Friday, November 15, 2013

Measuring evolutionary relationships in a folk tale

The family tree of Little Red Riding Hood stories. The Little 
Red  Riding Hood  European tales are the cluster at right 
labelled ATU 233.
Credit: Eurekalert
Since the time of Carl Linnaeus, biologists have worked on systems for classifying living beings. An ideal classification system will reflect evolutionary relationships between the organisms, so that great cats, for example, would be grouped near domestic cats, and gorillas and chimpanzees near to humans. Biologists who study these organizational principles,  collectively termed cladistics,  incorporate anatomical similarities, DNA sequences, and the fossil record with the goal of producing a "tree of life" reflecting evolutionary relationships. Drawing trees according to these rules is useful as a way of organizing data, but it may also suggest  underlying principles or relationships among the things classified. 
In last week's PLoS One, a cladistics approach was used to understand relationships among versions of a folk tale.  Jashmid Terhani at Durham University in England studied relationships between the many variations of the folk tale Little Red Riding Hood. The version in the Grimm's fairy tale, called ATU 233 in the Aarne-Thompson-Uther list of European fables,  has some plot similarities to folk tales from Africa and the far East. Quoting from Dr. Terhani's article,  "The East Asian tales also feature human protagonists (ATU 333), but they are usually a group of siblings rather than a single child (ATU 123). In most variants of the tale, they are attacked after being left at home by their mother (ATU 123), but in some cases they encounter the villain en route to their grandmother's house (as per ATU 333)." 
Figure 1 from the paper shows locations where 
Little Red Riding Hood type stories were collected.
Dr. Terhani generated a family tree using 72 variables present in the various stories. The different comparison methods yielded slightly different family trees; but the Western European tales consistently grouped together, as did the tales collected from East Asia.
The second question is whether these trees are reflecting a more fundamental resemblance between the stories grouped. A biologist looking at such a tree would be tempted to think of a single ancestor story, with an unknown assortment of elements present in the widely separated tales. But there need not be such a relationship-- a child's encounter with a hostile and brutal stranger would probably be a memorable tale  in many societies, and could have arisen independently.
The microbiologist Carl Woese encountered similar objections in the 70s and 80s, when he proposed that archaea were fundamentally different from bacteria and belonged in a separate domain of life. The resistance to Woese centered around his use of only one gene in his classification scheme. Since that time, additional support has come from many other genomic and cellular features. It's hard to think what analagous data trove would emerge to buttress analysis of folk tales.

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