Chapter 10: The Evolution of Behavior

Sapolsky examines how evolutionary forces sculpt not just anatomy and physiology but also behavior. Sapolsky begins with Evolution 101, clarifying misconceptions: evolution favors reproduction, not survival; it doesn’t preadapt traits for future needs; extinct species were just as adapted as living ones; and complexity is not inevitable, as bacteria remain evolutionarily successful. Evidence for evolution includes fossil intermediates, molecular similarities, geographic distributions, and vestigial traits like whale leg bones and goosebumps in humans. Evolution shapes behavior through natural selection (traits that aid survival and reproduction broadly) and sexual selection (traits enhancing mating success, such as peacock tails). Importantly, selection can preserve multiple strategies simultaneously, leading to frequency-dependent or balanced polymorphisms. Sapolsky then explores frameworks for the evolution of behavior. The outdated idea of group selection—that animals act “for the good of the species”—is replaced by individual selection, where organisms maximize reproductive success. Competitive infanticide in langurs, lions, and gorillas illustrates this logic, with females countering through pseudoestrus or coalition defense. Kin selection, formulated by W.D. Hamilton, explains altruism toward relatives, from babysitting in monkeys to alarm calls in squirrels, framed by Haldane’s quip about sacrificing for “two brothers or eight cousins.” Kinship recognition arises through pheromones (MHC genes), imprinting, or inference, but in humans it often depends on subjective feelings of relatedness, leading to pseudokinship and pseudospeciation. The chapter also details reciprocal altruism, where unrelated individuals cooperate if they expect reciprocation, modeled by game theory’s Prisoner’s Dilemma. Robert Axelrod’s round-robin tournaments revealed “Tit for Tat” as the most robust strategy—cooperate initially, retaliate against cheaters, but forgive when cooperation resumes. Variants like Forgiving Tit for Tat address real-world errors. Examples across animals include grooming, cooperative hunting, and food sharing, though humans excel uniquely at large-scale reciprocity supported by laws, norms, and moral codes. Together, individual selection, kin selection, and reciprocal altruism form the “three-legged stool” of behavioral evolution. Sapolsky further distinguishes pair-bonding versus tournament species. In pair-bonders (like gibbons or prairie voles), males and females are similar in size, males invest heavily in offspring, and cuckoldry rates are high. In tournament species (like baboons or mandrills), males are larger, fight intensely, and only a few reproduce, with little paternal care. Humans fall ambiguously between these categories—mildly dimorphic, socially monogamous but prone to polygyny. He also explores parent-offspring conflict, where mothers and infants clash over weaning, and maternal-fetal conflict, driven by paternal imprinted genes urging growth versus maternal genes curbing it. The concept of multilevel selection expands the framework. While Dawkins emphasized “selfish genes,” others stress phenotypes or group-level effects. Neo–group selection shows that traits harmful to individuals can benefit groups, as with cooperative flocks of chickens outperforming aggressive “superlayers.” Human warfare and cultural competition amplify group-level selection, fostering intragroup cooperation through ethnocentrism and parochial altruism. The chapter closes by addressing controversies: critics argue sociobiology overuses adaptationist “just-so stories” and ignores punctuated equilibrium, where evolution happens in rapid bursts. Others worry about the naturalistic fallacy, where “is” is conflated with “ought.” Sapolsky balances these debates, showing evidence for both gradualism and rapid change, adaptation and exaptation, while reaffirming that evolution is the bedrock of understanding human behavior. By synthesizing genes, kinship, cooperation, and culture, this chapter establishes how evolutionary principles explain the biological roots of both our noblest and most destructive actions.