prospered, and became more discrete and vehicle-like.
Originally biologists focused on the individual organisms, the vehicles in which genes were housed while the replicators, now known as genes, were seen as part of the machinery used by these individual organisms. As Dawkins reminds us, it requires a deliberate mental effort to turn biology the right way up again, and remind ourselves that the replicators come first, in importance as well as in history.
Phenotype and extended phenotype[8]
One way to remind ourselves is to reflect that, even today, not all the phenotypic effects of a gene (the sum total of all the bodily parts and functions that our genotype creates to advance its cause) are bound up in the individual body in which it sits. Certainly in principle, and also in fact, the gene reaches out through the individual body wall and manipulates objects in the world outside, some of them inanimate, some of them other living beings, some of them a long way away. With only a little imagination we can see the gene as sitting at the centre of a radiating web of extended phenotypic power. And an object in the world is the centre of a converging web of influences from many genes sitting in many organisms. The long reach of the gene knows no obvious boundaries. The whole world is criss-crossed with causal arrows joining genes to phenotypic effects, far and near.
Replicators are no longer peppered freely through the sea; they are packaged in huge colonies—individual bodies. And phenotypic consequences, instead of being evenly distributed throughout the world, have in many cases congealed into those same bodies. But the individual body, so familiar to us on our planet, did not have to exist. The only kind of entity that has to exist in order for life to arise, anywhere in the universe, is the immortal replicator.
So, all life on earth – from bacteria to elephants to plant life – are survival machines for the same kind of replicator (DNA molecules), but “there are many different ways of making a living in the world”, and the replicators have built a vast range of machines to exploit them”[9].
Why is the gene so ‘selfish’?
This is only to be expected in any entity that deserves the title of a basic unit of natural selection, says Dawkins. Some people regard the species as the unit of natural selection, others the population or group within the species, and yet others the individual, but Dawkins prefers to think of the gene as the fundamental unit of natural selection, and therefore the fundamental unit of self-interest.
It should be remembered that the selfish gene is not just one single physical bit of DNA. Just as in the primeval soup, it is all replicas of a particular bit of DNA, distributed throughout the world. Endowing it with conscious aims for the moment, a single selfish gene is trying to get more numerous in the gene pool, by helping to programme the bodies in which it finds itself to survive and to reproduce. It may even be able to assist replicas of itself that are sitting in other bodies, thus affording an example of individual altruism brought about by gene selfishness[10].
It is they who are the immortals
Natural selection in its most general form means the differential survival of entities. Some entities live and others die but, in order for this selective death to have any impact on the world, an additional condition must be met. Each entity must exist in the form of lots of copies, and at least some of the entities must be potentially capable of surviving in the form of copies for a significant period of evolutionary time. A gene is not indivisible, but it is seldom divided. It is either definitely present or definitely absent in the body of any given individual.
A gene travels intact from grandparent to grandchild, passing straight through the intermediate generation without being merged with other genes. If genes continually blended with each other, natural selection as we now understand it would be impossible. Another aspect of the “particulateness” of the gene is that it does not grow senile; it is no more likely to die when it is a million years old than when it is only a hundred. It leaps from body to body down the generations, manipulating body after body in its own way and for its own ends, abandoning a succession of mortal bodies before they sink into senility and death.
The genes are the immortals, or rather: they are defined as genetic entities that come close to deserving the title. We, the individual survival machines in the world, can expect to live a few more decades. But the genes in the world have an expectation of life that must be measured not in decades but in thousands and millions of years.
Sexual reproduction
In sexually reproducing species, the individual is too large and too temporary a genetic unit to qualify as a significant unit of natural selection. A population is not a discrete enough entity to be a unit of natural selection. It is not stable and unitary enough to be 'selected' in preference to another population. An individual body seems discrete enough while it lasts, but how long is that? Each individual is unique. You cannot get evolution by selecting between entities when there is only one copy of each entity!
Sexual reproduction is not replication. Just as a population is contaminated by other populations, so an individual's posterity is contaminated by that of his sexual partner. Your children are only half you, your grandchildren only a quarter you. In a few generations the most you can hope for is a large number of descendants, each of whom bears only a tiny portion of you, a few genes, even if a few do bear your surname as well.
