Why so fast? The relative rate of morphological evolution of secondary sex traits versus host-associated traits in seed beetles (Chrysomelidae: Bruchinae)

Studying how rapidly characters evolve is important to understanding the relative forces that affect morphological variation and therefore those features that allow for the definition and diagnosis of species. Astragalus-feeding seed beetles (Chrysomelidae: Bruchinae) in the Acanthoscelides aureolus species group, for example, have diversified rapidly and taxonomic boundaries are very difficult to determine. This is in large part because these beetles display both rapidly evolving secondary sex characteristics (femoral spines and abdominal sensoria), and stabilized ecologically adaptive characters (tarsal pads). In this study, we characterize these traits morphometrically using Scanning Electron Microscopy and evaluate their relative rates of morphological evolution using a molecular phylogenetic analysis. We discuss the implications of these varying rates for our current understanding of taxonomic boundaries within this difficult group.