Tempo and mode of morphological evolution in three neogene diatom lineages

Two late Pliocene planktic diatom morphotypes, Rhizosolenia praebergonii Mukhina and Rhizosolenia sigmoida Sorhannus, originated from Rhizosolenia bergonii Peragallo in the equatorial Pacific Ocean, after which R. praebergonii may have migrated into the Indian Ocean. An alternative interpretation is that R. praebergonii originated independently from R. bergonii in the Indian Ocean after which it underwent parallel evolution. The evolution of R. sigmoida was restricted to the eastern equatorial Pacific Ocean. Biotic factors in conjunction with paleoclimatic events may have played a role in the onset of the morphological divergence events between the lineages. A biometric analysis of the patterns of evolution indicates that both R. praebergonii and R. sigmoida underwent relatively rapid morphological change after their first appearances in the Pacific Ocean while R. bergonii evolved at a slower rate throughout the entire time period studied. As established morphotypes, which are morphologically distinct forms at a particular time level, R. praebergonii and R. sigmoida exhibit a slow down in the rate of morphological change relative to the time period immediately after their first appearance. In the Indian Ocean the change in morphology of R. praebergonii proceeded much slower after its first appearance in relation to the Pacific Ocean cores. The morphological evolution of R. praebergonii in the Peru water mass seems to have been iterative (repetitive) with respect to R. sigmoida.; Based on fitting a set of hierarchical linear models both R. praebergonii and R. sigmoida originated from R. bergonii in the Pacific Ocean through phyletic change in morphology possibly taking place at varying rates. To the contrary, if the first appearance of R. praebergonii in the Indian Ocean is a result of evolution as opposed to migration, it may be a case of punctuated character stasis which may reflect a speciation event. The relative frequency of character stasis is rather high in R. bergonii but also in R. praebergonii and R. sigmoida as they became established as distinct lineages in all the investigated sites. These observations are in agreement with the predictions of punctuated equilibrium hypothesis if the characters reflect species stasis.