The Selfish Gene is a book on evolution by Richard Dawkins, published in 1976. It builds upon the principal theory of George C. Williams's first book Adaptation and Natural Selection. Dawkins coined the term selfish gene as a way of expressing the gene-centered view of evolution, which holds that evolution is best viewed as acting on genes and that selection at the level of organisms or populations almost never overrides selection based on genes. An organism is expected to evolve to maximize its inclusive fitness—the number of copies of its genes passed on globally (rather than by a particular individual). As a result, populations will tend towards an evolutionarily stable strategy. The book also coins the term meme for a unit of human cultural evolution analogous to the gene, suggesting that such "selfish" replication may also model human culture, in a different sense. Memetics has become the subject of many studies since the publication of the book.
"Selfish", when applied to genes, doesn't mean "selfish" at all. It means, instead, an extremely important quality for which there is no good word in the English language: "the quality of being copied by a Darwinian selection process." This is a complicated mouthful. There ought to be a better, shorter word—but "selfish" isn't it.
There are other times when the implicit interests of the vehicle and replicator are in conflict, such as the genes behind certain male spiders' instinctive mating behaviour, which increase the organism's inclusive fitness by allowing it to reproduce, but shorten its life by exposing it to the risk of being eaten by the cannibalistic female. Another good example is the existence of segregation distortion genes that are detrimental to their host but nonetheless propagate themselves at its expense. Likewise, the existence of junk DNA that provides no benefit to its host, once a puzzle, can be more easily explained. A more controversial example is aging, in which an old organism's death makes room for its offspring, benefiting its genes at the cost of the organism.
These examples might suggest that there is a power-struggle between genes and their host. In fact, the claim is that there isn't much of a struggle because the genes usually win without a fight. Only if the organism becomes intelligent enough to understand its own interests, as distinct from those of its genes, can there be true conflict. An example of this would be a person deciding to use contraception, even though their genes lose out due to this decision.
When looked at from the point of view of gene selection, many biological phenomena that, in prior models, were difficult to explain become easier to understand. In particular, phenomena such as kin selection and eusociality, where organisms act altruistically, against their individual interests (in the sense of health, safety or personal reproduction) to help related organisms reproduce, can be explained as gene sets "helping" copies of themselves in other bodies to replicate. Interestingly, the "selfish" actions of genes lead to unselfish actions by organisms.
Prior to the 1960s, it was common for such behaviour to be explained in terms of group selection, where the benefits to the organism or even population were supposed to account for the popularity of the genes responsible for the tendency towards that behaviour. This was shown not to be an evolutionarily stable strategy, in that it would only take a single individual with a tendency towards more selfish behaviour to undermine a population otherwise filled only with the gene for altruism towards non-kin.
Most modern evolutionary biologists accept that the idea is consistent with many processes in evolution. However, the view that selection on other levels, such as organisms and populations, seldom opposes selection on genes is more controversial. While naive versions of group selectionism have been disproved, more sophisticated formulations make accurate predictions in some cases while positing selection at higher levels. Nevertheless, the explanatory gains of using sophisticated formulations of group selectionism as opposed to Dawkins's gene-centred selectionism are still under dispute.
Some biologists have criticised the idea for describing the gene as the unit of selection, but suggest describing the gene as the unit of evolution, on the grounds that selection is a "here and now" event of reproduction and survival, while evolution is the long-term trend of shifting allele frequencies.
Another criticism of the book, made by the philosopher Mary Midgley in her book Evolution as a Religion, is that it discusses philosophical and moral questions that go beyond the biological arguments that Dawkins makes. For instance, humanity finally gaining power over the "selfish replicators" is a major theme at the end of the book. Dawkins has pointed out that he is only describing how things are under evolution, not endorsing them as morally good (see appeal to nature).