Lecture 2: Behavioral Evolution

What this lecture covers

This lecture asks how evolutionary logic - the same reasoning that predicts a giraffe's heart size - can predict social behavior. It replaces the popular but wrong idea that animals act "for the good of the species" with the modern framework built on maximizing copies of one's genes in the next generation, then works through the three mechanisms that achieve this: individual selection, kin selection, and reciprocal altruism.

By the end you should be able to explain why "group selection" fails as an explanation for self-sacrifice, describe how game theory (especially the prisoner's dilemma and tit-for-tat) models when cooperation and cheating are favored, and use body-size dimorphism to predict a species' mating system, aggression levels, and parental behavior. It sets up the next lecture, which continues into competitive and group-level behavior.

Key ideas

• Not group selection: the "animals sacrifice for the good of the species" idea (associated with V.C. Wynne-Edwards) is rejected; apparent self-sacrifice, like a wildebeest pushed into a river by the herd, usually reflects individuals maximizing their own genetic payoff, not altruism for the group.
• Individual selection: behavior can be explained as maximizing an individual's own reproduction - summarized in the line that a chicken is an egg's way of making another egg.
• Natural selection vs. sexual selection: natural selection favors traits that aid survival and reproduction directly; sexual selection favors traits (like ornamental coloration) that succeed only because the opposite sex prefers them, and the two can pull in opposite directions.
• Kin selection (inclusive fitness): because relatives share genes statistically (identical twins 100%, siblings 50%, cousins 12.5%), it can be genetically "worth it" to sacrifice for relatives in proportion to relatedness - captured in Haldane's line about laying down his life for two brothers or eight cousins.
• Reciprocal altruism: cooperation between non-relatives is favored when it is reciprocated, requires the ability to recognize individuals and remember past exchanges, and comes with a constant tension between cooperating and cheating.
• Prisoner's dilemma and tit-for-tat: Robert Axelrod's tournaments showed that "tit-for-tat" (cooperate first, then mirror the other player's last move) beats other strategies, though it is vulnerable to signal error, which is why "forgiving tit-for-tat" and other refinements evolved in the models.
• Tournament vs. pair-bonding species: species with large male-female body size differences (tournament species) tend to show high male aggression, high variability in male reproductive success, little male parental care, and shorter male lifespan; species with similar-sized males and females (pair-bonding) show the opposite pattern.

Walkthrough

From giraffe hearts to Nash equilibria (5:07)

The lecture opens by showing that optimality reasoning used for anatomy (why a giraffe's heart or a desert rodent's kidney is exactly the size it needs to be) can be extended to behavior. It introduces the idea of an evolutionarily optimal strategy, likened to a Nash equilibrium in a simple game like tic-tac-toe, and names the fields of sociobiology and evolutionary psychology that grew out of applying this logic to behavior.

Darwin's basic logic and the "good of the species" myth (9:09)

The lecture reviews the minimal building blocks of Darwinian evolution: heritable traits, variability among them, differential adaptiveness, and occasional mutation. It then dismantles the popular myth (Wynne-Edwards' "group selection") that animals act for the good of the species, using the wildebeest-in-the-river anecdote to show that apparent self-sacrifice is usually not voluntary or altruistic in that sense.

Individual selection and sexual selection (18:18)

The first building block, individual selection, is behavior aimed at maximizing one's own reproduction. The lecture distinguishes natural selection (traits that aid survival/reproduction) from sexual selection (traits favored purely because the other sex prefers them), noting the two can conflict, as with bright coloration that attracts mates but also predators.

Kin selection (23:25)

Using degrees of genetic relatedness, the lecture explains kin selection: it can be genetically advantageous to help relatives reproduce instead of reproducing oneself, in proportion to shared genes. Evidence comes from vervet monkey studies (Seyfarth and Cheney) showing that monkeys track who is related to whom and respond accordingly, including displaced "revenge" on a relative of a wrongdoer.

Reciprocal altruism and game theory (34:34)

The third building block covers cooperation among non-relatives, which depends on reciprocity and vigilance against cheaters. The lecture introduces the prisoner's dilemma and Robert Axelrod's computer tournaments, which identified tit-for-tat (and its refinement, forgiving tit-for-tat) as a highly successful strategy, along with related strategies like Pavlov.

Tit-for-tat in real animals (1:02:06)

The lecture shows experiments demonstrating tit-for-tat-like behavior in vampire bats (which stop sharing regurgitated blood with a female made to appear as if she's hoarding it), stickleback fish tricked with mirrors, and sex-changing black hamlet fish. It then introduces complications from the real world, such as naked mole rats and lions that seem to violate simple reciprocity but turn out to be playing multiple simultaneous "games" or dividing roles.

Predicting mating systems from body size (1:18:18)

Applying the three building blocks together, the lecture works through how the degree of size difference between males and females in a species predicts aggression levels, variability in male reproductive success, female mate choice criteria, male parental investment, lifespan differences, twinning rates, and rates of female desertion, distinguishing "tournament species" from "pair-bonding species."

Where humans fit (1:32:31)

The lecture closes by placing humans between the tournament and pair-bonding extremes on most measures, and briefly discusses economically driven versus demographically driven polygamy across human cultures as evidence of this in-between status.

Before you watch

• This lecture builds directly on Lecture 1's introduction to the course; no other prior background is assumed, though basic familiarity with genetics (percent of genes shared between relatives) helps with the kin selection section.
• Familiarity with the general idea of natural selection is useful going in.

Check your understanding

1. Why does the "animals behave for the good of the species" explanation fail, and what replaces it?
2. Using Haldane's line about brothers and cousins, explain the mathematics behind kin selection.
3. What made tit-for-tat successful in Axelrod's tournaments, and why did "forgiving tit-for-tat" later outcompete it?
4. Using body size dimorphism, predict the aggression level, parental care, and lifespan pattern you would expect in a tournament species versus a pair-bonding species.
5. Give one animal example from the lecture where straightforward reciprocal altruism initially seemed to break down, and explain what turned out to be happening.