CONTRASTING THE UNDERLYING PATTERNS OF ACTIVE TRENDS IN MORPHOLOGIC EVOLUTION

Gastropod evolution during the early Paleozoic featured active trends (i.e., differential replacement of morphologies) for at least three sh ell characters. Selective sorting, either of individual organisms or o f whole species, is an obvious mechanism for such active trends. Sorti ng of individuals should result in a disproportionate number of ancest or to descendant transitions being in the same direction as the trend, whereas sorting of species should result in species with particular m orphologies producing more daughter species. Sorting of species can oc cur over long periods of time or it can be concentrated over a particu lar interval, such as an extinction event, Constraints on morphologic evolution also can drive trends. especially in cases where it is easie r to produce a particular morphology than it is to change it. Finally, active trends can be artifacts of unrelated differential diversificat ion within a clade (i.e., specie hitchhiking), which might result from sorting of species based on phylogenetically associated characters or simply by chance. Unlike other active trends, trends attributable to species hitchhiking do not support hypotheses about selection or evolu tionary constraints. From the latest Cambrian through the Silurian, ga stropods show significant increases in tile proportions of species wit h high shelf torques (congruent to high spire height). inclined apertu res, and narrow sinuses, Ancestor-descendant contrasts indicate a sign ificant bias towards daughter species with narrower sinuses than their ancestors. This is nor true for shell torque or apertural inclination . However, species with high shell torques appear constrained to produ ce daughter species with high shell torques. Analogous constraints do not exist for sinus width or apertural inclination. To separate the pr edictions of species hitchhiking from those of differential speciation and differential extinction, a bootstrapping procedure was used to si mulate the range of trends that phylogenetic effects alone could gener ate. The simulations indicate that the trend towards species with stro ngly inclined apertures is indistinguishable From a phylogenetic effec t, However, the simulations indicate that differential clade evolution affected the evolution of both shell torque and sinus width. The devi ations between observed and expected distributions an sudden and coinc ide with extinction events: species with low shell torques were signif icantly less likely to survive the Llandeilo/Caradoc (Middle Ordovicia n) turnover, and species with wide sinuses were significantly less lik ely to survive the end-Ordovician mass extinction. However, there is n ot a significant association between diversification and morphology fo r any of the characters studied here. These results suggest that selec tion on individual, evolutionary constraints, and differential survivo rship over extinction events all drove trends among early gastropods.