Dawkins elaborates upon this theme later in the text with his comment that our role as gene machines created to pass on our genes is destined to be forgotten in three generations:
“Your child, even your grandchild, may bear a resemblance to you, perhaps in facial features, in a talent for music, in the colour of her hair. But as each generation passes, the contribution of your genes is halved. It does not take long to reach negligible proportions. Our genes may be immortal but the collection of genes which is any one of us is bound to crumble away. Elizabeth II is a direct descendant of William the Conqueror. Yet it is quite probable that she bears not a single one of the old king's genes. We should not seek immortality in reproduction". [11]
Individuals are not stable things, they are fleeting. Chromosomes too are shuffled into oblivion, like hands of cards soon after they are dealt. But the cards themselves survive the shuffling. The cards are the genes. The genes are not destroyed by crossing-over[12], they merely change partners or rather habitats and march on. They are the replicators and we are their survival machines or habitats. When we have served our purpose we are cast aside. Genes are forever.
The life of any one physical DNA molecule may be quite short, perhaps a matter of months, certainly not more than one lifetime. But a DNA molecule could theoretically live on in the form of copies of itself for a hundred million years, and just like the ancient replicators in the primeval soup, copies of a particular gene may be distributed all over the world. The difference is that the modern versions are all neatly packaged inside the chromosomes the bodies of survival machines. It is the copying and the copies which make the genes virtually immortal.
The properties that a successful unit of natural selection must have are longevity, fecundity, and copying-fidelity. The gene (a piece of chromosome which is sufficiently short for it to last, potentially, for long enough for it to function as a significant unit of natural selection) is the largest entity which, at least potentially, has these properties. It is a long-lived replicator, existing in the form of many duplicate copies, but it is not infinitely long-lived. How long will depend on how much more likely a 'bad' genetic unit is to die than its 'good' allele (the gene that is successful in getting itself included in the instructions for building a new body on a chromosome).
A gene can live for a million years, but many new genes do not even make it past their first generation. The few new ones that succeed do so partly because they are lucky, but mainly because they have what it takes, and that means they are good at making survival machines – machines which develop to maturity and reproduce themselves by transmitting the genes to their off-spring.
At the gene level, altruism must be bad and selfishness good. Genes are competing directly with their alleles for survival – remember that their alleles in the gene pool are rivals for their slot on the chromosomes of future generations. Any gene that behaves in such a way as to increase its own survival chances in the gene pool at the expense of its alleles will, by definition, tautologously, tend to survive.
The gene is the basic unit of selfishness.
Before embarking on an analysis of the significance of the phenotype and its extended counterpart, it is as well to consider the story thus far and a few related aspects:
- the basic unit of natural selection is best regarded not as the species, nor as the population, nor even as the individual, but as some small unit of genetic material: the gene.
- Genes are potentially immortal, while bodies and all other higher units are temporary. This assumption rests upon two facts: the fact of sexual reproduction, crossing-over, and the fact of individual mortality.
- One theory is that senility represents an accumulation of deleterious copying errors and other kinds of gene damage which occur during the individual's lifetime.
- The individual is a survival machine built by a short-lived confederation of long-lived genes.
- The true 'purpose' of that portion of DNA which is never translated into protein is to survive, no more and no less. The simplest way to explain the surplus DNA is to suppose that it is a parasite, or at best a harmless but useless passenger, hitching a ride in the survival machines created by the other DNA. But here note John Mattick's work, considered on the previous page.
- Because of sex and crossing-over the gene pool is kept well stirred, and the genes partially shuffled.
- Evolution is the process by which some genes become more numerous and others less numerous in the gene pool.
- It is good to get into the habit, whenever we are trying to explain the evolution of some characteristic, such as altruistic behaviour, of asking ourselves simply: 'what effect will this characteristic have on frequencies of genes in the gene pool?'
- If we allow ourselves the license of talking about genes as if they had conscious aims, we can ask the question, what is a single selfish gene trying to do? It is trying to get more numerous in the gene pool.
