The genetic relationship between the daily circadian clock and the seasonal photoperiodic timer remains a subject of intense controversy. In Wyeomyia smithii, the critical photoperiod (an overt expression of the photoperiodic timer) evolves independently of the rhythmic response to the Nanda-Hamner protocol (an overt expression of the daily circadian clock) over a wide geographical range in North America. Herein, we focus on these two processes within a single local population in which there is a negative genetic correlation between them. We show that antagonistic selection against this genetic correlation rapidly breaks it down and, in fact, reverses its sign, showing that the genetic correlation is due primarily to linkage and not to pleiotropy. This rapid reversal of the genetic correlation within a small, single population means that it is difficult to argue that circadian rhythmicity forms the necessary, causal basis for the adaptive divergence of photoperiodic time measurement within populations or for the evolution of photoperiodic time measurement among populations over a broad geographical gradient of seasonal selection. Heredity (2012) 108, 473-479; doi:10.1038/hdy.2011.108; published online 9 November 2011Keywords: linkage; pleiotropy; photoperiodism; circadian rhythmicity; antagonistic selection INTRODUCTION Herein we are concerned with the genetic and evolutionary relationship between the photoperiodic timer that serves to organize seasonal events and the circadian clock that serves to organize daily events in the life histories of animals. 'Only model organisms live in a world of endless summer; most organisms in nature confront a seasonal environment. Fitness in a seasonal environment involves the abilities to exploit the favorable season, to avoid or mitigate the effects of the unfavorable season, and to make a timely transition between the two lifestyles' (Bradshaw et al., 2004). Timing is of the essence. Organisms cannot wait for the onset of winter but must have physiological mechanisms that enable them to prepare for the winter in advance. A wide variety of animals from rotifers to rodents use the length of the day (photoperiod) as an anticipatory cue in preparation for the changing seasons. Examples include the use of day length to cue the seasonal timing of migration in birds, to cue the seasonal timing of reproduction in mammals, and to cue the seasonal timing of diapause (dormancy) in arthropods (Bradshaw and Holzapfel, 2007a). Photoperiodic response is typically determined by exposing animals to a range of day lengths and plotting percent response as a function of hours of light per day to which they were exposed (Figure 1a). Photoperiodic response curves are usually sigmoid in shape and the day length that promotes a response in 50% of a sample population defines the critical photoperiod (hereafter, CPP) that is used as a proxy for the photoperiodic response curve.For more than 75 years, it has been hypothesized that the causal basis of the seasonal photoperiodic timer is controlled by ...