In prokaryotes then, genes are easily distinguished by their evolutionary conservation, while genomes are rather more difficult to characterize. Gene migrations between genomes can occur exceedingly quickly: the gene sets of different Escherichia coli strains can differ much more than those of elephants and mice. Some genes travel in tight pairs, such as the restriction enzymes and their protective methyltransferases: when arriving in a new genome as a doublet, they bring with them the ability to destroy intruders while providing self-immunity from that same destruction.

maritima genome from archaeal genomes, where they had collaborated with their fellow genomic travelers to survive in the same extreme conditions. A third of these genes have migrated into the T. A dramatic example is provided by the inhabitants of the genome of Thermotoga maritima, a bacterium that lives in extreme conditions such as hot springs and hydrothermal vents. There is often a rhythm and a rhyme to these gene migrations. Twenty-five years after the book’s publication, the biggest discovery of the Human Genome Project was that half of our genome is indeed plastered with a plethora of such selfish elements, including a million short interspersed elements (SINEs), which are parasites of genomic parasites. The most striking example may be Dawkins’ prediction of a phenomenon that turned out to be of truly genomic proportions: the existence of purely selfish DNA, genes that have no function other than to propagate themselves in the genome, for example, by copying and pasting themselves in a number of different chromosomes in our own genome and those of most walks of life. But what is a gene, anyway? When we interviewed Dawkins last November, he described his pragmatic definition of a gene in the Mendelian sense: “it’s what persists through the generations.” Remarkably, when Dawkins wrote The Selfish Gene, no genome sequence had yet been fully deciphered yet this did not stop him from deducing the genes’ workings at the genomic level.

The selfish gene