MOLECULAR VERSUS TAXONOMIC RATES OF EVOLUTION IN PLANKTONIC-FORAMINIFERA

Neogene planktonic foraminifera are among the most widely used microfo ssils in the study of tempo and mode of evolution. Comparisons of taxo nomic rates between the two major clades in this group have shown that the nonspinose globorotaliids have undergone a significantly more rap id evolutionary turnover than the spinose globigerinids (S. M. Stanley et al., 1988, Paleobiology 14, 235-249). In order to test if similar fluctuations are observed in molecular data, we have used different me thods to calculate absolute and relative rates of substitutions based on 16 partial SSU rDNA sequences from representatives of both groups, According to our data, rates of substitution are relatively constant w ithin the globigerinids with a mean value of 4.3 subst./site/10(9) yea rs, but vary in the globorotaliid clade with three species having a ra te of about 1 subst./site/10(9) years and two species evolving much fa ster with rates of more than 7 subst./site/10(9) years. Assuming that the fast rates result from recent accelerations, the globorotaliids ha ve basically much slower molecular evolutionary rates than the globige rinids, in opposition to the fossil data. (C) 1998 Academic Press.