EVOLUTION IN ORGANISMS THAT CHANGE THEIR NICHES DURING THE LIFE-CYCLE

Many organisms pass through a broad size range as they develop from independent juveniles to mature adults, and as a result most of them change their use of resources (their niches) during the life cycle. The different niches of the life stages often require different body plans to be effectively utilized. resulting in different selection pressures on each stage. However, in organisms with simple life cycles only a relatively restricted set of life-cycle phenotypes may be realized because genetic correlations between juvenile and adult morphology may constrain evolution. Such genetic correlations thus result in trade-offs between efficiencies of resource utilization of juveniles and adults. The evolutionary response to such trade-offs is examined with the help of evolutionarily stable strategy (ESS) models. Ultimately, the genetic correlations themselves may be modified by selection. A disruption of genetic correlations permits the life stages to respond independently to the different selection pressures they experience. It is argued that, when the niches of the life stages differ greatly, the most probable way to break up the ontogenetic covariance structure and hence the trade-offs in performance between the life stages is through evolution of complex life cycles (metamorphosis).