In the biological sciences, we tend to measure comparative differences rather than absolute values. We customarily compare treated versus untreated, exposed versus unexposed, susceptible versus resistant. Yet biologists too often neglect one of the most fundamental differences, male versus female. This distinction is clearly important; the mere fact that it has been adopted by so many species and preserved over so long a time testifies to its evolutionary advantage. By providing a powerful mechanism of diversity, sexual recombination forms a broadened substitute upon which selection pressure can act.