- Basically it does this by helping to program the bodies in which it finds itself to survive and to reproduce.
- The 'it' here is a distributed agency, existing in many different individuals at once. A gene might be able to assist replicas of itself that are sitting in other bodies.
- If so, this would appear as individual altruism but it would be brought about by gene selfishness[13].
The phenotype and its extended counterpart
Dawkins begins the last chapter of his book with the observation that an uneasy tension disturbs the heart of the selfish gene theory. It is the tension between gene and the individual body as the fundamental agent of life:
- On the one hand we have “the beguiling image of independent DNA replicators, skipping like chamois, free and untrammelled down the generations, temporarily brought together in throwaway survival machines - immortal coils shuffling off an endless succession of mortal ones as they forge towards their separate eternities”[14].
- And on the other, we look at the individual bodies themselves and each one is obviously a coherent, integrated, immensely complicated machine, with a conspicuous unity of purpose.
A body doesn't look like the product of a loose and temporary federation of warring genetic agents who hardly have time to get acquainted before embarking in sperm or egg for the next leg of “the great genetic diaspora”[15]. It has one single-minded brain which coordinates a cooperative of limbs and sense organs to achieve one end.
So, how do we resolve this paradox?
On any sensible view of the matter Darwinian selection does not work on genes directly. DNA is “cocooned in protein, swaddled in membranes, shielded from the world and invisible to natural selection”[16].
The bodily manifestation of the gene
If selection tried to choose DNA molecules directly it would hardly find any criterion by which to do so. All genes look alike. The important differences between genes emerge only in their effects. This usually means effects on the processes of embryonic development and hence on bodily form and behaviour. Successful genes are genes that, in the environment influenced by all the other genes in a shared embryo, have beneficial effects on that embryo. Beneficial means that they make the embryo likely to develop into a successful adult, an adult likely to reproduce and pass those very same genes on to future generations.
Our bodies may be important to us, but from our genes’ point of view, they are nothing more than vehicles to get themselves intact into the next generation. The entire collection of the genes that make up you and me is called our genotype. The sum total of all the bodily parts and functions that our genotype creates to advance its cause – you and me – is called our phenotype.
Phenotypes need not be limited to the boundaries of our bodies. They also include the behaviours brought about by our genes, for example a beaver’s brain circuits that lead it to gnaw trees and build dams to ensure that its habitat is free from predators. From an evolutionary perspective, the dam – and even the pond it creates – is as much an extension of the beaver’s genes as its own body is. This is an example of what is known as an extended phenotype[17].
Although genetic in origin, it is often convenient to see these phonotypic effects as grouped together in distinct vehicles such as individual organisms.[18] In other words, the technical word phenotype is used for the bodily manifestation of a gene, the effect that a gene, in comparison with its alleles (rivals for a spot on a chromosome), has on the body, via development. The phenotypic effect of some particular gene might be, say, green eye colour. In practice most genes have more than one phenotypic effect, say green eye colour and curly hair. Natural selection favours some genes rather than others not because of the nature of the genes themselves, but because of their consequences—their phenotypic effects[19].
Darwinians have usually chosen to discuss genes whose phenotypic effects benefit, or penalize, the survival and reproduction of whole bodies. They have tended not to consider benefits to the gene itself. This is partly why the paradox at the heart of the theory doesn’t normally make itself felt. For instance, a gene may be successful through improving the running speed of a predator. The whole predator's body, including all its genes, is more successful because it runs faster. Its speed helps it survive to have children; and therefore more copies of all its genes, including the gene for fast running, are passed on. Here the paradox conveniently disappears because what is good for one gene is good for all.
But what if a gene exerted a phenotypic effect that was good for itself but bad for the rest of the genes in the body? And what if a mutant gene arose that just happened to have an effect, not upon something obvious like eye colour or curliness of hair, but upon meiosis itself[20]?
