REVISITING THE EVIDENCE FOR A PUNCTUATED MODE OF EVOLUTION IN METRARABDOTOS

Is speciation a ‘special time’ in morphological evolution or are lineage splitting events just ‘more of the same’ where the end product happens to be two separate lineages? Eldredge and Gould’s rereading of the fossil record, suggesting stasis dominates lineage evolution and that phenotypic change mainly happens during rapid speciation events, initiated some of the fiercest debates in the history of evolutionary biology. Although some of the most heated debates over punctuated equilibrium have subsided, the dust has yet to settle regarding the relationship between speciation and morphological evolution. Data on evolutionary dynamics during anagenetic and cladogenetic events within a clade are rare, but the fossil record of the bryozoan genus Metrarabdotos is considered a textbook example of a clade where speciation causes rapid evolutionary change against a backdrop of morphological stasis within lineages. We point to some measurement theoretical issues in the original work on Metrarabdotos and reanalyze a subset of the original data that can be meaningfully investigated using quantitative statistical approaches. Neither the amount of morphological evolution, the strength of selection or the directions traveled in multivariate morphospace are different when comparing evolution within lineages and at speciation events in Metrarabdotos. Although widely considered the best example of a punctuated mode of evolution, morphological divergence and speciation are not linked in Metrarabdotos.