This post, originally published on January 16, 2005, was modified from one of my written prelims questions from early 2000.

EVOLUTIONARY PHYSIOLOGY OF BIOLOGICAL CLOCKS

Circadian clocks allow organisms to predict, instead of merely react to, cyclic (predictable) changes in the environment. A sentence similar to this one is the opening phrase of many a paper in the field of chronobiology. Besides becoming a truth by virtue of frequent repetition, such a statement appeals to common sense. It is difficult to imagine a universe in which it was not true. Yet, the data supporting the above statement are few and far-between. Believe it or not, the data are not always supporting it either.

This post will attempt to briefly review the literature on evolutionary and adaptive aspects of biological rhythmicity. Also, using the perspectives and the methodology of evolutionary physiology, I will try to suggest some ways to test the hypothesis stated in the first sentence above.

REASONING BEHIND THE ARGUMENT FROM COMMON SENSE

For outside observers of the field of chronobiology and its recent successes in molecular, neural and medical aspects of biological rhythmicity, it may come as a surprise that the field was founded by ecologists, ethologists and evolutionary biologists. When the statements about adaptive function of clocks were initially made, the authors were much more careful than is usually seen today. It was meant as a hypothesis to be tested, and elaborate reasoning was often offered to persuade the reader why it might be true (Daan 1981, Pittendrigh 1967,1993, Enright 1970).

One of the most common arguments that a clock must be adaptive (for one reason or another) was its ubiquity all plants, fungi, protista, invertebrates and vertebrates (more recently cyanobacteria, too) tested by the pioneers in the field showed circadian rhythmicity. The way those rhythms behaved in the laboratory in various experimental treatments was surprisingly similar over all species. Thus, the reasoning goes, if a physiological mechanism is found in every living thing, and it seems to work in the same way in all of them, then it must have originated early due to natural selection and was preserved over eons due to natural selection.

Some of the earliest experimental work was designed to test the genetic basis of biological rhythmicity. Many generations of laboratory organisms were raised and spent all their lives in aperiodic environments, yet the rhythms persist (Sheeba et al. 1999). Period of the rhythm was species -specific, highly heritable, and very amenable to artificial selection. So, if it is in the genes, the clock must have evolved due to some kind of selective pressure.

When reviewing evolutionary literature on biological rhythms, it is often difficult to distinguish between hypotheses of current utility from hypotheses of origin. It was often assumed that same selective pressures which keep the clocks ticking all over biosphere today, are the pressures responsible for the initial discovery of timing mechanisms by early forms of life.

The current adaptive functions of biological rhythms are often divided into two, mutually not exclusive categories. The Internal Synchronization hypothesis stresses the need for temporal separation of incompatible biochemical and physiological processes within a body (or cell), and for temporal synchronization of processes which need to coincide. An example of the former would be temporal separation of photosynthesis from nitrogen fixation. For the latter, surge of a hormone and availability of its receptor need to be synchronized for the generation of the endocrine effect. Evolution of such timing control mechanisms would presumably alleviate energetic costs of constant production of enzymes and their substrates.

Read the original:
Clock Evolution