Molecular phylogeny of arthropods and the significance of the cambrian "Explosion" for molecular systematics

Abstract

Accurate phylogcnetic reconstruction requires character systems that have evolved fast enough to have kept pace with cladogenesis but slowly enough to have conveyed the resulting phylogenetic signal to the present. Because stratigraphic evidence suggests that basal arthropod lineages arose rapidly during an ancient (Cambrian) phylogenetic radiation, the discovery of molecular sequences capable of resolving arthropod phytogeny may be a significant challenge for molecular systematists. This challenge is exemplified by our attempt to resolve arthropod phytogeny using the amino acid sequence of elongation factor-la (EF-la). Our fossil-based assessment of evolutionary rates indicates that EF-1α should be capable of resolving Cambrian-age divergences. However, phylogenetic analysis using EF-1α fails to establish relationships among most higher-level groups, although it does recover more recently derived clades. Here we propose two models to explain this incongruity. The Rapid Radiation Model maintains that fossil-based estimates of arthropod diversification are essentially accurate and that diversification occurred so rapidly during the Cambrian that few phylogenetically significant changes occurred in the slowly evolving EF-1α sequence. The Enhanced Preservation Model maintains that fossil-based estimates of Cambrian-age divergences reflect enhanced preservation of pre-existing lineages and that arthropod diversification occurred before the Cambrian. This model attributes lack of resolution to degradation of phylogenetic signal within EF-1α by subsequent evolution. Current evidence is more consistent with the Enhanced Preservation Model, which implies that fossil-based methods can be very misleading when attempting to gauge the phylogenetic information content of molecular sequences for Cambrian- and Precambrian-age divergences.