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Super-species in the calcareous plankton
de Vargas, C.; Sáez, A.G.; Medlin, L.K.; Thierstein, H.R. (2004). Super-species in the calcareous plankton, in: Thierstein, H.R. et al. Coccolithophores: from molecular processes to global impact. pp. 271-298. http://dx.doi.org/10.1007/978-3-662-06278-4_11
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| Auteurs | | Top |
- de Vargas, C.
- Sáez, A.G.
- Medlin, L.K., meer
- Thierstein, H.R.
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| Abstract |
The most successful groups of pelagic protists in the modern Ocean have evolved convergent phenotypic traits, including the presence of hard skeletons enclosing the cell. These micro-skeletons – tests, coccoliths, frustules, theca – have accumulated kilometers of deep-sea sediments since the Jurassic, the most complete and continuous fossil record widely used for reconstructing Earth systems dynamics and microbial evolution. The use of the traditional morphological species concepts in those groups indicates that the relatively few species living at a given time have huge, often circum-global biogeographic distributions, and commonly last for many million years in the sediment record, which contrasts with the hectic biological pace of life occurring in the oceanic water masses, leading to one of the highest organismic turnover that any ecosystem sustains. Here we review all recent genetic data on coccolithophore and foraminifer biodiversity. In both groups, the sequencing of various genes shows that the morphological ‘species’ are in fact monophyletic assemblages of sibling species which diverged several million years ago according to molecular clock calculations. Furthermore the sibling species within a morphological entity may systematically occupy restricted geographic or temporal allopatric subdivisions of the total ecological range attributed to the traditional morphospecies. They display also stable and subtle – despite million years of genetic isolation – morphological differences that have been previously overlooked or interpreted as ecophenotypic variations. Obviously, various selective forces related to life in the marine planktic realm impose a strong stabilizing selection on pelagic organisms that maintains “optimal” phenotypes through the origination and possibly extinction of sibling species. We propose that this mode of evolution is characteristic of most marine planktic taxa, including metazoans, and we introduce a concept of ‘planktic super-species’ to describe these constrained morphological monophyletic entities that include several sibling species adapted to different ecological niches. Two different evolutionary models displaying different degrees of complexity in the spatio-temporal disconnection between morphological and genetic/ecologic differentiations are discussed in the frame of the existing morphometric and DNA data sets. The design of experimental protocols at the boundary between molecular phylogenetics and micropa-leontology will be a necessary condition to test which of our models reflect the real world. This will be also a crucial step to reveal the full potential of microfossil applications in paleoecology and stratigraphy, and to understand, at the level at which adaptation and selection operate, how pelagic biodiversity reacted to climatic changes in the past oceans and how it may react to the severe warming events projected in the near future. |
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