Two genes control seasonal isolation in sibling species.

Interspecific hybridization tests between Chrysopa carnea and Chrysopa downesi show that single allele differences at two unlinked autosomal loci cause large differences in photoperiodic responses. These differences produce asynchronous seasonal reproductive cycles, thus forming an effective temporal reproductive barrier between the two sympatric species. The results subserve the development of a genetic model for allochronic speciation.