The magical number of seven

Neubert & Janssen (2004) made an interesting observation. They write (p. 242): “Although the system of the Orthalicoidea was intensively reviewed by Breure, it still remains in an unsatisfactory state. On the one hand, the enormous number of species level taxa were affiliated to a quite low number of genera, on the other hand the taxonomic rank and nomenclatural status of many of these names remained unsolved”.

Recently, Richard Zander, remarked on the Taxacom list that “the issue of how many species per genus is perhaps more a psychological or cognitive issue in so far as genera are man-made constructs. Eight is (today) the number of bits in a byte. Eight is the lucky number in many countries. Even more compelling, perhaps, is that the human mind on average has the capacity to operate simultaneously with seven words, concepts, sounds or the like (Miller 1956). So, perhaps the human mind has an innate urge to break up genera when they exceed that number?”.

The Orthalicoidea is a very diversified group, with over 1750 available names at the species level. Given the number of species in the catalogues by Richardson (1993, 1995 in Tryonia), the number of species per genus varies from 1 to 100+. 
Still, Neubert & Janssen have a point when they say that some genera seem to be too undifferentiated in my 1979 thesis. Bostryx, Naesiotus, and Drymaeus are the obvious candidates. If I would have had the opportunity to continue my research during the 1980s and beyond, I’m sure they wouldn’t have made this remark. Ongoing research involving these groups already indicate that they should be divided.

At the same time one could wonder about the number of genera per family. Also a man-made construct, although there is a useful ‘rule of thumb’ that a family should be monophyletic. Starting with one large family, Bulimulidae, at the time that Pilsbry wrote his monumental Manual of Conchology (1895-1902), my latest analysis counts no less than seven families. That sounds like a magical number…


BTW: Zander’s remark was made in a discussion that started with this link:

Miller, G.A., 1956. The magical number seven, plus or minus two. – Psychological Review 63: 81-87.
Neubert, E. & Janssen, R., 2004. Die Typen und Typoide der Natur-Museums Senckenberg, 84: Mollusca: Gastropoda: Pulmonata: Orthalicoidea: Bulimulidae (2), Orthalicidae, Placostylidae. – Archiv für Molluskenkunde 133: 193-297.

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