Physiological Differentiation of Vertebrate Populations

Abstract

Although studies of geographic variation are relatively common, the vast majority have dealt with morphometric or meristic traits (13, 120, 297, 327), allozymes (150, 233, 327), or most recently, mitochondrial DNA (9). The relative rarity of studies of geographic variation in physiological traits (see 4 2, 245, 312 for reviews of early studies) is somewhat surprising, considering that a major focus within vertebrate "physiological ecology," "comparative physiology," "environmental physiology," and "biophysical ecology" is the study of adaptation (14,15, 44, 75, 102, 112, 156, 181, 244, 246, 298, 299). Of course, populations may show differentiation on other than large geographic scales, such as "microgeographic" (150, 188, 213, 233, 290), temporal (164, 165, 213, 253, 278, Armitage, this volume) or altitudinal; the last has received considerable attention from vertebrate physiologists. Why are population differences in physiology not studied more often? We suggest four reasons: (i) Physiological measurements require living organisms. Collecting from multiple populations is formidable enough without the: additional difficulty of making physiological measurements. "Expeditionary physiology" is possible, but requisite equipment often is considerably less portable than that needed for sampling tissues or for obtaining study skins or skeletons. Moreover, physiological measurements can be quite time-consuming and require maintaining animals in a healthy state, sometimes for weeks or months, to achieve a common state of acclimation. (ii) Physiologists and evolutionary biologists often share little in the way of training, interests, perspectives, techniques, and study organisms. Population geneticists routinely study Drosophila because they are small and so can be bred in large numbers. Physiological ecologists rarely study Drosophila because they are small and hence intractable for most physiological measurements (but see 63, 137, 158, 166). Nonetheless, some studies of population differentiation in physiological traits have been done. Many involve invertebrates, including Drosophila (42, 63, 137, 158, 223, 233, 245, 312 refs. therein). Another cohort of researchers has come from a background in population genetics, having moved toward physiology in an attempt to discover the adaptive significance of population differences in allozymes (104, 137, 191, 223, 224, 243, refs. therein). (iii) To increase the likelihood of finding differences and exemplary cases of adaptation, comparative physiologists have tended to focus on species expected to display extremes in physiological function (102, 255). Recognizing problems introduced by rampant acclimatization, physiologists may have been inhibited from searching for relatively small population differences. Consequently, the focus on proximate mechanisms of coping with environmental change (i.e. acclimatization) drew more attention than the possibility of genetic differentiation among populations. (iv) "Typological thinking" still exists among many physiologists (102). (The coexistence of typological thinking and ardent adaptationism is somewhat surprising!)