Assessing canalisation of intraspecific variation on a macroevolutionary scale: the case of crinoid arms through the Phanerozoic

Abstract

Clades that represent a new ‘Bauplan’ have been hypothesised to exhibit more variability than more derived clades. Accordingly, there is an expectation of greater variation around the time of the origin of a clade than later in its evolutionary history. This ‘canalisation’ has been tested in terms of morphological disparity (interspecific variation), whereas intraspecific variation in macroevolution is rarely studied. We analysed extensive data of brachial counts in crinoid populations from the Ordovician to the Recent to test for canalisation in morphological intraspecific variation. Our results show no support for the canalisation hypothesis through the Phanerozoic. This lack of pattern is maintained even when considering crinoid subclades separately. Our study is an example of the lack of universality in such macroevolutionary patterns both in terms of organisms and in terms of modules within them. It is also an example on the challenges and limitations of palaeontological studies of macroevolutionary processes.