EVOLUTIONARY CHANGE IN THE MORPHOLOGICAL COMPLEXITY OF THE MAMMALIAN VERTEBRAL COLUMN.

The notion that morphological complexity increases in evolution is widely accepted in biology and paleontology. Several possible explanations have been offered for this trend, among them the suggestion that it has an active forcing mechanism, such as natural selection or the second law of thermodynamics. No such mechanism has yet been empirically demonstrated, but testing is possible: if a forcing mechanism has operated, the expectation is that complexity would have increased in evolutionary lineages more frequently than it decreased. However, a quantitative analysis of changes in the complexity of the vertebral column in a random sample of mammalian lineages reveals a nearly equal number of increases and decreases. This finding raises the possibility that no forcing mechanism exists, or at least that it may not be as powerful or pervasive as has been assumed. The finding also highlights the need for more empirical tests.