Body size is a central trait of organisms, as nearly all traits of organisms scale with it.
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.