It has been postulated that body size is determined by an interaction of natural and sexual selection. I quantified the selection pressures on body size in two island populations of marine iguanas. There was no predation and interspecific competition to confound the analysis. Natural selection acts through energy limitation, the strength of which differs between habitats and seasons. Sexual selection, on the other hand, always favors large body sizes, the effect being strongest in males.
Sexual selection acts through the higher mating success for larger males.
Large body size is the major determinant of territorial acquisition in the lek of marine iguanas, the first lek mating pattern described for reptiles. By knowing the quantitative amount of selection pressures and the mechanisms of selection, I can proximately explain body sizes in marine iguana island populations from just two environmental parameters, namely effective temperature and food supply. Wikelski, M. 1999. Evolution of body size in Galapagos marine iguanas. All Rights Reserved.