Segregation and speciation in the Neogene planktonic foraminiferal clade Globoconella

The origin of the Neogene planktonic foraminifer Globorotalia (Globoconella ) pliozea in the subtropical southwest Pacific has been attributed to its i solation resulting from intensification of the Subtropical Divergence (Tasm an Front). Oxygen isotopic analyses suggest that, although the Subtropical Divergence may have played a role, the evolution of Gr. (G.) pliozea was fa cilitated by depth segregation of Gr: (G.) conomiozea morphotypes (low and high conical) during an interval of near-surface warming and increasing the rmal gradient. Oxygen isotopic analyses suggest that low conical morphotype s of Gr. (G.) conomiozea inhabited greater depths than high conical morphot ypes. Low conical forms of Gr. (G.) conomiozea are considered ancestral to the low conical species, Gr. (G.) pliozea. Oxygen isotopes indicate that Gr . (G.) pliozea inhabited greater depths than its ancestor, Gr. (G.) conomio zea. These data are consistent with depth-parapatric and depth-allopatric models , but not with a sympatric model of speciation. In the allopatric model, re production at different water depths acts as a barrier between morphotypes. In the parapatric model, clinal variation along a depth gradient acts as a barrier between morphotypes living at the limits of the gradient. Depth se gregation in both models results in genetic isolation and evolutionary dive rgence. Our data support a correlation between morphological evolution and habitat changes in the Globoconella clade, implying separation of populatio ns as a driving force for morphological evolution. Ecological segregation of morphotypes and species map be related to morphol ogy (height of the conical angle), based on the data from Gr. (G.) conomioz ea and Gr. (G.) pliozea. However, morphological differences alone do not ne cessarily produce depth differences. Large morphological differences betwee n Gr. (G.) pliozea and closely related Gr. (G.) puncticulata did not result in isotopic and therefore depth differences between these species. These s pecies coexisted at the same water depths for nearly 1 m.y. Thus, it is unl ikely that the extinction of Gr. (G.) pliozea in the middle Pliocene result ed from competition with Gr. (G.) puncticulata, as previously suggested.