Biological Clocks at the End of the 20^th Century

Abstract

Biological systems regulate their internal environments as well as their relationships to the outside world. The regulatory mechanism usually involves negative feedback. This leads to the generation of oscillations because of delay in the feedback loops. Natural selection has often acted to dampen the amplitude of these oscillations but a few have been enhanced and their periods have been adjusted to match closely the periods of astronomical events that are of importance to living things (daily, tidal, lunar and annual cycles). They function as endogenous clocks built into organisms at the most fundamental level of gene transcription and translation. Best studied are the so-called circadian clocks with natural periods of about a day, normally synchronized to the day-night cycle, and used by their owners in an extraordinary variety of adaptations, many of which are discussed in this volume. In a little more than forty years, the progress has been made from superb observational natural history and ingenious behavioral experiments, to identification of clock components at organ, tissue, cellular and molecular levels, and a reasonable understanding of mechanism at each of these levels about how circadian (and other) clocks evolved and how they function in real-world ecological communities. The study of temporal programs and in particular those that generate clocks, will occupy an increasingly important place in biological research of the 21^st century.

Some of this material was presented at the Royal Society Meeting, The Measurement of Time, September 20–21, 2000.