Lecture 3: Behavioral Evolution II

What this lecture covers

This lecture continues applying individual selection, kin selection and reciprocal altruism to real animal behavior, then pushes further into more unsettling territory: competitive infanticide, intersexual conflict at the genetic level, and the circumstances under which group selection can legitimately reappear in evolutionary thinking. It closes with a sustained look at the scientific and political criticisms leveled against this entire style of reasoning.

After this lecture you should be able to explain why competitive infanticide occurs in some species and not others, describe how imprinted genes reveal a genetic conflict between mothers and fathers over fetal growth, and lay out the main criticisms of adaptationist evolutionary reasoning (heritability, adaptiveness, and gradualism) including Gould and Lewontin's "spandrel" argument. It sets up the next lecture on punctuated equilibrium and molecular genetics.

Key ideas

• Competitive infanticide: in species where the average female interbirth interval is longer than a dominant male's average tenure, incoming males benefit from killing infants sired by the previous male, which both eliminates a rival's offspring and brings the female back into ovulation sooner.
• Female countermeasures: females can defend infants (more strenuously if older, since they have less future reproductive potential), miscarry in response to a new male's scent (documented in rodents and horses), or fake ovulation (pseudoestrus) while already pregnant to prevent a new male from later recognizing and killing her offspring.
• Sex-ratio manipulation: because male reproductive success is riskier and more variable in tournament species, high-ranking females tend to skew toward sons and low-ranking females toward daughters; poor ecological conditions also shift birth ratios toward the cheaper sex (female).
• Imprinted genes: some genes are expressed only when inherited from one parent and silenced when inherited from the other. In tournament species, paternally active imprinted genes tend to push for more fetal growth while maternally active ones restrain it, reflecting a genetic tug-of-war between a father's interest in this offspring and a mother's interest in her future reproduction (David Haig's theory); this pattern is largely absent in pair-bonding species.
• Adelphic polyandry: stable cases of one female mating with multiple males occur almost only when the males are brothers, as seen in some lion prides and in traditional Tibetan fraternal marriage, both explained by kin selection.
• Group selection, reconsidered: rejected as "for the good of the species," but reintroduced narrowly through founder effects (small inbred populations fixing a cooperative trait that then outcompetes a larger population) and multilevel selection (traits disadvantageous to an individual but advantageous when carried by a whole group).
• Three core criticisms of the sociobiological framework: the assumption that behavior is heritable (challenged by "show me the gene"), the assumption that every trait is adaptive (challenged by Gould and Lewontin's concept of "spandrels," non-adaptive byproducts of other selected traits), and the assumption of gradual evolutionary change (challenged by the "punctuated equilibrium" model).

Walkthrough

Recap and predicting behavior from body size (3:08)

The lecture reviews individual selection, kin selection and reciprocal altruism from the previous session, and restates how tournament versus pair-bonding body-size patterns predict aggression, parental behavior, mate choice, and lifespan across species.

Competitive infanticide (12:15)

Drawing on Sarah Hrdy's langur monkey studies and later findings in lions and mountain gorillas, the lecture explains why males kill infants sired by other males when female interbirth intervals outlast typical male tenure, and why this threatens mountain gorillas with extinction. It notes that infanticide is suppressed when the incoming male is a close relative.

Female counterstrategies (19:18)

The lecture covers olfaction-induced miscarriage in rodents and horses, age-graded maternal and grandmaternal defense of infants, and pseudoestrus as a way pregnant females manipulate incoming males into treating a later-born infant as potentially their own.

Kidnapping, dominance and sex ratios (26:28)

Using baboon studies, the lecture describes males "kidnapping" infants likely to be their own as leverage against aggressors, nepotistic inheritance of female dominance rank, and how relatedness and social rank predict whether mothers bias toward sons or daughters, including a discussion of sex-ratio shifts under ecological stress.

Imprinted genes and intersexual conflict (41:40)

The lecture introduces imprinted genes as evidence of an evolutionary conflict between paternal and maternal genetic interests over fetal growth, with examples including insulin-like growth factor genes and diseases (choriocarcinoma, implantation failure) that appear when one parent's genetic "voice" is lost. It also covers sperm competition experiments in flies (William Rice) showing toxic sperm evolving alongside female detoxification.

Group selection and multilevel selection (59:52)

The lecture distinguishes the discredited "for the good of the species" group selection from two legitimate modern versions: founder effects, where small inbred populations fix a cooperative trait that later outcompetes a larger population, and multilevel selection (via David Sloan Wilson and E.O. Wilson's later reconciliation), where a trait harmful to an individual can be advantageous at the group level.

Criticizing the framework (1:17:00)

The lecture closes with three critiques of the whole sociobiological approach: doubts about assumed heritability without identified genes, Gould and Lewontin's "spandrel" argument that many traits (like the human chin) are non-adaptive byproducts rather than selected features, and the challenge to gradualism from the punctuated equilibrium model, plus a discussion of the field's political controversies, including the 1977 incident where protesters confronted E.O. Wilson.

Before you watch

• Watch Lecture 2 first; this lecture assumes familiarity with individual selection, kin selection, reciprocal altruism, tournament versus pair-bonding species, and the prisoner's dilemma/tit-for-tat framework introduced there.
• Basic Mendelian genetics (dominant/recessive alleles, degrees of relatedness) is assumed for the imprinted genes section.

Check your understanding

1. Under what specific demographic condition does competitive infanticide become an evolutionarily favored male strategy, and why?
2. Explain how pseudoestrus in pregnant langur monkeys can reduce the risk of a new male killing her future offspring.
3. What are imprinted genes, and how does David Haig's theory explain why paternally and maternally active imprinted genes push fetal growth in opposite directions?
4. What is a "spandrel," and how does Gould and Lewontin's argument use it to challenge adaptationist explanations of traits like the human chin?
5. Describe the two legitimate modern versions of group selection discussed in the lecture, and explain how each differs from the "good of the species" idea rejected earlier in the course.