As far as I know there are no extinction phases that have been connected to a magnetic reversal. From this one can argue that there is no or only minor changes in the amount of radiation that reaches the Earth's surface. Life in the oceans and underwater anyway has an extra protective layer, i.e. water (but also see this as a grain of salt because most extinction events are recorded in the oceans, which are anyway protected).
There are also some physics-models that say that a reversal is most likely not harmful. And migratory animals with magnetic sensors will adapt to the changes by simply migrating with their parents (The changes are so slow that the animals just slightly correct their compass bearing each generation)
Source:
http://www.geomag.bgs.ac.uk/education/reversals.html#6