The article "Evaluating the Adaptive Fitness of Circadian Clocks and their Evolution" by Maria Luísa Jabbur, Chitrang Dani, Kamil Spoelstra, Antony N. Dodd, and Carl Hirschie Johnson explores the adaptive significance of circadian clocks and the criteria for evaluating their fitness. The authors discuss the concept of "fitness" and "adaptation," emphasizing that while circadian clocks are widespread, their adaptive value is not guaranteed. They highlight the importance of phase angle, the relationship between the biological clock and environmental cycles, in determining fitness. The article reviews various methods to assess the adaptive fitness of circadian clocks, including latitudinal clines, survival and longevity, growth and developmental rates, and fecundity. It also introduces the competition assay as a rigorous test of adaptive fitness, using cyanobacteria, plants, and mice as models. The authors conclude that while circadian clocks may enhance fitness in certain contexts, their adaptive value is complex and influenced by various environmental and genetic factors.The article "Evaluating the Adaptive Fitness of Circadian Clocks and their Evolution" by Maria Luísa Jabbur, Chitrang Dani, Kamil Spoelstra, Antony N. Dodd, and Carl Hirschie Johnson explores the adaptive significance of circadian clocks and the criteria for evaluating their fitness. The authors discuss the concept of "fitness" and "adaptation," emphasizing that while circadian clocks are widespread, their adaptive value is not guaranteed. They highlight the importance of phase angle, the relationship between the biological clock and environmental cycles, in determining fitness. The article reviews various methods to assess the adaptive fitness of circadian clocks, including latitudinal clines, survival and longevity, growth and developmental rates, and fecundity. It also introduces the competition assay as a rigorous test of adaptive fitness, using cyanobacteria, plants, and mice as models. The authors conclude that while circadian clocks may enhance fitness in certain contexts, their adaptive value is complex and influenced by various environmental and genetic factors.