The evidence for adaptation Drosophila as a model

Variation in physiological traits among populations and species occupying different environments has long been regarded as evidence for adaptation, but this correlational approach has been subject to criticism (Garland and Adolph 1994). The use of phylogenetic information in comparative studies can show whether variation among taxa reflects adaptation to their environment or is, perhaps, an artefact of their evolutionary history. Alternatively, laboratory selection studies are proving a powerful tool in evolutionary physiology (Bradley et al. 1999; Gibbs 1999). Selected populations can be replicated, under strictly defined conditions, and unselected control populations are available for direct comparison with the ancestral condition. Desiccation is a strong agent of selection in small insects, and laboratory populations of Drosophila melanogaster which have undergone selection for increased desiccation resistance provide some unexpected information about the physiological traits contributing to this resistance. Analysis of the water budget of five desiccation-resistant populations (D flies) and five control populations (C flies) showed adaptive changes in some mechanisms but not in others (Gibbs et al. 1997). D flies are larger than C flies because they contain more water, and they also lose water less rapidly, but there is no difference in water content at death. As for the components of water loss, surface lipids do not differ significantly, and excretion (Fig. 4.14) accounts for less than 6 per cent of total water loss (Gibbs et al. 1997, 2003a). Reduced water loss rates must, therefore, be achieved through reductions in respiratory losses. Adult D flies contain 34 per cent more water than C flies, and the main storage site

Figure 4.14 Water loss recording for paired control (C) and desiccation-resistant (D) populations of Drosophila melanogaster (groups of 20 female flies).

Figure 4.14 Water loss recording for paired control (C) and desiccation-resistant (D) populations of Drosophila melanogaster (groups of 20 female flies).

Note: Trace shows both evaporative and excretory water losses; peaks of water vapour recorded by the humidity sensor indicate defaecation by individual flies.

Was this article helpful?

0 0

Post a comment