MORPHOLOGICAL VARIATION IN FROG PECTORAL GIRDLES: TESTING ALTERNATIVES TO A TRADITIONAL ADAPTIVE EXPLANATION

In a recent article, Gould and Lewontin (1979) criticize evolutionary biologists for their widespread acceptance of the "omnipotence of natural selection in forging organic design and fashioning the best among possible worlds," and they urge the exploration of other explanations in addition to that of the 'adaptationist programme' for the distribution of traits among organisms. Although their appeal seems reasonable, if a pluralistic approach is to provide meaningful insights into the origin and distribution of traits, it is important to devise tests of these alternative explanations. Otherwise, they will be invoked in an ad hoc manner similar to the strict adaptationist interpretations (see section on telling stories, Gould and Lewontin, 1979). This paper explores alternative explanations to those of the 'adaptationist programme' for the presence of two different types of pectoral girdles among frogs. The importance of this study, I believe, is that it provides actual tests of two of these alternative explanations, those of functional equivalence and mechanically forced correlation. When morphological differences are found among species, it has usually been assumed, under the 'adaptationist programme,' that each configuration represents a best solution and that such differences indicate that either each morphology is adapted for a different biological role (sensu Bock and von Wahlert, 1965), or, if they serve the same biological role, that one type is better adapted than another for that role. Another possible explanation (and one suggested by Gould and Lewontin) would be that such morphologies are equally adaptive and that differences among them are a result of the particular phylogenetic histories of the involved taxa. Unlike some of the other proposed alternative explanations, like pleiotropy or chance, where it appears difficult, if not impossible, to gather supporting data, the 'equally effective but different' hypothesis can be tested. Three pieces of information are necessary: (1) the phylogenetic distribution and history of the morphologies involved, (2) the demonstration that the different morphologies perform the same biological role(s), and (3) some measure or test of functional equivalence, and those data are relatively easy to obtain. Unfortunately, in the past it has been rare for biologists to provide information in all three categories and the missing piece has almost always been the last, a measure of functional equivalence. However, since the functional equivalence of structures can be directly measured through comparative performance, the 'equal but different' explanation seems to be a potentially fruitful avenue to explore in attempts to broaden our understanding of evolutionary patterns and processes. Presence or absence of an element, under the strict adaptationist programme, has most commonly been viewed as adaptive (or nonadaptive) in its own right, with little or no consideration of the biomechanical relationship of that element to others or to overall structure. An alternative view (again suggested by Gould and Lewontin, but not original with them) is that some patterns, such as presence/ absence of an element, arise not as adaptations, but rather as consequences of the design or material composition of a