Abstract:
Majority of life forms have been observed to exhibit circadian (Latin: circa =
about/approximately; diēs = day) rhythmicity in behaviour and physiological processes
driven by underlying circadian clocks, and has been the topic of study for several
decades. Even though the earliest mention of daily rhythms dates back to Androsthenes
around 4th century BC, other evidence suggest that such rhythms were extensively
noticed and their importance realised much before Androsthenes (Box 1). Today, it is
commonplace to encounter statements such as “circadian clocks evolved in response to
rhythmic selection pressures imposed by environmental variables as a consequence of
earth’s rotation about its axis” implying that it probably is a well-established knowledge.
Given the prevalence of circadian rhythms in multiple facets of human lives as well as
that of other life forms, it may be quite an arduous exercise for chronobiologists to even
consider that circadian clocks might not be adaptive. Nevertheless, if one were to
critically assess the large number of studies in chronobiology, it would become evident
that studies on this topic are relatively few, and the existing body of evidence only
demonstrate that circadian clocks provide certain advantages to its bearers under specific
contexts. However, from the perspective of an evolutionary biologist’s definition of
adaptation, being advantageous may not necessarily mean being adaptive. Here we will
discuss the evolutionary significance of circadian clocks by reviewing literature on
theories and the evidence about their origin and the adaptive value. In later sections, we
will discuss the pitfalls of some of the experimental methodologies used and suggest
improvements that we believe can fine tune experimental designs for future studies.
