1. Life history theory generally predicts a trade-off between short juvenile development and large adult size, assuming invariant growth rates within species. This pivotal assumption has been explicitly tested in few organisms.
2. We studied ontogeny in 13 populations of Omocestus viridulus grasshoppers under common garden conditions. High-altitude populations, facing short growing seasons and thus seasonal time constraints, were found to grow at a similar rate to low altitude conspecifics.
3. Instead, high-altitude grasshoppers evolved faster development, and the correlated change in body size led to an altitudinal size cline mediating a trade-off with female fecundity.
4. An additional juvenile stage occurred in low- but not high-altitude females. This difference is probably due to the evolution of lowered critical size thresholds in high-altitude grasshoppers to accelerate development.
5. We found a strikingly lower growth rate in males than females that we interpret as the outcome of concurrent selection for protandry and small male size.
6. Within populations, large individuals developed faster than small individuals, suggesting within-population genetic variation in growth rates.
7. We provide evidence that different time constraints (seasonal, protandry selection) can lead to different evolutionary responses in intrinsic growth, and that correlations among ontogenetic traits within populations cannot generally be used to predict life history adaptation among populations. Moreover, our study illustrates that comparisons of ontogenetic patterns can shed light on the developmental basis underlying phenotypic evolution.