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theses

Morphologic variability is a major theme in microevolutionary studies. Variations provide the raw material from which opportunities for selection are generated, leading to speciation. Three modes of evolution have been described from the paleontological record— Gradualism (gradual evolution implying continuous changes over a long time span), Punctuated Equilibrium (stable morphologies evolving through abrupt jumps), and Punctuated Anagenesis (gradual shifts between stable morphologies). There are only a handful of papers that illustrate unambiguously these models, and none involved the coccolithophores. The latter are, however, an ideal taxonomic group for evolutionary studies, with rapid generational turnovers, high morphological diversity and abundance, and long fossil record. I present three tests in this dissertation. In the first study, I test the significance of morphological variability in the species Ellipsolithus macellus in terms of genetic versus phenotypic expression, represented by, respectively, cryptic speciation versus adaptation to oligotrophication of the early Paleocene (Danian) Ocean. In the second study, I test for gradualism versus stasis in the short lived (370 kyr) early Eocene species Heliodiscoaster mahmoudii, and show that morphological fixation was achieved after 83 kyr (20% of the range of the species). In the third study, I first follow up on the previous study to confirm morphologic fixation in two other species that are part of the early Eocene Tribrachiatus lineage, T. bramlettei being ancestral to T. contortus. I then test for abrupt versus gradual morphological change during the evolutionary divergence of T. contortus from T. bramlettei. I show that speciation in this lineage follows the model of Punctuated Gradualism, or Punctuated Anagenesis. The T. bramlettei-T. contortus transition occurs over 73 kyr during which high morphologic variability occurs and after which the T. contortus morphology is fixated. Prior to the transition the T. bramlettei morphology was also fixated. This thesis unambiguously illustrates examples of Punctuated Anagenesis in the Coccolithophores. Stasis (low morphologic variability) is the primary mode that characterizes species during most of their life span. Speciation events are short episodes of high morphologic variability (25% higher than during stasis) during which morphologic fixation of the new species is progressively achieved.

ETD_5101

Ph.D.

Includes bibliographical references

by David Bord

doi:10.7282/T3639MRQ

ETD doctoral

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