734 THE STRUCTURE OF EVOLUTIONARY THEORY
8 - 8. Top: Clear speciational trends in Lower Paleozoic gastropods towards higher spires (A—
measured as shell torque), more inclined apertures (B) and narrower sinuses (C). The bottom
diagram demonstrates that the first two trends obey Wright's Rule in showing no bias in species
origins in the direction of the trend. However, the trend for sinus width does show a bias for new
species to originate in the direction of narrower sinuses—thus yielding a complex trend, partly
produced by directional speciation, and not entirely by species selection. From Wagner, 1996.