Evolutionary Developments in the Leafhoppers, the Insect Family Cicadellidae

Abstract

ONE of the least appreciated yet most remarkable facts in the living world is the great number of insect species, genera, and families which exist today. This number is usually considered a sort of freak phenomenon, as if it had no real connection with the natural laws affecting other organisms, or perhaps was to be attributed to some overstimulation of the descriptive powers of insect taxonomists. Actually the tremendous number of kinds of insects is simply an expression of the continuous speciation which has occurred in insects, combined with the survival of an unusually high percentage of the total number of phyletic lines produced. As a result of this survival we find preserved in living species of many insect groups an array of different steps in the evolution of specialized morphological conditions which permits us to reconstruct the course of evolution with considerable confidence. This is not the case in every family or genus of insects. In certain cases either survival rate of phyletic lines has been low, or our collecting has as yet unearthed only a few dissimilar types. In many large families or genera, however, we have sufficient primitive species and annectant types to form a remarkably cohesive picture of evolutionary events. In such an analysis it is necessary to establish which are the specialized groups and which the primitive, and to ascertain finally what assemblage of primitive characters was possessed by the ancestral form of the category in question. If one is studying a certain family, information only from within that family may not give an answer to these questions. Suppose, for example, one finds a spine series ranging from a large, heavy spine, through a small, slender one, to complete absence of a spine. How can one know the direction which evolution took? Did this line start out with a big spine which became smaller and finally atrophied; with no spine, then gain a small one which subsequently increased in size; or with a medium-sized spine which became larger in one line and atrophied in another? Two lines of evidence may be brought to bear on this problem. In one approach, examination of closely related families may indicate what condition is ancestral to the assemblage of families involved. In the other approach, examination within the family may show that the spine series parallels changes in some other character for which the primitive condition is known, allowing through this correlation the probable determination of the primitive condition of the spine character. In this latter case when we say that the primitive condition is well known we tacitly imply that we are drawing on studies from groups other than the family under scrutiny, hence in reality both lines of evidence owe their value to knowledge from outside the family. It is also true that as more characters are used in a phylogenetic analysis, the more detail can be embodied in the family tree. It has not been unusual, at least among the insects, for one character to remain virtually unchanged while another evolved at a rapid rate. Thus many branchings of the tree might occur for which no tell-tale changes would be shown by the first character. In the insects this