Complex sentence: Severe bipolar disorder reduces a person's ability to function within society and has a high morbidity rate, therefore it would be expected by evolutionary theory that the genes responsible for bipolar disorder would have been naturally selected out, effectively culling the disorder.
Split into simpler sentences: Severe bipolar disorder reduces a person's ability to function within society and has a high morbidity rate. Therefore it would be expected by evolutionary theory that the genes responsible for bipolar disorder would have been naturally selected out, effectively culling the disorder.