The extended phenotype and its effects on the world outside the body
The phenotypic effects of a gene are normally seen as all the effects that it has on the body in which it sits. This is the conventional definition. But the phenotypic effects of a gene need to be thought of as all the effects that it has on the world. The phenotypic effects of a gene are the tools by which it levers itself into the next generation, and these tools may reach outside the individual body wall. What might it mean in practice to speak of a gene as having an extended phenotypic effect on the world outside the body in which it sits? Examples that spring to mind are artefacts like beaver dams, bird nests and caddis houses[21]. So genes in one organism can have extended phenotypic effects on the body of another organism such as in the case of snail shells which are secreted by the snail's own cells.
In all cases in which natural selection has favoured genes for manipulation, it is legitimate to speak of those same genes as having (extended phenotypic) effects on the body of the manipulated organism. It doesn't matter in which body a gene physically sits. The target of its manipulation may be the same body or a different one.
Natural selection favours those genes that manipulate the world to ensure their own propagation, and an animal's behaviour tends to maximize the survival of the genes 'for' that behaviour, whether or not those genes happen to be in the body of the particular animal performing it. The theorem could apply, of course, to animal behaviour, colour, size, shape, anything.
To state it in bland terms, the doctrine of the extended phenotype is that the phenotypic effect of a gene (genetic replicator) is best seen as an effect upon the world at large and only incidentally upon the individual organism – or any other vehicle – in which it happens to be. [22]
The organism and the gene as rival candidates for the central role in natural selection
Remember that the fundamental units of natural selection, the basic things that survive or fail to survive, that form lineages of identical copies with occasional random mutations, are called replicators. DNA molecules are replicators. They generally, and for reasons that we shall come to, gang together into large communal survival machines or 'vehicles'. The vehicles that we know best are individual bodies like our own. A body is not a replicator. It is a vehicle. Vehicles don't replicate themselves; they work to propagate their replicators. Replicators don't behave, don't perceive the world, don't catch prey or run away from predators; they make vehicles that do all those things[23].
For many purposes it is convenient for biologists to focus their attention at the level of the vehicle. For other purposes it is convenient for them to focus their attention at the level of the replicator. Gene and individual organism are not rivals for the same starring role in the Darwinian drama. They are cast in different, complementary and in many respects equally important roles: the role of replicator and the role of vehicle.
- So why did genes come together into large vehicles, each with a single genetic exit route?
- And why did genes choose to gang up and make large bodies for themselves to live in?[24]:
In The Extended Phenotype, Dawkins divides the question up into three.
- Why did genes gang up in cells?
- Why did cells gang up in many-celled bodies?
- And why did bodies adopt a 'bottlenecked' life cycle?
The answer to the first two questions derives from the benefits to be achieved from co-operation. Part of the answer comes by looking at how modern DNA molecules cooperate in the chemical factories that are living cells. DNA molecules make proteins. Proteins work as enzymes, catalysing particular chemical reactions. Often a single chemical reaction is not sufficient to synthesise a useful end-product. In a human pharmaceutical factory the synthesis of a useful chemical needs a production line. The starting chemical cannot be transformed directly into the desired end-product. A series of intermediates must be synthesized in strict sequence. Each gene flourishes only in the presence of the right set of other genes. In other words, cells came together (or failed to separate after cell division) to form many-celled bodies.
And why did cells gang together? Why the “lumbering robots” (the bodies or vehicles)? This is another question about cooperation. But the domain has shifted from the world of molecules to a larger scale. Many-celled bodies outgrow the microscope. They can even become elephants or whales. Being big is not necessarily a good thing: most organisms are bacteria and very few are elephants. But when the ways of making a living that are open to small organisms have all been filled, there are still prosperous livings to be made by larger organisms: large organisms can eat smaller ones, for instance, and can avoid being eaten by them.
And the advantages of being in a club of cells don’t stop with size. The cells are a clone. All contain the same genes, although different genes will be turned on in the different specialist cells. Genes in each cell type are directly benefiting their own copies in the minority of cells specialized for reproduction, the cells of the immortal germ line.
