Rates of Evolution and Selection Intensity in Species Transitions Within the Ordovician Bryozoan Genus Peronopora

Joseph F. Pachut
Department of Geology, Indiana University-Purdue University at Indianapolis, 723 West Michigan Street, Indianapolis, IN 46202-5132,
Robert L. Anstey
Department of Geological Sciences, Michigan State University, 206 Natural Science Building, East Lansing, MI 48824-1115

Abstract.--Heritabilities, evolutionary rates, and selection intensities were calculated for 34 morphometric measurements across 211 specimens representing 14-16 species and metaspecies of the Ordovician bryozoan genus Peronopora. Cladistic branching pattern and stratigraphic position permitted an estimate of the average time of separation (42,640 yrs.) between all pairs of ancestor and descendant species. Statistically significant differences in estimated heritability existed between metaspecies and species across characters and within zooecial, acanthostyle, and wall character subsets. There is no genetic significance to heritability differences; they resulted from wider ecophenotypic responses in the ancestral metaspecies and reduced ecophenotypy in descendant species.
     Rates of between-species character evolution in Peronopora, in darwins (r_d), were similar to those near the lower limit of rates measured in living organisms, falling between 10^-3 and 10¹. In contrast, they are typically at least two orders of magnitude higher than rates measured by previous workers in 93 other fossil invertebrate taxa. Rates in haldanes (r_h), ranging between 10^-9 and 10^-3, overlap those measured in both living and fossil species. Selection intensities (i) were weak with values ranging between 10^-9 and 10^-5.
     Three indices evaluated the likelihood of random mutation and genetic drift producing observed changes between species. Lynch's (1990) delta values (all £ 10^-5) suggest that the species differences in Peronopora were developed so sluggishly that they reflect the action of stabilizing selection, with no support for mutation-drift or directional selection. Effective population sizes (Lande’s [1976] N*) for species of Peronopora, averaging 2 million across characters and 1.8 million to 310 million for character subsets, indicate the action of mutation-drift. Turelli et al.’s (1988) upper and lower confidence limits on the rate of selection [(s²_m*(U)/s)² and (s²_m*(L)/s)²] 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; stabilizing selection is indicated for 10% of characters and directional selection for 4%.
     Finally, the intensity of selection required (Lande’s [1976] b) and minimum mortality rates necessary to explain observed species transitions solely by directional selection have means of 5.5 standard deviation units from the average phenotype, and 17 selective deaths per million individuals per generation, respectively. These results confirm the action of very weak directional selection.
     A consensus of results indicates that mutation and random genetic drift acted as the primary evolutionary forces that operated on most characters with much smaller contributions from both stabilizing and directional selection. Therefore, the neutral model of phenotypic evolution is supported in the fossil record of speciation in Peronopora.