New publication: Revisiting a Landmark Study System: No Evidence for a Punctuated Mode of Evolution in Metrarabdotos
By Kjetil L. Voje, Emanuela Di Martino and Arthur Porto in The American Naturalist. Open Access.
Is speciation generally 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? Data on evolutionary dynamics during anagenetic and cladogenetic events among closely related lineages 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. Here, we point to some methodological and measurement theoretical issues in the original work on Metrarabdotos. We then reanalyze a subset of the original data that can be meaningfully investigated using quantitative statistical approaches similar to those used in the original studies. We consistently fail to find variation in the evolutionary process during within-lineage evolution compared with cladogenetic events: the rates of evolution, the strength of selection, and the directions traveled in multivariate morphospace are not different when comparing evolution within lineages and at speciation events in Metrarabdotos, and genetic drift cannot be excluded as a sufficient explanation for the morphological differentiation within lineages and during speciation. Although widely considered the best example of a punctuated mode of evolution, morphological divergence and speciation are not linked in Metrarabdotos.
Published online: March 17th, 2020
* Centre for Ecological and Evolutionary Synthesis (CEES), Department of Biosciences, University of Oslo, Oslo, Norway
§ Natural History Museum, University of Oslo, Blindern, PO Box 1172, Oslo 0318, Norway