The Selfish Gene

by Richard Dawkins

If the genetic unit we are considering is only 1 per cent of the length of the chromosome, we can assume that it has only a 1 per cent chance of being split in any one meiotic division.

The average life-expectancy of a genetic unit can conveniently be expressed in generations, which can in turn be translated into years.

Suppose it is your chromosome number 8a, inherited from your father. It was created inside one of your father’s testicles, shortly before you were conceived.

This particular sperm (unless you are a non-identical twin) was the only one of the flotilla which found harbour in one of your mother’s eggs—that is why you exist.

The life-span of a chromosome is one generation.

What about the life-span of a smaller genetic unit, say 1/100 of the length of your chromosome 8a? This unit too came from your father, but it very probably was not originally assembled in him. Following the earlier reasoning, there is a 99 per cent chance that he received it intact from one of his two parents.

Eventually, if we trace the ancestry of a small genetic unit back far enough, we will come to its original creator. At some stage it must have been created for the first time inside a testicle or an ovary of one of your ancestors.

The smaller sub-units which make up the genetic unit we are considering may well have existed long before.

Remember too that an individual’s descendants constitute not a single line but a branching line.

Whichever of your ancestors it was who ‘created’ a particular short length of your chromosome 8a, he or she very likely has many other descendants besides you.

A point mutation is an error corresponding to a single misprinted letter in a book. It is rare, but clearly the longer a genetic unit is, the more likely it is to be altered by a mutation somewhere along its length.

Then natural selection may tend to favour the new ‘genetic unit’ so formed, and it will spread through the future population.

It looks as though a single gene determines whether an individual will mimic species A or species B. But how can a single gene determine all the multifarious aspects of mimicry—colour, shape, spot pattern, rhythm of flight? The answer is that one gene in the sense of a cistron probably cannot.

I am using the word gene to mean a genetic unit that is small enough to last for a large number of generations and to be distributed around in the form of many copies.

The more likely a length of chromosome is to be split by crossing-over, or altered by mutations of various kinds, the less it qualifies to be called a gene in the sense in which I am using the term.

To say the least this is not a catchy title so, defining a gene as a little bit of chromosome which potentially lasts for many generations, I call the book The Selfish Gene.

selfishness is to be expected in any entity that deserves the title of a basic unit of natural selection. We saw that some people regard the species as the unit of natural selection, others the population or group within the species, and yet others the individual. I said that I preferred to think of the gene as the fundamental unit of natural selection, and therefore the fundamental unit of self-interest.

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.

Mendel’s discovery had already been published, and it could have rescued Darwin, but alas he never knew about it: nobody seems to have read it until years after Darwin and Mendel had both died.

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, not stable and unitary enough to be ‘selected’ in preference to another population.

Sexual reproduction is not replication.

Individuals are not stable things, they are fleeting. Chromosomes too are shuffled into oblivion, like hands of cards soon after they are dealt.

The genes are not destroyed by crossing-over, they merely change partners and march on. Of course they march on. That is their business. They are the replicators and we are their survival machines.

Genes, like diamonds, are forever, but not quite in the same way as diamonds. It is an individual diamond crystal that lasts, as an unaltered pattern of atoms.

the potential near-immortality of a gene, in the form of copies, as its defining property.

To define a gene as a single cistron is good for some purposes, but for the purposes of evolutionary theory it needs to be enlarged. The extent of the enlargement is determined by the purpose of the definition.

The gene is defined as 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.

A gene can live for a million years, but many new genes do not even make it past their first generation.

each successive body in which they find themselves, such that that body is a little bit more likely to live and reproduce than it would have been under the influence of the rival gene or allele.

However independent and free genes may be in their journey through the generations, they are very much not free and independent agents in their control of embryonic development.

So it is with single genes in the development of an embryo.

Embryonic development is controlled by an interlocking web of relationships so complex that we had best not contemplate it.

All parts of a baby have a near infinite number of antecedent causes.

It is differences that matter in the competitive struggle to survive;

As far as a gene is concerned, its alleles are its deadly rivals, but other genes are just a part of its environment, comparable to temperature, food, predators, or companions.

The effect of the gene depends on its environment, and this includes other genes.

The whole set of genes in a body constitutes a kind of genetic climate or background, modifying and influencing the effects of any particular gene.

Many a good gene gets into bad company, and finds itself sharing a body with a lethal gene, which kills the body off in childhood. Then the good gene is destroyed along with the rest. But this is only one body, and replicas of the same good gene live on in other bodies which lack the lethal gene. Many copies of good genes are dragged under because they happen to share a body with bad genes, and many perish through other forms of ill luck, say when their body is struck by lightning. But by definition luck, good and bad, strikes at random, and a gene that is consistently on the losing side is not unlucky; it is a bad gene.

natural selection may unconsciously ‘edit’ a gene complex by means of inversions and other gross movements of bits of chromosome, thereby bringing genes that cooperate well together into closely linked groups.

A gene that cooperates well with most of the other genes that it is likely to meet in successive bodies, i.e. the genes in the whole of the rest of the gene pool, will tend to have an advantage.

This is a subtle, complicated idea. It is complicated because the ‘environment’ of a gene consists largely of other genes, each of which is itself being selected for its ability to cooperate with its environment of other genes.

An analogy adequate to cope with this subtle point does exist, but it is not from everyday experience. It is the analogy with human ‘game theory’, which will be introduced in Chapter 5 in connection with aggressive contests between individual animals.

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 which it is convenient to label the gene.

The cornerstone of the argument, as given earlier, was the assumption that genes are potentially immortal, while bodies and all other higher units are temporary.

We have already asked what are the most general attributes of a ‘good’ gene, and we decided that ‘selfishness’ was one of them. But another general quality that successful genes will have is a tendency to postpone the death of their survival machines at least until after reproduction.

A gene that makes its possessors die is called a lethal gene.

A semilethal gene has some debilitating effect, such that it makes death from other causes more probable.

The aspect that Medawar himself emphasizes is that selection will favour genes that have the effect of postponing the operation of other, lethal genes, and it will also favour genes that have the effect of hastening the effect of good genes.