Circadian rhythms (~24 h) in biochemistry, physiology and behaviour are found in almost all eukaryotes and some bacteria. The elucidation of the molecular components of the 24 h circadian clock in a number of model organisms in recent years has provided an opportunity to assess the adaptive value of variation in clock genes. Laboratory experiments using artificially generated mutants reveal that the circadian period is adaptive in a 24 h world. Natural genetic variation can also be studied, and there are a number of ways in which the signature of natural selection can be detected. These include the study of geographical patterns of genetic variation, which provide a first indication that selection may be at work, and the use of sophisticated statistical neutrality tests, which examine whether the pattern of variation observed is consistent with a selective rather than a neutral (or drift) scenario. Finally, examining the probable selective agents and their differential effects on the circadian phenotype of the natural variants provides the final compelling evidence for selection. We present some examples of how these types of analyses have not only enlightened the evolutionary study of clocks, but have also contributed to a more pragmatic molecular understanding of the function of clock proteins.

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