Molecular phylogeny of arthropods and the significance of the Cambrian "explosion" for molecular systematics

Accurate phylogenetic reconstruction requires character systems that have e volved fast enough to have kept pace with cladogenesis but slowly enough to have conveyed the resulting phylogenetic signal to the present. Because st ratigraphic evidence suggests that basal arthropod lineages arose rapidly d uring an ancient (Cambrian) phylogenetic radiation, the discovery of molecu lar sequences capable of resolving arthropod phylogeny may be a significant challenge for molecular systematists. This challenge is exemplified by our attempt to resolve arthropod phylogeny using the amino acid sequence of el ongation factor-1 alpha (EF-1 alpha). Our fossil- based assessment of evolu tionary rates indicates that EF-1 alpha should be capable of resolving Camb rian-age divergences. However, phylogenetic analysis using EF-1 alpha fails to establish relationships among most higher-level groups, although it doe s recover more recently derived clades, Here we propose two models to expla in this incongruity, The Rapid Radiation Model maintains that fossil-based estimates of arthropod diversification are essentially accurate and that di versification occurred so rapidly during the Cambrian that few phylogenetic ally significant changes occurred in the slowly evolving EF-1 alpha sequenc e, The Enhanced Preservation Model maintains that fossil-based estimates of Cambrian-age divergences reflect enhanced preservation of pre-existing lin eages and that arthropod diversification occurred before the Cambrian, This model attributes lack of resolution to degradation of phylogenetic signal within EF-1 alpha, by subsequent evolution. Current evidence is more consis tent with the Enhanced Preservation Model, which implies that fossil-based methods can be very misleading when attempting to gauge the phylogenetic in formation content of molecular sequences for Cambrian- and Pre-cambrian age divergences.