ADAPTATION AND CONSTRAINT IN THE COMPLEX LIFE-CYCLES OF ANIMALS

Life cycles that incorporate discrete, morphologically distinct phases predominate among animals. One explanation for the abundance and long -term persistence of complex life cycles is that they represent adapti ve mechanisms for decoupling the developmental processes that underlie morphological traits of alternative phases, thereby allowing phases t o respond independently to different selective forces. Another explana tion is that complex life cycles persist due to developmental constrai nts acting on particular phases; in particular, larvae may represent t he conservation of traits of early development while adult traits evol ve more freely in response to selection. A role of adaptive decoupling is generally supported by comparative data on the relative extent of morphological evolution of larvae and adults and on the frequency of e limination of phases from complex life cycles. Adaptive decoupling cou ld result from developmental compartmentalization, and within such dev elopmental systems, heterochrony could be a route to the deletion of a life cycle phase. The extent to which complex life cycles represent a daptive mechanisms for severing developmental linkages could be elucid ated both by quantitative genetic studies comparing levels of genetic correlation among phase-specific traits and by molecular developmental studies of gene expression in alternative phases.