Sexual Selection and Insect Mating Behavior

Abstract

Randy Thornhill is associate professor of biology at University of New Mexico, Albuquerque 87131. He received his B.S. and M.S. degrees in entomology from Auburn University and his Ph.D. degree in zoology from the University of Michigan. His research interests include the diversity of animal mating systems (particularly insect systems) and human social behavior. This research has led to a number of articles dealing with the evolution of insect and human behavior, and a new book (co-authored with John Alcock), The Evolution of Insect Mating Systems (Harvard University Press, 1983). Gary Dodson is assistant curator of the Museum of Southwestern Biology, Division of Invertebrates, University of New Mexico. He holds a B.S. degree in zoology (University of Tennessee) and an M.S. degree in entomology (University of Florida). His research has centered around mating systems in general, especially reproduction and sexual selection in insects. Larry D. Marshall is a Ph.D. candidate at the University of New Mexico. He holds B.S. and M.S. degrees in biology, both from Arizona State University. He is conducting research on the effects of male nutrient investment on the structure of insect mating systems and the reproductive strategies of desert tenebrionid beetles. The evolutionary process consists of the following components: selection, mutation, inheritance, drift (random genetic change), and isolation of populations. These components can be called facts of evolution, because their action in every organism can be demonstrated at will. These five factors acting together have led to the diversity of life on Earth. It has also been demonstrated that the chief factor in guiding evolutionary change is selection, which is differential reproduction of individuals. Charles Darwin did not invent the idea of selection; he discovered it, as did Alfred R. Wallace independently at about the same time. Selection has acted continuously on all living things throughout the history of life and continues to do so today. This omnipotence of selection provides the foundation for scientific study of all life. It says that the features of life are what they are largely because of selection in the past, and thus all features of all living things are expected to ultimately promote reproduction or genetic propagation of individuals. This theoretical framework tells the biologist how to ask questions and proceed in order to gain further understanding of life through experiment and observation. Since life itself is a product of selection, an understanding of this process and how it can be applied to elucidate living things provides the practicing biologist with the best direction and insight, regardless of whether he or she is interested in molecules, behavior, physiology, morphology, etc. See R.D. Alexander's paper (1978) for a detailed treatment of the nature of the evolutionary process. Selection is of two types: natural and sexual. Both types stem from differential reproduction of individuals, but the differential reproduction is affected in different ways. Natural selection is differential reproduction of individuals due to differences in survival. Since reproduction is necessary for selection to act, natural selection includes differential reproduction of individuals in the contexts of obtaining a mate of the right species, proper fertilization, etc. Sexual selection is differential reproduction of individuals in the context of competition not just for mates, but for the best mates. Although both forms of selection involve competition between individuals for genetic representation, competition for mates is a key factor for distinguishing natural vs. sexual selection. Consider the of the male stag beetle, which are absent in females. In this species the horns of a male are actually mandibles that are elaborated into antler-like structures. If horns increase survival prospects for a male, perhaps by use in stabbing predators, then they are the product of natural selection. Natural selection is also the cause of horns if they function in increasing the probability that a male can inseminate a female, perhaps by giving him better balance during copulation. But these two hypothesized selective