GALIS, F.; STEIGENGA, M.; RUINARD, S.; Leiden University, The Netherlands: THE CONSTANCY OF CERVICAL VERTEBRAL NUMBER IN MAMMALS: EVIDENCE FOR STRONG STABILIZING INTERNAL SELECTION

Mammals have a remarkably constant number of 7 cervical vertebrae, unlike the number of vertebrae of other regions which have a variable number. We proposed a hypothesis some years ago that negative pleiotropic effects (including cancer) are the indirect cause of the strong evolutionary constraint on cervical vertebrae. To further test this hypothesis we investigated human fetal deaths and determined the association between abnormalities in cervical vertebral number and congenital abnormalities. Furthermore we investigated the association between congenital abnormalities and the number of thoracic vertebrae. The results show strong support for our hypothesis. The relevance for the role of modularity in evolutionary conservation is discussed.