Implications of intracolonial variation in a Paleozoic bryozoan

Abstract

Relative differences between environmentally controlled variation and genetically controlled variation are important when investigating morphologic variation in general, especially when establishing species concepts. The colonial nature of bryozoans provides a means for distinguishing between the two sources; variation can be partitioned into within-colony (microenvironmental) and among-colony (environmental + genetic) components. For the Paleozoic order Cryptostomata, biologically and taxonomically significant morphologic characters are well defined and methods for recognizing morphotaxa are well established.The importance of within-colony variation to the morphometric treatment of fenestrate species was assessed after the discovery of a large specimen of Hemitrypa sp. Variation within the colony was compared to variation among and within two congeneric species. The distribution of study segments across the colony allowed assessment of variation both along the growth axis and laterally between segments of approximately equivalent generational age. Repeatability of methods was assessed using data measured independently from identical segments by three workers.Variation within the large colony is less than variation among congeneric species, indicating that genetic differences among species exceed variation resulting from combined phenotypic and genotypic sources within species. Neither astogenetic nor ontogenetic morphologic gradients are recognized. Variation between data collected from identical segments by pairs of workers falls within the range of variation for the entire colony. Thus, multiple workers can reproduce data to the finest level of meaningful resolution. Cryptostome morphospecies concepts are validated.The potential for partitioning genotypic versus environmental variation in reduced, multidimensional morphospace is reinforced. Studies of microevolution and speciation may be designed that account for these factors.