COMMENTS 1624 Evolution, 53(5), 1999. pp. 1624-1627 DON’T FORGET THE BIOLOGY: A REPLY TO GREEN W. G. EBERHARD, 1 B. A. HUBER, 2 and R. L. RODRIGUEZ 3 1 Escuela de Biología, Universidad de Costa Rica, Ciudad Universitaria, Costa Rica, and Smithsonian Tropical Research Institute E-mail: weberhar@cariari.ucr.ac.cr 2 Department of Entomology, American Museum of Natural History, New York, New York 10024 3 Department of Entomology, Hawarth Hall, University of Kansas, Lawrence, Kansas 66045 Key words.—Allometry, genitalia, sexual selection. Received May 19, 1999. Accepted May 21, 1999. In our 1998 paper (Eberhard et al. 1998), we tested several hypotheses regarding the possible selective factors involved in the evolution of animal genitalia. We compared the slopes of log-log ordinary least squares (OLS) regressions of gen- italia on indicators of body size with the slopes of other body parts on the same indicators in 20 species of insects and spiders. Our major conclusions regarding the rejection of male-female conflict, good viability genes, and lock and key arguments (see Abstract) are unaffected by the reanalysis proposed by Green (1999), even if it were more appropriate than ours, which we doubt (see below). In neither our OLS regressions nor Green’s reduced major axis (RMA) regressions are the slopes of genitalia greater than those of other body parts, as would be expected if gen- italia were used as weapons in forceful intraspecific battles (the male-female conflict hypothesis) or as signals of male size (good viability genes hypothesis). Instead, the slopes show a statistically significant trend to be lower in both an- alyses. In addition, neither we nor Green found differences on comparing the slopes of the genitalia of the species in which lock and key considerations might be important (spe- cies in which male genitalia fit against rigid female genitalic structures) with the slopes in species in which lock and key can be ruled out because of the mechanical mesh of the male’s genitalia with those of the female. So, conservatively, we conclude that the major conclusions of our paper are not affected under Green’s reanalysis. It is important not to lose sight of the biological questions being tested in debates over statistical methods. There are a number of reasons, however, to doubt several of Green’s points. Green’s claim that reanalysis is needed, the analysis he performed, and the additional explanations that he proposed in preference to those we mentioned in our original paper all have serious problems. We will discuss first the statistical questions, and then the more directly biological questions. Is RMA Regression More Appropriate? Many of Green’s objections (see the first half of his final, summary paragraph) hinge on the different values he ob- tained when he used RMA regression analyses. We struggled with the question of whether we should use RMA rather than OLS regressions while we were preparing our original paper, and finally decided against RMA for two reasons. The first is that it is not obvious which of the two types of regression is more appropriate, despite the impression given by Green. For instance, Sokal and Rohlf (1995, p. 544) mention that the RMA regression technique has been the subject of “se- rious criticisms” (see also reservations expressed by Mc- Ardle 1988; Martin and. Barbour 1989). LaBarbera (1989) states that “OLS regression is appropriate only when the goal is to . . . allow prediction of expected values given one of the two variables” (which is the “good genes” hypothesis we wanted to test). Sokal and Rohlf also note that if a cau- sality relation exists between variables, one should employ OLS rather than RMA regressions (p. 545). While, as Green noted, we were careful in our paper to avoid claiming that we could establish cause-effect relations between the partic- ular body size indicators and the other variables we used, we think that a complete lack of cause-effect between overall body size and the sizes of both genitalic and nongenitalic traits is highly improbable. Green’s proposal that there is equal justification for presenting regression slopes of y re- gressed on x and x regressed on y does not make biological sense (the size of an animal’s body does not seem likely to be determined by the size of its genitalia). Inasmuch as our indicator variables are indeed indicators of overall body size, then at least indirect cause effect relationships with other variables probably do exist. The analyses that we performed using alternative indicators of body size (p. 418, second par- agraph of Results) generated nearly identical results, sug- gesting that our indicators may indeed be reasonable esti-