Thirdly, why do bodies participate in a 'bottlenecked' life cycle? [25]
Dawkins defines bottlenecked in this way: no matter how many cells there may be in the body of an elephant, the elephant began life as a single cell, a fertilised egg. The fertilised egg is a narrow bottleneck which, during embryonic development, widens out into the trillions of cells of an adult elephant. And no matter how many cells, of no matter how many specialised types, cooperate to perform the unimaginably complicated task of running an adult elephant, the efforts of all those cells converge on the final goal of producing single cells again—sperms or eggs. The elephant not only has its beginning in a single cell, a fertilised egg. Its end, meaning its goal or end-product, is the production of single cells, fertilised eggs of the next generation. The life cycle of the broad and bulky elephant both begins and ends with a narrow bottleneck. This bottlenecking is characteristic of the life cycles of all many-celled animals and most plants. Why? What is its significance?
If life cycles become ‘bottlenecked’, living material seems bound to become boxed into discrete, unitary organisms, and the more that living material is boxed into discrete survival machines, the more will the cells of those survival machines concentrate their efforts on that special class of cells that are destined to ferry their shared genes through the bottleneck into the next generation. The two phenomena, bottlenecked life cycles and discrete organisms, go hand in hand. As each evolves, it reinforces the other. The two are mutually enhancing[26].
Dawkins returns to these themes in his 2009 book The Greatest Show on Earth, when he opines that natural selection is the means by which those breeds or species which survive in the natural world of competition between themselves and each other reproduce and pass on the genes that helped them to survive. The individuals thus “chosen” to survive by their superior equipment are the most likely to reproduce, and therefore most likely to pass on the genes (lengths of DNA code) for possessing superior equipment. In this way, every gene pool, or mixture of genes in every breed or species tends to become filled with genes for making superior equipment for survival and reproduction.
The result is “a great evolutionary armament race, whose results for defence, are manifested in such devices as speed, alertness, armour, spinescence, burrowing habits, nocturnal habits, poisonous secretions, nauseous taste, and procryptic, aposematic, and mimetic coloration; and for offence, in such counter-attributes as speed, surprise, ambush, allurement, visual acuity, claws, teeth, poison fangs and anticryptic and alluring coloration. Just as greater speed in the pursued has developed in relation to greater speed in the pursuer; or defensive armour in relation to aggressive weapons; so the perfection of concealing devices has evolved in response to increased powers of perception[27].
The race in evolutionary time is a race to build up equipment for races run in real time. And what that actually means is that genes for making the equipment to outsmart or outrun the other side build up in gene pools, on the two sides. Secondly, the equipment for running fast is used to outrun rivals of the same species who are fleeing from the same predator. “It is all about gene survival. Natural selection cares only for the survival and reproduction of individual genes, that is, the survival of self-replicating instructions for self-replication, and along the way nature is neither kind nor unkind, neither against suffering nor for it, unless it affects the survival of DNA”[28].
In other words, genes evolve to make copies of themselves more successfully[29]. But how do mutations and natural selection operate in the context of a parasites’ power to manipulate their hosts? If the power of a gene can extend to manipulation of the physical world, Dawkins wondered, could it not extend as well to the manipulation of another living creature? Dawkins argues that it could, and he pointed to parasites as the prime example. The ability of a parasite to control the behavior of a host is encoded in its genes. If one of those gene’s mutated, the host’s behavior would change: if a wasp acquires a mutation that compels its ladybug host to begin to act as a bodyguard, for example, its offspring carrying that trait will thrive, because fewer of them will be killed by predators[30].
A twist in the tail of the gene: the meme
Daniel Dennett, a philosopher begins the last paragraph of his Afterword to the 1999 edition of Dawkins’ Extended Phenotype (first published 1982) with the comment that “(t)he logic of Darwinian thinking is not just about genes”[31]. Dawkins himself had touched upon this theme in a somewhat surprising fashion in The Selfish Gene when he said that Darwinism is too big a theory to be confined to the narrow context of the gene, and that “for an understanding of the evolution of modern man, we must begin by throwing out the gene as the sole basis of our ideas on evolution”. [32]
Dawkins named his “new replicator” the meme, a concept which involves the mimicry of ideas such as tunes, ideas, catch-phrases, clothes fashions, ways of making pots or of building arches”. One example of a propagated meme is the idea of God and “blind faith”. Like their genetic counterpart, to be a successful replicator, memes also possess longevity, fecundity and copying-fidelity[33]. A “good meme” spreads because brains are receptive to it, and the receptiveness of brains is ultimately shaped by (genetic) natural selection. “The very fact that animals imitate other animals at all must ultimately be explicable in terms of their Darwinian fitness” [34].
