The most popular textbook example of punctuated evolution does not hold up to further scrutiny

Zoom Scanning electron micrographs of the seven species of Metrarabdotos analyzed in this study. All specimens are from the upper Miocene (about 7 Ma) of the Dominican Republic.

(Credit: Voje et al., © The University of Chicago) © University of Chicago Metrarabdotos

The fossil record is our only direct source of information on how evolution proceeds on timescales beyond a few centuries. How we analyze and interpret fossil data is therefore fundamental for our understanding of the processes that govern the evolution of life on Earth.

Stephen J. Gould and Niles Eldredge’s rereading of the fossil record – claiming species remain more or less unaltered during their existence with major and rapid evolutionary change happening during speciation events – started a heated debate within paleontology and evolutionary biology, which holds to this day. The bryozoan genus Metrarabdotos is generally considered the best example of punctuated evolution in the fossil record. Gould called Metrarabdotos “the most brilliantly persuasive, and most meticulously documented, example ever presented for predominant (in this case, exclusive) punctuated equilibrium in a full lineage” (Gould 2002, page 827).

Evolutionary biologists Kjetil L. Voje and Arthur Porto at the University of Oslo, together with bryozoologist and paleontologist Emanuela Di Martino at the Natural History Museum in Oslo, have reanalyzed the original Metrarabdotos data.

The authors point out critical methodological issues in the original work on Metrarabdotos. One issue is related to what is called measurement theory. For example, biologists use different scale types when they describe the morphology of a species. The length of a character in millimeters is a different scale type than the scoring of a character as present or absent. The scale type restricts what kind of statistical analyses and inferences that can be drawn from the numbers, but this principle was violated in much of the original work on Metrarabdotos.

The authors find no evidence for punctuated evolution within Metrarabdotos in their reanalysis of the data, when taking the methodological issues into account.

Abstract

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 measurement theoretical and methodological issues in the original work on Metrarabdotos. We then reanalyze a subset of the original data that can be meaningfully investigated using similar quantitative statistical approaches as in the original studies. We consistently fail in finding variation in the evolutionary process during within-lineage evolution compared to cladogenetic events: Neither the rates of 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, 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.