Cladogenetic and anagenetic changes in the morphology of Rhizosolenia praebergonii Mukhina

Abstract

Biometric analysis of a late Pliocene planktic diatom morphospecies, Rhizosolenia praebergonii Mukhina, from the equatorial Pacific Ocean indicates that it represents a lineage which underwent relatively rapid change in morphology after its first appearance (3–1–2–7 Ma) followed by a period of time where the rate (as expressed as a slope) of morphological change was not significantly different from zero (2–7–1–7 Ma). Based on criteria for identifying ancestral‐descendant relationships (Prothero and Lazarus, 1980), we conclude that R. praebergonii evolved from R. bergonii Peragallo via a cladogenetic event. The observed pattern of evolution conforms to the shifting balance model of evolution, rather than to a punctuated model. Isotopic evidence suggests that paleoclimatic events may have played a role in the onset of the period of stasis.

Key words:

• diatoms
• ancestral‐descendant relationships
• evolutionary patterns
• morphological rates of evolution
• shifting balance theory
• punctuated equilibrium

Notes

Corresponding author: Ulf Sorhannus, Biology Department, Queens College, 65–30 Kissena Boulevard, Rushing, New York 11367.