Fig. 6

Hypothesized relationship of selection and modularity in the axial skeleton. The strength of selection may vary along the vertebral column (length of arrows, a, c), but the evolutionary response to that selection is modulated by integration patterns (shading on arrows). Four different evolutionary scenarios are hypothesized. a Modularity and gradational selection. Selection varies along the column and the selective regimes of different modules may diverge (stronger in the red zone than the yellow zone) due to limited integration between red and yellow modules. Such a scenario is proposed to explain the heterogenous vertebral column of mammals. Homogeneous vertebral columns may result from either increased integration, uniform selection or both. b Integration and gradational selection. Selection regimes vary along the column, but morphological variation is muted by strong integration between vertebrae. c Modularity with uniform selection. Although the potential for generating evolutionary variation between vertebrae exists, selection maintains uniformity along the column. d Integration with uniform selection. Both factors limit craniocaudal variation. Circles: vertebrae; shaded boxes (yellow, red, grey): modules; colored arrows: selective pressure; greyed-out arrows: selective pressure muted by serial integration