The evolution of complexity: a multi-component weapon system in the superfamily of leaf-footed bugs

Sexually selected weapons, such as deer antlers and crab claws, are strikingly diverse across species, even among closely related species. Existing phylogenetic comparative studies have typically taken a broad, but simplified, view of weapons, examining gains and losses, shifts in types, or changes in location. What is lacking is how individual elements evolve to address questions such as whether complexity increases over time and whether certain sets of elements evolve convergently due to selection for coordinated functions during battle. We examined 15 elements of the highly complex weapon system in the Superfamily Coreoidea (Hemiptera: Heteroptera), which includes the charismatic leaf-footed bugs. Males often have spiny, enlarged hindlegs used to fight for territories and mating opportunities. We estimated a phylogeny for 248 species using ultraconserved elements, which we used to infer evolutionary transitions among character states and test correlations among weapon elements. Our results showed extreme lability in weapon elements. For example, we estimated projections off the ventral side of the hind femur were independently gained 18 times and subsequently lost 26 times. Overall, weapons showed patterns of increasing complexity over time, with some evidence of a cyclical evolutionary pattern (gains followed by losses and then gains again). Finally, we found intriguing patterns of trait co-occurrences revealing, in some cases, the evolutionary assembly of traits into highly specialized piercing or crushing weapons.