In our hands are 33 versions of a chain letter, collected between 1980 and 1995, when photocopiers, but not e-mail, were in widespread use by the general public. These letters have passed from host to host, mutating and evolving. Like a gene, their average length is about 2,000 characters. Like a potent virus, the letter threatens to kill you and induces you to pass it on to your "friends and associates"--some variation of this letter has probably reached millions of people. Like an inheritable trait, it promises benefits for you and the people you pass it on to. Like genomes, chain letters undergo natural selection and sometimes parts even get transferred between coexisting "species." Unlike DNA, however, these letters are easy to read. Indeed, their readability makes them especially suitable for classroom teaching of phylogeny (evolutionary history) free from the arcana of molecular biology.

The letters are an intriguing social phenomenon, but we are also interested in them because they provide a test bed for the algorithms used in molecular biology to infer phylogenetic trees from the genomes of existing organisms. We believe that if these algorithms are to be trusted, they should produce good results when applied to chain letters. Using a new algorithm that is general enough to have wide applicability to such problems, we have reconstructed the evolutionary history of our 33 letters. The standard methods do not work as well on these letters. Originally developed for genomes, our algorithm has also been applied to languages and used to detect plagiarism in student assignments: anything involving a sequence of symbols is grist for its mill.

This article was originally published with the title Chain Letters and Evolutionary Histories.

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