Figure 5.6 The relationship between walking speed and temperature in males and females of three lines of Drosophila melanogaster evolving at different temperatures.

Source: Physiological Zoology, Gilchrist et al. 70, 403-414. © 1997 by The University of Chicago. All rights reserved. 0031-935X/97/7004-9686$03.00

curves can potentially be modified either by acclimation or by developmental switches (Fig. 5.6).

The evolution of performance curves is dealt with in various sections of this book, and has been reviewed in detail elsewhere (Huey and Kingsolver 1993; Gilchrist 1995). Nonetheless, it is worth noting that any discussion of physiological performance curves immediately shows up the difficulty, and artificial nature, of distinguishing between 'resistance' and 'capacity' adaptations. In some cases, the long- or short-term (plastic) responses of these curves might involve a change in critical limits, but not the optimum, while in others the shape, limits, and optimum values may all change in concert (Huey and Kingsolver 1993).

Beneficial acclimation?

Because physiologists have largely concentrated on the mechanisms underlying acclimation (Kingsolver and Huey 1998), they have often assumed that acclimation is advantageous for the organism concerned when it is subsequently exposed to the conditions under which it was acclimated. This beneficial acclimation hypothesis has recently been questioned on the basis mainly of laboratory selection trials using both D. melanogaster and Escherichia coli (see Huey and Berrigan 1996; Huey et al. 1999 for review). These investigations have not only revealed considerable complexity in the acclimation response, but have also shown that it is not necessarily adaptive. For example, in some cases there appears to be an optimal temperature which results in a phenotype that performs best over all conditions (Tantawy and Mallah 1961; Huey et al. 1999; Gibert et al. 2001). In addition, acclimation also appears to be associated with considerable physiological costs (Hoffmann 1995) (Section 5.2.2). However, beneficial acclimation has been demonstrated, at least partially, in some insect species not only in the laboratory (Scott et al. 1997; Huey et al. 1999; Hoffmann and Hewa-Kapuge 2000), but also in elegant field trials. Moreover, the field trials have not only assessed performance, but have also assessed the fitness of individuals characterized by a given phenotype, either by examining survival (Kingsolver 1995a,b, 1996), or by directly determining egg-laying ability (Thomson et al. 2001).

More recently, the question of the extent to which studies involving development at different temperatures, and under stressful conditions, represent tests of developmental plasticity rather than the beneficial acclimation hypothesis has been raised (Wilson and Franklin 2002). Moreover, Woods and Harrison (2002) argue that exposure to non-optimal temperatures will degrade performance in all environments (which can be called the deleterious acclimation hypothesis), and a focus on fitness as a whole rather than the contribution of the individual trait of interest, both bias tests against findings of beneficial acclimation. They suggest that focusing on the fitness consequences of individual traits is essential for tests of the beneficial acclimation hypothesis and its alternatives. These complexities of acclimation, and the realization that the extent and nature of acclimatization are likely to depend considerably on the degree and predictability of environmental variation, are cogent reminders of the need for a strong inference approach, involving multiple hypotheses, in evolutionary physiology (Huey and Berrigan 1996; Huey et al. 1999; Woods and Harrison 2002).

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