Anagenesis or Cladogenesis? New Evidence for Widespread Cladogenesis in Marine Microfossils

Oceanic microfossils have long provided a counter-example to the dominant mode of evolution observed in larger fossils from benthic marine and terrestrial environments. Where most species appear to speciate in geologically brief, cladogenetic events, microfossil species in groups like the planktonic foraminifera, have been repeatedly shown to arise through gradual anagenetic morphological change from an ancestor to a descendent morphology (without cladogenesis). Historically, marine plankton have been considered to be largely cosmopolitan with distinct phenotypes within different oceanic environments. However, phylogenetic studies over the past decade have revealed the presence of ubiquitous cryptic speciation in marine species; ecophenotypic morphospecies are often found to be genetic cryptic species clusters. While genetic techniques have revolutionized our perception of modern morphological species in the open ocean, the historic patterns of speciation have yet to be similarly reconsidered. We have reanalyzed a classic case of anagenetic evolution, the Globorotalia plesiotumida-G. tumida transition, using a more powerful technique --semi-landmark thin-plate splines. Using this technique we are able to separate sister species of planktonic foraminifera in morphospace for the first time. Notably, we find nuanced evidence for cladogenesis. Our current perception of evolutionary mode in marine microfossils may simply reflect the power of the techniques that we have historically used.