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This question is related to this question about the cause of the Earth's magnetic field switching polarity.
My question is: How does this switch in the polarity of the magnetic field affect life on Earth? Will it have detrimental effects to living organisms?
While this does not directly answer the question, these two ideas are relevant: Rare Earth hypothesis and Mediocrity principle.
More focused to the question, This letter to nature discusses the purposed link between evolution/mutation rate during of life and magnetic pole reversal periods. You see, when the magnetic fields reversed, it widely thought that it occurs over a period on the order of 1 - 10,000 years. During this reversal, the intensity of the magnetic field degrades to 0, and then increases from there in a reversed polarity. It is not a contentious position that during these field reversals the mutation rate (not sure about speciation rate) dramatically increases.
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
