Studies reveal the evolutionary origin of unique traits in pitcher plants and marine snails

In a pair of studies, researchers use different approaches to investigate how complex and innovative phenotypic traits evolve in plants and animals. “The amazing breadth of plant and animal diversity across the globe has evolved by circuitous paths, and resolving the complex history of genomes and traits unlocks new depths for understanding evolution,” writes Kathryn Elmer in a related Perspective. Although biological traits are constantly changing in populations, the emergence of a trait that provides a unique function is a far rarer occurrence. These “game-changing” evolutionary innovations can result from multiple independent biological adaptations that, in combination, produce complex phenotypic traits. However, the origin of composite traits has remained largely a mystery as it requires coordinated evolution of components that might not be beneficial or even functional on their own or in disparate combinations.

Here, in two studies, Guillaume Chomicki and colleagues and Sean Stankowski and colleagues, respectively, use different approaches to investigate how complex composite traits can arise in unexpected ways – one in carnivorous pitcher plants and another in marine periwinkle snails. Using field experiments, microscopy, chemical analysis, laser Doppler vibrometry, and comparative phylogenetic analyses, Chomicki et al. examined the evolution of the so-called springboard trapping feature in two carnivorous pitcher plant species, Nepenthes gracilis and Nepenthes pervillei. This unique composite trapping mechanism results from three distinct structural, chemical, and mechanical innovations. Chomicki et al. discovered that the new trait evolved separately in both species of pitcher plant and suggest that this new trait evolved convergently through “spontaneous coincidence” of the required trait combination rather than directional selection in the component traits. In the other study, Stankowski et al. investigated the evolutionary origin of the recent transition from egg-laying to live-bearing offspring in a clade of marine snails (Littorina spp.). Using whole-genome sequences of otherwise indistinguishable live-bearing and egg-laying snails, the authors identified the genomic regions associated with each reproductive mode. They found that the genomic regions associated with egg laying or live birth have a different evolutionary history than does the genome overall. According to the findings, the genetics of live birth are not reflected in most of the genome despite the trait being under selection and fundamental to the species’ biology. According to Stankowski et al., these results suggest that new biological functions can evolve through the recruitment of many alleles rather than in a single evolutionary step. “There has been a long-standing debate about whether the accumulation of small stepwise changes or big leaps is more important in the evolution of diversity,” writes Elmer in the Perspective. “Chomicki et al. and Stankowski et al. did not identify a single big evolutionary step or large-impact mutation that moved the species to a new level of phenotypic innovation.”