Mosaic evolution in the middle Miocene planktonic foraminifera Fohsella lineage

Abstract

AbstractRecent studies have shown that modes of evolution, namely directional trend, random walk, and stasis, vary across morphologic traits and over the geographic range of a taxon. If so, is it possible that our interpretation of evolutionary modes is actually driven by our selection of traits in a study? In an attempt to answer this question, we have restudied the middle Miocene planktonic foraminifera Fohsella lineage, an iconic example of gradual morphologic evolution. In contrast to previous studies that have focused on the gross morphology as embodied by the edge view of tests, we analyze here multiple phenotypic traits chosen because their biologic and ecologic significance is well understood in living populations. We find that traits in the lineage did not evolve in concert. The timing and geographic pattern of changes in shape, coiling direction, size, and ecology were different. The evolution of this lineage is a mosaic combination of different evolutionary modes for different traits. We suggest that overemphasis on the evolution of some single trait, such as the edge-view outline, from narrow geographic ranges has significantly underestimated the dynamic evolutionary history of this group.