Several forms of selection motivate and in turn become amplified by the gestational phenomena described in part I. The chapters in part II address these evolutionary forces. Chapter 6 explains how natural selection can affect mammalian (including human) pregnancies; chapter 7 discusses how sexual selection emanates from and affects piscine pregnancies; and chapter 8 reexamines these topics from the vantage point of comparative evolution. The chapters in part II have the following overarching themes: (a) various forms of pregnancy represent extreme expressions of gender-biased parental care that in effect amplify the inherent intersexual (male versus female) tensions that originated millions of millennia ago during the evolutionary emergence of anisogamy and sexual reproduction; (b) pregnancy (like many other aspects of ontogeny) entails compromises and trade-offs between opposing selective forces; and (c) pregnancy from an evolutionary perspective is as much about conflict as it is about unbridled cooperation among mother, child, father, and copies of their respective genes.