Fig. 2.

Graphical depiction of morphological evolution at the levels of (A) morphospecies, (B) lineage, and (C) biological species, for a hypothetical lineage. (A) Darker shading corresponds to morphotypes of higher abundance in morphospace for an evolving population. Dashed vertical lines represent typological divisions of the morphospace into discrete morphospecies. Based on assumptions i–iv, we identify the transitions between morphospecies 1 and 2, and between morphospecies 2 and 3, as potentially anagenetic, because the FAD and LAD values overlap within our error margins. Typological errors may broaden the estimated temporal overlap between ancestor and descendant, and must therefore also be accounted for. As the LAD of morphospecies 3 and FAD of morphospecies 4 do not overlap, and the two forms occupy distinguishable peaks in morphospace, they represent distinct morphospecies, implying that a cladogenetic event occurred. (B) All four morphospecies are assigned to the same lineage, despite morphospecies 3 and 4 having distinguishable frequency distributions in morphospace, because adjacent frequency distributions exhibit some overlap. Had there been unoccupied morphospace between morphospecies 3 and 4, this would have represented a cladogenetic event at the lineage level (24). (C) Selected “cryptic” biological species within morphospecies are represented as thin lines. Molecular evidence indicates that a morphospecies generally comprises multiple cryptic biological species (30). Consequently, the cladogenetic event at the morphospecies level may occur at the same time as the cladogenetic event at the biological species level or may occur later, as depicted in the figure.