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 shell characters. Selective sorting, either of individual organisms or of whole species, is an obvious mechanism for such active trends. Sorting of individuals should result in a disproportionate number of ancestor to descendant transitions being in the same direction as the trend, whereas sorting of species should result in species with particular morphologies producing more daughter species. Sorting of species can occur over long periods of time or it can be concentrated over a particular interval, such as an extinction event. Constraints on morphologic evolution also can drive trends, especially in cases where it is easier to produce a particular morphology than it is to change it. Finally, active trends can be artifacts of unrelated differential diversification within a clade (i.e., species 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 evolutionary constraints.