Sukuan Liu, Stacey D. Smith

The evolution of carnivorous pitcher traps across multiple angiosperm lineages represents a classic example of morphological convergence. Nevertheless, no comparative study to-date has examined pitcher evolution from a quantitative morphometric perspective. In the present study, we used comparative morphometric approaches to quantify the shape space occupied by Heliamphora pitchers and to trace evolutionary trajectories through this space to examine patterns of divergence and convergence within the genus. We also investigated pitcher development, and in particular, how the packing of pitchers is affected by crowding, a common condition in their natural environments. Our results showed that Heliamphora pitchers have diverged along three main axes in morphospace: pitcher curvature, nectar spoon elaboration, and pitcher stoutness. Both curvature and stoutness were correlated with pitcher size, suggesting structural constraints in pitcher morphological evolution. Among these four traits (curvature, spoon elaboration, stoutness and size), all but curvature lacked phylogenetic and showed marked convergence across the phylogeny. We also observed tighter packing of pitchers in crowded conditions, and this effect was most pronounced in curved, slender pitchers. Overall, our study demonstrates that diversification and convergent evolution of carnivory-related traits extends to finer evolutionary timescales, reinforcing the notion that ecological specialization may not necessarily be an evolutionary dead end.