What would have happened to to the evolutionary process had the Earth's axis of rotation had no tilt? What comes to me first would be a total lack of anything based on the year calendar. There would be no deciduous trees or dormant grasses, certain animal species would have no particular mating seasons (since there would be no seasons), and other animals would be prey to predatory life forms all year round. All species would have little tolerance for changes in temperature, so biological diversity would vary greatly with the latitude. The next question: would any place on the surface of the Earth be inhabitable if the tilt were 90 degrees?