Abstract:
The phrase as sure as night comes after day' highlights the most conspicuous phenomenon on earth (i.e. the daily cycling of geophysical factors). It is hypothesized that life-forms on earth have evolved timekeeping mechanisms (circadian clocks) as adaptations to cope with such cyclic variations in their environment and, to test this hypothesis, diverse strategies have been employed. In this review, these different approaches, including comparative, clinal, ecology and/or trait manipulation and laboratory selection strategies are discussed, aiming to evaluate the adaptive value of circadian clocks. The limitations of each of these methods are assessed, and it is suggested that laboratory selection is an ideal, potent and suitable strategy for examining whether circadian clocks are indeed adaptations. In support of this, laboratory selection strategies are highlighted and critically reviewed in a discussion of studies that demonstrate the evolution of circadian clocks and life-history traits in response to selection for the timing of rhythmic behaviours, as well as those studies that demonstrate the evolution of circadian clocks in response to selection for life-history traits. Finally, newer approaches are proposed that involve the use of mutants, simultaneous manipulation of multiple environmental factors and genomic technologies in conjunction with laboratory selection to further explore the adaptive significance of circadian clocks.
