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 Sexual Selection and been demonstrated that the chief factor in guiding evolutionary change is selection, which is differential Insect reproduction of individuals. Charles Darwin did not Mating invent the idea of selection; he discovered it, as did Alfred R. Wallace independently at about the same Behavior 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 Randy Thornhill the foundation for scientific study of all life. It says that Downloaded from http://online.ucpress.edu/abt/article-pdf/45/6/310/40963/4447713.pdf by guest on 25 September 2021 Gary Dodson the features of life are what they are largely because LarryMarshall of selection in the past, and thus all features of all liv- ing 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 obser- vation. Since life itself is a product of selection, an understanding of this process and how it can be ap- plied 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. Alex- ander'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 in- dividuals, but the differential reproduction is affected in different ways. Natural selection is differential reproduction of individuals due to differences in sur- vival. 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 selec- tion is differential reproduction of individuals in the context of competition not just for mates, but for the Randy Thornhill is associate professor of biology at University of New Mexico, Albuquerque 87131. He received his B.S. and M.S. bestmates. Although both forms of selection involve degrees in entomology from Auburn University and his Ph.D. degree competition between individuals for genetic represen- in zoology from the University of Michigan. His research interests tation, competition for mates is a key factor for include the diversity of animal mating systems (particularly insect systems) and human social behavior. This research has led to a distinguishing natural vs. sexual selection. number of articles dealing with the evolution of insect and human Consider the "horns" of the male stag beetle, which behavior, and a new book (co-authored with John Alcock), TheEvolu- are absent in females. In this species the horns of a tionof InsectMating Systems (Harvard University Press, 1983). Gary Dodson is assistant curator of the Museum of Southwestern Biology, male are actually mandibles that are elaborated into Division of Invertebrates, University of New Mexico. He holds a B.S. antler-like structures. If horns increase survival pro- degree in zoology (University of Tennessee) and an M.S. degree in spects for a male, perhaps by use in stabbing predators, entomology (University of Florida). His research has centered around mating systems in general, especially reproduction and sexual selec- then they are the product of natural selection. Natural tion in insects. Larry D. Marshall is a Ph.D. candidate at the Univer- selection is also the cause of horns if they function in sity of New Mexico. He holds B.S. and M.S. degrees in biology, both increasing the probability that a male can inseminate from Arizona State University. He is conducting research on the ef- fects of male nutrient investment on the structure of insect mating a female, perhaps by giving him better balance dur- systems and the reproductive strategies of desert tenebrionid beetles. ing copulation. But these two hypothesized selective 310 THE AMERICAN BIOLOGY TEACHER, VOLUME 45, NO. 6, OCTOBER 1983 FIGURE 1. The horns of male forked-fungus beetles are used in male-male contests for females which lack horns. (Drawing courtesy of Don Luce.) Downloaded from http://online.ucpress.edu/abt/article-pdf/45/6/310/40963/4447713.pdf by guest on 25 September 2021 contexts are incorrect. Instead, horns serve males in importance in bringing about evolutionary change. sexual competition for mates. Male stag beetles use Their criticisms were largely directed at Darwin's view their horns in contests with other males for females. of the important role of female choice in evolution. Bet- It has recently been shown in a diversity of homed bee- ween Darwin's time and recent years, the study of sex- tle species that horn morphology for each species is ual selection was neglected. The present enthusiasm suited to the nature of fighting conducted by males among behaviorists about sexual selection stems from (Eberhard 1980) (fig. 1). Horns require energy that the contributions of George Williams, Geoffrey Parker, could be used in body maintenance and hence sur- and Robert Trivers, which revived interest and provid- vival, and beetle horns are sometimes so massive that ed research direction, and the numerous studies in the the male cannot feed. Thus the horns are contrary to last ten years showing that sexual selection is real and survival-maladaptive under natural selection-but important. evolved because they contribute to greater mating suc- Williams' book Adaptationand Natural Selection(1966) cess in male-male competition for females. Features initiated a revolution in the study of behavior, because such as the horns of male beetles, the gaudy plumage it identified the flaw in much of the thinking about how of some male birds, and the energetically demanding behavior has evolved; that is, the notion that behavior courtship of many male animals led Darwin to his functions for the good of the population or species. discovery of sexual selection. He reasoned that as long Because selection acts on individuals, it can only favor as the benefit of a trait to reproduction via sexual com- traits that promote individual genetic propagation. If petition exceeds its cost to survival, the trait will the group survives better as the result of some adap- evolve. tation of individuality it is merely an incidental effect Darwin outlined two forms of sexual selection: in- of individual selection. trasexual selection, competition among members of Trivers (1972) identified why the male is typically the one sex (usually males) for individuals of the opposite charming (courting) sex and the female the choosy sex. sex; and intersexual selection, preferential choice by The answer lies in the sexual asymmetry in parental members of one sex (usually females) for certain contribution to offspring, what Trivers calls parental members of the opposite sex. Both forms of sexual investment. The sexual disparity in investment begins selection ultimately involve competition for females. with gametes; males produce many small, streamlined Intersexual selection or female choice occurs because gametes containing only genes, the females produce some males are better than others in providing ap- a few large, nutritive gametes. Eggs contain the propriate courtship stimuli and thereby coaxing nutrient necessary for offspring survival (and ultimate- discriminating females to mate. ly reproduction). Male reproduction is not limited by For reasons that are still not completely clear, sex- gamete production but by male ability to fertilize ual selection was controversial upon its discovery by female gametes. Female reproduction is limited by Darwin (Thornhill 1980a). Biologists A.R. Wallace and gamete number and not by difficulties in getting Julian Huxley and Anthropologist E. Westermarck, gametes fertilized. It is typically also the female that staunch natural selectionists, argued in the late 1800s provides postzygotic care of young when this occurs, and early 1900s that sexual selection is of minor or no and this too is an important factor contributing to the SEXUAL SELECTION 311 sexual asymmetryin parentalinvestment. Parentalin- The sequence of sexual behavior in H. apicalisbegins vestment ultimately determines whether an offspring when a male catches for himself or steals from another survives and reproduces and thus females, who are male a prey insect and begins to feed on it. After a brief the majorparental investors, become the objectof male feeding males usually discard the prey and obtain competition. The more parentalinvestment a male can another if the prey is small (<16mm2in surface area). secure via copulation with multiple females the higher Only about 10% of males do not discard small prey his reproductive success. Because females are the ob- prior to premating flights and use these prey as nup- ject of competitionamong males, and because they lose tial gifts. If the prey is 16 mm2or larger, after the brief more reproductive potential from an improper mate feeding and while still holding the prey in his hind tar- choice,