Phylogeny reconstruction and the tempo of speciation in cheilostome Bryozoa

Abstract

We compared phylogenies derived from morphological data for two cheilostome bryozoan genera,StylopomaandMetrarabdotos,with genetic differences between species (Stylopoma) and the stratigraphic occurrence of fossils (both genera). Correspondence between species ofStylopomadefined by protein electrophoresis and on preservable skeletal morphology is excellent, despite great morphological variability within colonies and the predominance of quantitative over discrete characters. Moreover, agreement between genetic and morphological classifications increased greatly when morphological discrimination was pushed to the limit, despite inability to consistently assign all specimens to species with high confidence. This “splitting” strategy also maximized the correlation between genetic distances and the distances between species in cladistically derived phylogenies.Fossil and living species of both genera are sufficiently abundant and widespread to provide credible limits for inferred ancestral relationships. Inclusion of fossils in cladistic analyses ofStylopomaincreased the consistency of cladistic hypotheses by up to 30% and provided a more effective means of rooting trees than comparison with living species of the most closely related genus (“outgroup”). Moreover, in the case ofMetrarabdotos,failure to incorporate stratigraphic information turned the cladogram virtually upside down, so that postulated ancestors first appear in the fossil record 6–16 m.y. after their putative descendants became extinct.Stratigraphically rooted trees suggest that most well-sampledMetrarabdotosandStylopomaspecies originated fully differentiated morphologically and persisted unchanged for > 1 to > 16 m.y., typically alongside their putative ancestors. Moreover, the tight correlation between phenetic, cladistic, and genetic distances among livingStylopomaspecies suggests that changes in all three variables occurred together during speciation. All of these observations support the punctuated equilibrium model of speciation.