Some Thoughts on Angiosperm Phylogeny and Taxonomy

Abstract

Since the time of Darwin, taxonomists have been concerned with phylogenetic relationships as well as with the formal taxonomic system. Some definitions of taxonomy have essentially equated it with phylogeny, as a study devoted to determining the evolutionary relationships among organisms. More recently, particularly in England, some botanists have tried to divorce taxonomy from phylogeny, reverting in this respect to pre-Darwinian days. Theirs is a natural reaction to the fact that the marriage of taxonomy to phylogeny has proved to be a rather difficult one. It has become progressively clearer that a precise correlation of taxonomy with phylogeny is an unattainable goal. The more abundant the phylogenetic and other data, the more obvious the impossibility. Over and over again it turns out, when we have enough evidence, that before we can trace the members of a particular group back to a common ancestor, we are outside the confines of the group. The mammals, for which we have a very good fossil record, provide a case in point. No matter what set of criteria one chooses, there was never an original species of mammal, from which all other mammals are descended. The mammals originated as a set of more or less parallel evolutionary lines from reptiles-not just any old reptile, but from a particular group of reptiles during a particular span of geologic time. George Gaylord Simpson has for thirty years been using these facts to point out that the monophyletic criterion must be interpreted loosely if it is to be taxonomically useful. One way to put it is to say that if all the members of a particular taxon are descended from another taxon of lesser rank, the taxonomic criterion of monophylesis has been sufficiently met. Once we admit the necessity for a loose interpretation of the monophyletic requirement, we are committed to the position that similarities due to evolutionary parallelism, as well as those due strictly to inheritance from a common ancestor, provide some indication of relationship and should be considered in the formulation of a taxonomic system. Insofar as the nature of the supply of mutations is a controlling force in evolution, the greater the genetic similarity between two groups, the greater the likelihood that they will produce similar mutations, have similar evolutionary potentialities, and undergo parallel evolutionary change. On phenotypic and genotypic bases, as well as on the basis of the nature of the supply of mutations, different groups have different evolutionary potentialities, and not all evolutionary channels are open to any one group. Insofar as natural selection is a controlling -force in evolution, the greater the phenotypic and genotypic similarity between two groups, the greater their potentiality to undergo parallel evolutionary change. If the similarities resulting from parallelism are numerous and pervasive, then the ancestors were probably very similar to begin