Rates of anagenetic evolution and selection intensity in Middle and Upper Ordovician species of the bryozoan genusPeronopora

Abstract

Evolutionary rates and selection intensities in eight cladistically defined species-level evolutionary sequences of the Middle and Upper Ordovician bryozoan genusPeronoporawere calculated for comparison with values published for fossil and living taxa. Calculations were restricted to statistically significant unidirectional segments of anagenetic series to minimize the mixing of different modes, directions, and rates of evolutionary change.Rates and selection intensities ranged from 10−7to 10−6darwins and from 10−6to 10−5haldanes. Across characters, the weighted mean evolutionary rate equaled 5.86 × 10−7darwins and the mean selection intensity was 6.44 × 10−7. Mean rates of 2.15 × 10−6, 4.31 × 10−6, and 8.61 × 10−6haldanes, and corresponding mean selection intensities equaling 2.39 × 10−6, 4.78 × 10−6, and 9.56 × 10−6, were calculated for generation lengths of 0.5, 1, and 2 years, respectively.The magnitudes of positive and negative evolutionary rates and selection intensities do not differ statistically, individual characters display no consistent pattern of positive or negative values, and no character complexes were detectable. A mosaic pattern of change occurs across characters in evolutionary sequences.Eighty percent of analyzed evolutionary series were multispecies lineages. Both individual and mean values provide direct estimates of the rates of evolution within those lineages at the moment of speciation.Rates of anagenetic evolution inPeronoporawere low and similar to published rates for a variety of fossil protists, invertebrates, and vertebrates. However, earlier rate calculations did not isolate the effect of unidirectional anagenesis from that of stasis, random walks, trend reversals, or rate variations. Eight percent of characters inPeronoporaproduced anagenetic series that were statistically significant, a percentage similar to the 5% calculated in a study of 251 sequences of evolving traits in 53 fossil lineages (Hunt 2007). Stasis and mutation-drift are the most common patterns detectable in the fossil record, although anagenesis remains a potentially important force in shaping the course of both micro- and macroevolution.