Adaptive significance of a circadian clock: temporal segregation of activities reduces intrinsic competitive inferiority in Drosophila parasitoids

Most organisms show self-sustained circadian oscillations or biological clo cks which control their daily fluctuations in behavioural and physiological activities. While extensive progress has been made in understanding the mo lecular mechanisms of biological clocks, there have been few clear demonstr ations of the fitness value of endogenous rhythms. This study investigated the adaptive significance of circadian rhythms in a Drosophila parasitoid c ommunity. The activity rhythms of three sympatric Drosophila parasitoids ar e out of phase, the competitively inferior parasitoid species being active earlier than the superior competitor. This temporal segregation appears at least partiality determined by endogenous periods of the clock which also v ary between species and which correlate the time of activity. This earlier activity of the inferior competitor significantly reduces its intrinsic com petitive disadvantage when multiparasitism occurs, thus suggesting that nat ural selection acting on the phase of the rhythm could substantially deviat e the endogenous period from the optimal ca. 24h period. This study demonst rates that temporal segregation of competing species could be endogenously controlled, which undoubtedly favours their coexistence in nature and also shows how natural selection can act on biological clocks to shape daily act ivity patterns.