What are the ramifications to life on Earth when the Earth's magnetic poles switch?
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.