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Undisciplined thinking: morphology and Hennig’s unfinished revolution

There was a time, not long ago and prior to Hennig (1966), when taxonomy was widely dismissed as a mere service to ‘real’ – read experimental – sciences. Taxonomists were regarded by many to have nothing more to contribute to modern biology than the pragmatic role of identifying species and keeping track of their names. This was a legacy of the conflation of systematics with genetics by Huxley (1940), Mayr (1942) and others (see Wheeler, 1995, 2008a). Hennig re-elevated taxonomy, as phylogenetic systematics, to its rightful place as a rigorous, free-standing and central field of the biological sciences. Taxonomy is typically performed best when it is carried out for its own sake. Taxonomists are motivated to explore species, character diversity and phylogenetic relationships within monophyletic groups. The ultimate goal of taxonomists is a phylogenetic classification with associated scientific names, what Hennig described as biology’s general reference system. Oh yes, they make species identifiable, too. Current molecular initiatives, including DNA barcoding and DNA taxonomy, threaten to reduce species discovery as well as classifications to nothing more than a service. Because ‘new’ species would be ‘discovered’ on the basis of phenetic distances only, DNA barcoding might be described better as a disservice to biology (Prendini, 2005; Wheeler, 2005). After all, it offers only arbitrary averages, by contrast with explicitly testable alternatives, such as the phylogenetic species concept (Wheeler & Platnick, 2000). DNA taxonomy (in the sense of Tautz et al., 2003) is another flawed approach that would diminish the information content of classifications (e.g. Lipscomb et al., 2003). The trend in molecular phylogenetics (‘phylogenetic biology’) has been to increasingly marginalize the evidential basis of taxonomy and to treat the creation of ‘trees’ largely as a service to those same ‘real’ sciences. It is not coincidental that central to taxonomy ‘done for its own sake’ is a detailed study of characters, whereas most molecular phylogenies are published without characters. Morphology is regarded increasingly as an optional adjunct to molecular phylogeny, rather than an essential component of our understanding of species and their diversification. It is said that morphology provides few characters by comparison with molecular sources, and that its complex characters are inconveniently difficult to interpret and unacceptably subjective in their analysis. As a result, we are educating few young comparative morphologists and homogenizing ‘phylogenetic biology’ to be effectively little more than molecular ‘trees’ and morphology little more than literature reviews. The number of projects in which serious original morphological study is undertaken is alarmingly few, meaning that existing morphological hypotheses are untested, mistaken homology assessments are perpetuated and new morphological characters are undiscovered. The tragic loss represented by this neglect of comparative morphology has yet to be felt. Molecular phylogenies have concerned themselves largely with the reinterpretation of known taxa, and especially those bearing controversial ‘key’ morphological achievements, such as the evolution of flight in insects, vascular tissues in plants and, of course, the great tritomy of human, chimp and gorilla. Increasingly, it seems that molecular phylogenies are performed in service to others, to provide chronological patterns to biologists studying some attribute of organisms of sufficient complexity to be of inherent interest. The same putative virtue of simplicity that is claimed to make the interpretation of phylogeny from molecules less subjective deprives amino acids of sufficient complexity to store historical information. That only emerges from the next step in complexity: the sequence of amino acids in strands of DNA. Just as this minimal complexity adds some imperfect historical information, increasingly complex characters – genes, functionally linked genes, proteins, folding patterns, tissues, cells, ontogenetic pathways and morphology – each in turn potentially stores more and more historical information. This information is more difficult to interpret. Nevertheless, as has been well established, it can be made fully explicit and testable (Hennig, 1966; Farris, 1979; Gaffney, 1979; Nelson & Platnick, 1981; Wiley, 1981; Schuh, 2000). It takes time to begin to interpret the techniques used by Leonardo da Vinci to capture the mystique of Genevre D’Benci, and considerably Correspondence: Quentin D. Wheeler, College of Liberal Arts and Sciences, Arizona State University, PO Box 876505, Tempe, AZ 85287-6505, U.S.A. E-mail: quentin.wheeler@asu.edu