Circadian clocks: an overview on its adaptive significance

The metabolic, physiological, and behavioral processes in most of the living organisms exhibit an approximate 24-h rhythmic, self-sustained circadian (L. circa = about, dies = day) oscillations. These oscillations, apart from passively persisting in synchrony with external cues, exist in their absence as well, conferring adaptive advantage by following environmental cycle (extrinsic adaptation) and physiological harmony (intrinsic advantage), and tackling the adverse seasonal changes. Interestingly, these circadian perturbations generated at cellular level as rhythms are pretty much hierarchically similar across unicellular organisms to complex mammals. The biological clocks (master clock along with peripheral clocks) are able to influence biological fitness, life-history traits, physiology and behavior of organism. Given that the circadian clocks influence various biological processes, its evolutionary significance remained the subject of several laboratory studies with the view that imposing artificial selection for various clock properties such as phase, period, etc., might lead to the co-evolution of associated other clock-controlled evolutionary traits. In this review, we have presented key hypotheses pertaining to circadian rhythms, where by its presence to the owner can confer adaptive advantage intrinsically and extrinsically and enable endogenous clock to perform better in synchrony with the environment.