GRADES in the EVOLUTION and CLASSIFICATION of INSECTS By

3 GRADES IN THE EVOLUTION AND CLASSIFICATION OF INSECTS [Presidential Address to the Australian Entomological Society, Melbourne, January 15, 19671 By I. M. MACKERRAS* Abstract Grades of organization, of the kind recognized in vertebrates by de Beer and J. S. Huxley, are a conspicuous feature of the insects also. Four can be distinguished by well-defined anagenetic gaps: aptery ote; palaeopterous and exopte gote; neopterous and exoptery- gote; neopterous and enffopterygote.As grades can be dezed more clearly than ph logeny at higher taxonomic levels and the reverse is true at lower levels, it is suggestelthat a classification can be made to reflect both without confusing the user. Insects have been remarkably successful animals. They probably differentiated from other lowly terrestrial arthropods about 350 to 400 million years ago, as small, soft-bodied hexapods that lived in moist, sheltered situations and crawled or ran among the primitive vegetation that was then be ning to develop. Some of them began to fly about 300 million ears ago. Therear ter they radiated widely, evolving aripassu with the developing rand flora, until today about three-fourths of all the own species of animals are insects, while the numbers of individuals defy com- Lputation. Their success in exploiting a wide range of environments, from high latitudes to the equator, from rain forest to desert, from mountains to shore, even including a limited invasion of the sea, and in adapting themselves to varied ways of living-saprophagous, phytophagous, carnivorous, parasitic, social-has been one of the outstanding events in the whole course of animal evolution. It can be brought into focus sharply enough for our purpose by recalling that, whereas the Australian mammalogist has 22 families and about 250 native species to think about, the Australian entomologist must cope with some 700 families and 54,000 known species, including nearly 4,000 species of the family Curculionidae alone. This superabundance of material has naturally brought many problems in its train, not the least of which is that it becomes diflticult to see the wood for the trees. It may be appropriate, therefore, that the first presidential address to this Society should be an attempt, however imperfect, to look at the evolutionary and taxonomic picture as a whole. It is, in part, a sequel to more general examination of taxonomic principles (Mackerras 1964), into which I was led by my first contact with numerical taxonomy. SCE~CMETHOD AND TAXONOMY As this discussion is concerned with taxa and their evolutionary relationship to one another, we must be clear at the outset about the taxonomic principles upon which the classification we use is founded. In the first place, it is essential to remember that taxonomy can be regarded as a scientific discipline in its own right only if those who practise it conform to the basic rules that are held, by common consent, to govern any kind of scientific en- deavour: (l), that the scientist should describe and interpret nature objectively (i.e., without bias) as it is, and not merely chop it up into tidy little watertight com- partments to suit his convenience-though he may do so as a preliminary to closer examination; and (2), that the collection and sorting of facts (“descriptive science”) is not an end in itself, but a stepping stone to theoretical knowledge. It might, indeed, be salutary, in the li t of some retrogressive modern trends, to define facts (information)as the materia of science, science as the use that is made of the facts to construct and test hypotheses.?kh Second1 , the basic premise of evolutionary animal taxonomy is that animals have evolved by descent with modification, diversity being the result of speciation, and the purpose of the classification is to express that relationship. In its classical form, every taxon is a statement of the hypothesis that its members are more nearly related to each other by propinquity of descent than to members of any other taxon, so the classification is called “phylogenetic” or “Darwinian”. As every episode of *Division of Entomology, C.S.I.R.O.,Canberra, A.C.T. ?It follows that empirical or Adansonian taxonomy, as expressed in a “phenetic” or “numerical” classification, is not a scientific discipline. It is purely a system of filing information, so it need not concern us here. J. Ausr. enr. SOC.,1967, 6:3-1 I. 4 I. M. MACKERRAS speciation is a point of origin of diversity of unpredictable extent, monophyly may be defined, strictly, as the evolution of a taxon from a single ancestral species. It follows, too, that the relative times at which splitting occurred are the fundamental criteria in constructing phylogenetic hierarchies (Hennig’s principle of “sister groups”-see his 1965 review). Thirdly, there are two com onents in diversity: the multiplication of phyletic lines, and the development of pf: enetic divergence between them. The two are not necessarily co-ordinate, and a strictly