  The discovery of millisecond pulsations from neutron stars in low mass X-ray
binary (LMXB) systems has substantiated the theoretical prediction that links
millisecond radio pulsars (MSRPs) and LMXBs. Since then, the process that
produces millisecond radio pulsars from LMXBs, followed by spin-down due to
dipole radiation has been conceived as the 'standard evolution' of millisecond
pulsars. However, the question whether all the observed millisecond radio
pulsars could be produced by LMXBs has not been quantitatively addressed until
now.
  The standard evolutionary process produces millisecond pulsars with periods
(P) and spin-down rates (Pdot) that are not entirely independent. The possible
P-Pdot values that millisecond radio pulsars can attain are jointly
constrained. In order to test whether the observed millisecond radio pulsars
are the unequivocal descendants of millisecond X-ray pulsars (MSXP), we have
produced a probability map that represents the expected distribution of
millisecond radio pulsars for the standard model. We show with more than 95 %
confidence that the fastest spinning millisecond radio pulsars with high
magnetic fields, e.g. PSR B1937+21, cannot be produced by the observed
millisecond X-ray pulsars within the framework of the standard model.
