Circadian clocks shape how organisms schedule their physiology and behaviours across the time of day. While the underlying molecular basis for clocks is now extensively investigated, we know little about how evolutionary forces shape waveforms. Using a laboratory selection approach, we have created populations of fruitflies Drosophila melanogaster which have now diverged dramatically in their time of emergence from pupal cases 'eclosion'. These unique populations have allowed us to discover that in the process of adapting to new temporal niches circadian clock organization is altered in multiple ways including changes in the splicing machinery for core circadian clock genes. I will present some of our recent findings in this direction.