2.5 What does relationship mean in systematics? W. Hennig
Activity 4
Timing: 0 hours 5 minutes
In this clip, Dr. Patterson introduces his third systematist, a German entomologist named Willi Hennig. This offers a third meaning of ‘relationship’, which is illustrated through a diagram showing Hennig’s conception of the relationship between phylogeny and classification (Figure 6). At the end of the clip, Dr. Patterson refers back to Simpson’s diagrams (Figure 4). This figure is repeated below the clip.
Figure 6 Diagram to show Hinnig's conception of the relationship between phylogeny (II) and classification (I), redrawn from Hinnig, W. (1966) Phylogenetics Systematics, Figure 19, p. 75. I shows a cladistic classification and Ia shows the classification a pheneticist would adopt
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Dr. Colin Patterson
The third answer to the meaning of ‘relationship’ comes from Willi Hennig, a German entomologist, who lived from 1913 to 1976. Here's a quotation from Hennig's 1966 book, "Phylogenetic Systematics" … "The concept of 'relationship’ may be defined as follows: a species x is more closely related to another species y, than it is to a third species z if, and only if, it has at least one stem species in common with species y, that is not also a stem species of z," end quote.
Hennig's definition sounds formal and Germanic, but it's easy enough to follow with the help of a picture, which comes from the page facing the definition in Hennig's book. I've added the letters X, Y and Z so that we can match his definition to the diagram. He says that X is more closely related to Y than to Z, because X and Y share a stem species - which I've labelled C - which is not also a stem species of Z.
Above Hennig's tree are two Venn diagrams .- patterns of nested ellipses. It's a feature of Hennig's view of classification that the tree and the classification should be exact images of each other. His upper Venn diagram, labelled 1, is an exact match with the relationships shown in the tree. His lower Venn diagram, labelled la, shows a different pattern, a pattern we should get if we followed Mayr's definition of relationship - shared genes - because the distance between stem species A and B is much less than that between B and C. The shared similarity of species W and Z would make Mayr classify them together, with the result shown by the dotted line in the tree, and by the dotted ellipse in the lower Venn diagram. The pattern of relationship shown by the tree can't be recovered from the classification shown by the lower Venn diagram, which gives a different tree.
As for Simpson's concept of relationship, his right-hand diagram, number 2B, is presented both as a tree and a Venn diagram, and you might try copying out the Venn diagram part of it with these seven species, and seeing what tree you recover from it. As you'll find, the tree you get is quite different from his original … but Simpson's left-hand diagram, the concept that he called ‘flatly false’, exactly matches a Venn diagram expressing the relationships in the tree.
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Figure 4 The relationship between phylogeny and higher and lower taxa, according to Simpson, G. G. (1961) Principles of Animal Taxonomy, p. 190, Figure 19 (redrawn). (a) 'Phylogenetic tree with stems and branches incorrectly conceptualized as corresponding with taxa at different levels'. (b) 'Same correctly subdivided into taxa.'