Evidence for abrupt speciation in a classic case of gradual evolution.

Abstract

In contrast with speciation in terrestrial organisms, marine plankton frequently display gradual morphological change without lineage division (e.g., phyletic gradualism or gradual evolution), which has raised the possibility that a different mode of evolution dominates within pelagic environments. Here, we reexamine a classic case of putative gradual evolution within the Globorotalia plesiotumida-G. tumida lineage of planktonic foraminifera, and find both compelling evidence for the existence of a third cryptic species during the speciation event and the abrupt evolution of the descendant G. tumida. The third morphotype, not recognized in previous analyses, differs in shape and coiling direction from its ancestor, G. plesiotumida. This species dominates the globorotaliid population for 414,000 years just before the appearance of G. tumida. The first population of the descendant, G. tumida, evolves abruptly within a 44,000-year interval. A combination of morphological data and biostratigraphic evidence suggests that G. tumida evolved by cladogenesis. Our findings provide an unexpected twist on one of the best-documented cases of within-lineage phyletic gradualism and, in doing so, revisit the limitations and promise of the study of speciation in the fossil record.