But whether gene or meme, Dawkins manages to finish up on a note of optimism: “We are built as gene machines and cultured as meme machines, but we have the power to turn against our creators. We, alone on earth, can rebel against the tyranny of the selfish replicators…. by deliberately cultivating an nurturing pure, disinterested altruism - something that has no place in nature, something that has never happened before in the whole history of the world” [35].
[1] From a letter to the pioneering botanist, Asa Gray, a minister, 22 May 1860 in The Life and Letters of Charles Darwin. Nowadays Darwin might have added, the loa loa eye burrowing worm, a nematode transmitted to humans through fly bites, which burrows through the eyeballs of children in east Africa, blinding them. "The worm cannot live in any other way, except by burrowing through eyeballs": David Attenborough at https://www.newscientist.com/blogs/shortsharpscience/2009/01/eye-burrowing-worms-national-t.html ;orthe Ommatokoita elongata crustacean which permanently attaches itself to the front of the eyes of the Greenland Shark, permanently damaging their corneas and rendering 90% of the species blind: http://www.bbc.com/earth/story/20141028-the-mystery-shark-of-the-arctic
[2] This overview is drawn from Chapters 2, 3 and 13 of The Selfish Gene (SG).
[3] My references are to the 1989 New version, Oxford University Press, Oxford.
[4] Ibid, 11.
[5] Ibid, 13, 264.
[6] EP 84.
[7] This is explained in more detail in Chapter 13 of SG and is the subject of later elaboration here.
[8] These terms are also the subject of later elaboration.
[9] Ibid 21.
[10] SG 89.
[11] Ibid 199.
[12] The process of taking bits and pieces of matching paternal and maternal genes, and cutting and exchanging matching portions, regardless of what may be written on them. This is the subject of elaboration in the sub-page on "DNA, genes and their ilk".
[13] SG 88.
[14] Ibid 234.
[15] Ibid.
[16] Ibid 235.
[17] These definitions are drawn from Dawkins 2009 work The Greatest Show on Earth.
[18] EP 133.
[19] SG 235.
[20] These interesting questions are explored at 235 ff.
[21] The larvae of the caddis fly are especially interesting: “Using cement of their own manufacture, they skilfully build tubular houses for themselves out of materials that they pick up from the bed of the stream. The house is a mobile home, carried about as the caddis walks, like the shell of a snail or hermit crab except that the animal builds it instead of growing it or finding it”: Ibid 238.
[22] EP 117.
[23] SG 254.
[24] Ibid at 256, 257 (Dawkins’ summary appears in the The Selfish Gene, Chapter 13). .
[25] Ibid at 258.
[26] Ibid at 264
[27] Hugh Cott, Adaptive Coloration in Animals (1940), cited in Greatest Show, 382.
[28] Source: Greatest Show 383, 384, 390, 391. On the suffering point, Dawkins uses the example of the Ichneumon wasps, which paralyse their prey, say a caterpillar, so that they can lay their eggs in it, but keep it alive so that the newly hatched larvae will have fresh meat upon which to feed as hey gnaw it hollow from within. For their part, the larvae leave the vital organs until last to prolong their supply of fresh meat. Darwin himself used the same example in the Origin of Species – we should not wonder at such cruelty; it is simply part of nature.
[29] Source: drawing upon and explaining Richard Dawkins’ The Selfish Gene (1976) and The Extended Phenotype (1982), “In many respects, a book far ahead of its time”. : Carl Zimmer, “Meet nature’s nightmare: Mindsuckers”, National Geographic, November 2014, 36-55.
[30] A list of this and other similar examples may is reproduced in Appendix 8.
[31] EP 268.
[32] SG 191.
[33] SG 193-4.
[34] EP 110.
[35] SG 201.