Not all possible biological shapes are actually seen in nature. Despite much experimental work, the developmental basis explaining the presence or absence of certain biological shapes remains poorly understood. We studied
Drosophila melanogasterdevelopment using the sex comb, a group of modified bristles exhibiting spectacular morphological diversity among Drosophilaspecies. We provide several lines of evidence suggesting that increasing D. melanogastersex comb length produces a mechanical blockage, affecting comb shape and position. We infer that simple physical principles acting on tissues can influence the direction of evolution, and comparative studies of other fly species are consistent with this hypothesis. This work highlights the fundamental role of development for understanding biodiversity and evolution.