Size Related Bias in planktonic Foraminifera

The second mechanism tested by Arnold et al. (1995) was that new species were founded at a small size and due to minimum size constraints, it was necessary for them to diversify towards a larger size (Arnold et al., 1995). The reasoning behind this hypothesis was that since smaller species were less specialized they had more structural flexibility and were able to branch towards new lineages (Arnold et al., 1995). Since it is difficult to measure the structural complexity of an specimen, Arnold et al. (1995) predicted

Globigerinita uvula (image by Natalia Vazquez Riveros)
that if this hypothesis were the mechanism behind Cope’s rule that speciation rates of smaller species should be much higher than those for larger species. In the data for the foraminifera, there was no difference found between the origination rates in large and small species (Arnold et al. 1995). However, the authors did find that the foraminifera of the Cenozoic era were likely founded by small founders (Arnold et al., 1995).