Survival, growth, and fecundity are considered as the three main components of the life cycle of any living organism. The relative contribution of these three components to average fitness may vary drastically among populations of single species. Examining this interpopulation variation and understanding its causes can provide insight on the particular selection pressures that drive phenotypic divergence among populations. We conducted a demographic study of eight distinct populations of the viviparous lizard Sceloporus grammicus in central Mexico. We estimated variation among populations in stage-specific survival, growth, and fecundity. Using these data we constructed site-specific population projection matrices to estimate population growth rates, which we interpreted as measures of the average fitness of each population. Elasticity analysis was used to calculate the relative contribution of the three life-cycle components to population growth rates. The three life-cycle components had relatively high elasticity values, which meant that survival, growth, and fecundity contributed similarly to fitness. However, some variation was found among populations. We searched for potential associations between the observed interpopulation variation in these measures of relative importance for average fitness and interpopulation variation in temperature, rainfall, population density, microhabitat availability, degree of human-induced disturbance, and overall mortality. None of these environmental factors or their interactions could explain the observed interpopulation variation in the relative importance of the life-cycle components. Our results provide insight about how these viviparous lizards allocate resources to survival, growth, and fecundity in different environments.