Comments on Hennig's “Phylogenetic Systematics” and its Influence on Ichthyology

Gareth J. Nelson (Dept. of Ichthyology, the American Museum of Natural History, New York, N.Y. 10024) 1972. Hennig's "Phylogenetic Systematics" and its influence on ichthyology. Syst. Zool., 21:364-374.-Mayrian relationships are concepts that embody overall genetic and phenotypic similarity rather than kinship. Simpsonian relationships embody kinship partitioned into horizontal and vertical aspects, defined in relation to ancestors presumed to be empirically unknown (hypothetical) and empirically known (nonhypothetical), respectively. Hennigian relationships correspond only to the horizontal relationships of Simpson, but are generally applicable-to fossil as well as recent species. Hennigian relationships are consistent with, and acceptable to, ichthyological usage. Phylogenetic classification in Hennig's sense is consistent with, and is a modern expression of, the Darwinian tradition. Recent work in systematic ichthyology indicates progress toward a phylogenetic classification in the Darwin-Hennig sense. [Classification; Hennig; ichthyology.] For better or worse, creators of biological classifications have traditionally been unwilling to explain, discuss and weigh the relative merits of their purposes and achievements. But during the last ten years or so, the niche of expositor-discussant-arbiter has not remained empty, being filled (one might even say overpopulated) by theoreticians, experimentalists and mathematicians (Darlington, 1971:341). One result, perhaps ultimately beneficial, is that the taxonomist is now under some pressure to be explicit about his purposes and precise about his methods (Hull, 1970:49-50). Among other taxonomists, Hennig (1966, 1969, 1971; also Schlee, 1971) has written on the subject of methods and purposes. His view is that many different classificatory systems are possible, and perhaps useful in one way or another, but that among them there is one best suited to serve as a general reference system for the science of biology. He defined the concept of phylogenetic relationship (kinship, or genealogical relationship) on which this system of classification is based and termed it the "phylogenetic system." It demands, axiomatically, that species related by common ancestry be classified together. Whereas some commentators consider Hennig's work a modern expression of the Darwinian tradition in biology (e.g., Crowson, 1970:95; Nelson, 1971; see also Crowson, 1958), and value it as such, others consider it absurdly extreme or too rigid, to be of general applicability (Darlington, 1970). Some persons contest its claim to be "phylogenetic" (e.g., Mayr, 1965a: 167, 1968:547, 1969:70), and some warn of the "disastrous" results that would follow its application to classification (e.g., Ashlock, 1971:64). Critics of Hennig's system of classification have attacked, among others, the principles that species (or groups of them) are to be grouped according to the branching sequence of their ancestry, and ranked (1) according to their relative time of origin or (2) such that sister-groups are given equal rank. Each of these three principles may appear in itself rather arbitrary and unconnected with the other two, in short, a convenient target for hostile criticism. All three, however, are simple corollaries of the axiom that species related by common ancestry be classified together (grouped in the same taxon), and logically follow from it. So far there have been few explicit attempts to refute directly the axiom itself, probably because Hennig's concept of re-