Higher ribosomal RNA substitution rates in Bacillariophyceae and Dasycladales than in Mollusca, Echinodermata, and Actinistia-Tetrapoda.

Molecular evolutionary rates within two protistan and three metazoan taxa were estimated using divergence times derived from fossil records. The results indicate that the small-subunit rRNA sequences within Dasycladales (Chlorophyta) and Bacillariophyceae evolved at a rate approximately two to three times faster than that estimated within Echinodermata, Mollusca, and Actinistia-Tetrapoda. It was concluded that this twofold discrepancy demonstrates actual taxonomic differences in the fixation rate of mutations in the small-subunit rRNA.