RATES OF EVOLUTION AND SELECTION INTENSITY IN SPECIES TRANSITIONS WITHIN THE ORDOVICIAN BRYOZOAN GENUS PERONOPORA

Heritabilities, evolutionary rates, and selection intensities were calculated for 34 morphometric measurements across 211 specimens representing 14 species and metaspecies of the genus Peronopora. Cladistic branching pattern and stratigraphic position permitted estimates of the average time of branching (42,640 yrs.) between ancestor and descendant species pairs. Statistically significant differences in estimated heritability existed between species and metaspecies across characters and within several zooecial, acanthostyle, and wall character subsets.

The rate of character evolution, in darwins, was very low (10^-9 to 10^-5), considerably lower than those measured in living organisms but comparable to rates found in other fossil lineages. Rates in haldanes (10^-9 to 10^-3) overlap those measured in living and fossil species. Selection intensities were weak with values ranging between 10^-9 and 10^-5. Similar percentages of characters experienced negative vs. positive rates of evolution.

Three indices evaluate the likelihood of random mutation and genetic drift producing observed changes between species. Lande’s (1976) delta values (all £ 10^-5) suggest the action of stabilizing selection. Delta values for species of two genera of Neogene to Recent bryozoans were higher but indicated the action of mutation-drift and stabilizing selection. Effective population sizes for species of Peronopora, equaling 2 million across characters and 1.8 million to 310 million for character subsets, indicate the action of mutation-drift. Upper and lower confidence limits also suggest divergence by mutation-drift; individual and character subset means are less than the lower limit of 10^-2. Upper limit means for 86% of characters indicate mutation-drift with stabilizing selection suggested for 10% and directional selection for 4%.

The intensity of selection required and minimum mortality rates necessary to explain observed species transitions are 5.5 standard deviation units from the average phenotype and 17 selective deaths per million individuals per generation, respectively. This indicates very weak directional selection.

Mutation and random genetic drift appear to be the primary evolutionary forces operating on species of Peronopora with much smaller contributions from both stabilizing and directional selection.