phylogenetic classification is concerned only with the first. Much confusion has resulted from failure to appreciate this principle. Finally, it must be emphasized a ain that taxa are statements of hypothesis. Like all scientific hypotheses, they are %ased on a critical analysis and judgement of the evidence available at the time, and they are tested by analysis of new evidence as it becomes available. Again like other hypotheses, they are modified when necessary, and abandoned when found untenable. The evidence that is used, both for formulating and testing, is primarily anatomical (including ultrastructure), and it can be analysed in a variety of wa s. No modern taxonomist would depend on one set of structures or one method or analysis when he can use more. The essential point is that the classification is dynamic: it unfolds a little more with every increase in our knowledge, and becomes progressively more illuminating as time goes on. No one can ex ect that finality will ever be reached (Mackerras 1964). With this gackground we may pass to a consideration of the complications that arise from the existence of grades of organization in animals. THE NATUREOF GRADES Julian Huxley (1957, 1958) has pointed out that there are three main processes of evolution, “leading respectively to biological improvement, to diversification, and to persistence”. He adopted the terms anagenesis, cladogenesis, and stasigenesis for these processes, and he designated the steps in anagenetic advance as grades and monophyletic lines as clades. In this way he crystallized ideas which had been developed by Darwin and T. H. Huxley a century earlier. Phylogenetic classifications are composed of clades, and have been considered in the preceding section. We need not concern ourselves with persistence, except to note that it is treated as positive when the taxon survives, negative when it declines. Failure to advance, however, does not necessarily mean extinction. Unspecialized animals may survive, either because they can evade competition, or because they are very competent at something (Manton 1958), and it is a common observation that animals that are extremely primitive in most respects also have compensating specializations in a few. That biolo ‘cal improvement is the consequence of natural selection is the core of Darwinian #eory, and Huxley has defined it as including “detailed ada tation to a restricted niche, specialization for a particular way of life, increased eli ciency of a given structure or function, greater differentiation of functions, improvement of structural and physiological plan, and higher general organization”. He noted, too, that Darwin had recognized that diversification is a special form of general biological improvement. Clearly, ana enesis OCCUTS at all taxonomic levels and may be of all degrees of magnitude. If t%at were always so, grades would be merely arbitrary divisions in a continuum, and would have little special si cance. But it is not always so. Observations have shown that particular structur modifications have, from time to time, been associated with the appearance ofP extensive new adaptive radiations. Events of this kind have long been known in the vertebrates, and we shall be examining their occurrence in the insects. They are clearly reco nizable, at least at higher taxonomic levels, and it is to them that the term gra%- e may be usefully restricted. Thus, though anagenesis is continuous, grades are dis- continuous. De Beer (1954) showed that the transition from one grade to the next took place by what he called mosaic evolution. Archaeopteryx, for example, had a mixture of characters, primitive ones shared by reptiles and birds, some reptilian characters that are lost in birds, and some truly avian characters that reptiles do not possess. Also, as perhaps we might expect with mosaic evolution, the transition often seems to depend, not on a change in any one structure or function, but on what amounts 6 I. M. MACKERRAS and agility would have come into operation, leading to the development of longer, stronger legs carried on a larger, more powerful thorax. The mechanical benefits of the hexapod gait could be fully realized; functional division of the trunk into a thoracic tagma specialized for locomotion and an abdomen specialized for tro hic and reproductive functions would be facilitated; and raising the anterior end of! the substrate would have facilitated evolution of the varied types of hypognathous mouth-parts that occur in the insects. Even more important, from the point of view of subsequent evolution, were the provision of a point of balance in the anterior third of the body and the results of selection pressures for perception of, and reaction to, more distant as well as nearby objects in the environment. These two features, hexapody and emancipation, thus combined to provide the foundation on which the rest of insect evolution was built, so the Apterygota may be regarded as the base grade of the class.

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