2.6 Three schools of classification
Activity 5
Timing: 0 hours 10 minutes
This clip explores the three kinds of relationships that have been explained so far, in terms of the work of Simpson, Mayr and Hennig, which are referred to as Simpsonian, Mayrian and Hennigian relationships.
Dr. Patterson links each of the systematists with a particular school of classification – phenetics, cladistics and evolutionary systematics, or eclectics, and establishes which one of these most directly matches the ideas of molecular systematists.
This clip refers to Figures 4, 5 and 6. You may want to review these diagrams before listening to the clip.
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Dr. Colin Patterson
Now the three kinds of relationship I've been talking about have been called Simpsonian, Mayrian and Hennigian relationship. And they can be fitted very neatly to the three different schools of classification that developed during the 1960s, and were much disputed through the 1970s. These three schools are called phenetics, cladistics and evolutionary systematics, or eclectics.
Phenetics relies on overall similarity as a measure of relationship, and so it classifies similar organisms together. This matches Mayr's definition of relationship as shared genotype. Cladistics aims to classify by inferred recency of common ancestry, and so it matches Hennig's definition of relationship. And eclectics, or evolutionary systematics, classifies by a mixture of similarity and inferred common ancestry, using taste or judgement as to when one criterion's given precedence. And so it matches Simpson's discussion of relationship, and how one ought to classify.
But notice that there are only two criteria of relationship - the phenetic one of similarity, and the cladistic one of inferred common ancestry. Eclectics merely uses a mixture of the two. Well, which school is correct, or is best?
The overwhelming consensus, after twenty years of argument, is that cladistics is best, and it's unusual these days to find a systematist, who has given any thought to the fundamentals, who isn't a cladist.
Cladistics has won through, I think, for two main reasons. First, it has a consistent and coherent philosophy, and second, it developed at the same time as the early work in molecular systematics, when protein sequences and other molecular evidence was first brought to bear on problems of relationship and classification.
Let me explain quickly why this is so. A molecular biologist can only sample living taxa. He ends up with a set of data, let's say DNA sequences, from which he wants to recover a tree, or a classification. The sequences are necessarily seen as terminals of the tree. No-one would dream of seeing a DNA sequence from one species as ancestral to a sequence from another species. And, in a tree, it's ancestors that occupy the internal nodes and branches.
So molecular systematists, from the beginning, worked with samples from the tips of the tree, and tried to reconstruct the tree by one method or another. Now, if we look back to the diagrams explaining Simpson's, Mayr's and Hennig's ideas about relationship, we see that Hennig's is the only one that deals just with terminals of the tree. He's trying to classify species W, X, Y and Z, four terminals of the tree in his diagram. Simpson's trying to classify ancestors, as we can see from his diagram recommending combining three ancestral species into a genus ancestral to the four living or terminal species. And, in Mayr's diagram, the three terminals, B, C and D, are labelled with their percent genetic difference from A, the ancestor. Obviously, if we can tell that C differs from A by only 10%, whereas D differs from it by 70%, we must have access to A.
So among these three, Hennig is the only one whose definition of relationship treats the terminals of the tree as real, and the internal part of the tree as hypothesis or conjecture. And that's why his system directly matches the ideas of molecular systematists.
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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.'
Figure 5 Relationships according to 'inferred percentual difference from ultimate ancestor (A)', following Mayr, E. (1974) Cladistic analysis or cladistic classification?, Z. Zool. Syst. Evolut.-forsch., 12, 94-128, Figure 1 (redrawn). Mayr states, 'Taxon C is more closely related to B than to D, even though it shares a more recent common ancestor with D'
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