Difficulty: Intermediate | Prerequisites: Natural selection, sexual selection, basic understanding of fitness.
Behaviour is a phenotype, shaped by both genes and environment, and subject to natural selection just like any physical trait. The puzzle this unit tackles is altruism: why would an organism reduce its own fitness to help another? The answer lies in inclusive fitness and reciprocal altruism, two frameworks that explain how seemingly selfless behaviour can be favoured by selection. This material builds on everything in the course so far, particularly the logic of fitness and natural selection.
Social interactions can be classified by their effect on actor and recipient fitness (selfish, cooperative, spiteful, altruistic). Altruism evolves through two main routes: inclusive fitness (helping relatives who share your alleles, governed by Hamilton's Rule) and reciprocal altruism (helping unrelated individuals who are likely to return the favour).
Social interactions
Any interaction between individuals that affects the fitness of both the actor and the recipient. Classified into four types based on fitness consequences.
Selfish behaviour
Benefits the actor at the expense of the recipient. In simple terms, "I win, you lose."
Cooperative behaviour (mutualism)
Benefits both the actor and the recipient. Both individuals gain fitness.
Spiteful behaviour
Harms both the actor and the recipient. Rare in nature because it is costly to everyone involved.
Altruistic behaviour
Benefits the recipient at a fitness cost to the actor. The puzzle of evolutionary biology: why would selection favour a trait that reduces the actor's own reproductive success?
Group selection
The idea that natural selection can act on groups of organisms rather than individuals, favouring groups with more cooperative members. Largely rejected as a primary explanation for altruism because selfish individuals within the group would still outcompete altruists.
Inclusive fitness
An individual's total fitness, combining its direct fitness (own offspring) and indirect fitness (offspring produced by relatives, weighted by relatedness). This is the framework that explains kin-directed altruism.
Kin selection
Natural selection favouring behaviours that increase the reproductive success of relatives, even at a cost to the actor's own reproduction. The mechanism behind inclusive fitness.
Hamilton's Rule (Br > C)
Altruism is favoured when the benefit to the recipient (B), multiplied by the relatedness between actor and recipient (r), exceeds the cost to the actor (C). In plain terms: help your relative if the genetic payoff through them outweighs what it costs you.
Relatedness (r)
The probability that two individuals share an allele inherited from the same recent ancestor. Siblings share r = 0.5, half-siblings r = 0.25, cousins r = 0.125.
Eusociality
The most extreme form of social organisation, characterised by overlapping generations, division of labour with specialised reproductive and non-reproductive castes, and cooperative care of offspring. Found in ants, bees, termites, and naked mole-rats.
Reciprocal altruism
Altruistic behaviour between unrelated individuals, maintained because the favour is likely to be returned in the future. Think of it as "I help you now, you help me later."
Classifying social interactions
Every social interaction affects the fitness of the actor and the recipient. The four categories:
Selfish: actor gains, recipient loses.
Cooperative (mutualistic): both gain.
Spiteful: both lose.
Altruistic: actor loses, recipient gains.
Natural selection readily explains selfish and cooperative behaviours. The challenge is explaining altruism.
Inclusive fitness and kin selection
An individual's genes can reach the next generation through its own offspring (direct fitness) or through the offspring of relatives who share those alleles (indirect fitness).
Inclusive fitness = direct fitness + indirect fitness.
Kin selection is the process by which behaviours that increase relatives' reproductive success are favoured, even at a personal cost.
Hamilton's Rule: Br > C
B = benefit to the recipient (in units of offspring or fitness)
r = coefficient of relatedness between actor and recipient
C = cost to the actor
Altruism is favoured when the genetic benefit through relatives exceeds the personal cost.
Example: saving three siblings (r = 0.5 each) at a cost to yourself. Br = benefit x 0.5 x 3 relatives. If this exceeds C, the behaviour is favoured.
The closer the relationship, the more likely altruism evolves. This is why alarm calls, food sharing, and self-sacrifice are most common among close kin.
Eusociality
The extreme end of kin-selected cooperation.
Three defining features: overlap of parent and offspring generations living together, reproductive division of labour (queens and workers), and cooperative care of young.
Found in Hymenoptera (ants, bees, wasps), termites, and naked mole-rats.
In Hymenoptera, the haplodiploid sex-determination system means sisters share r = 0.75, making it genetically advantageous for workers to raise sisters rather than produce their own offspring (r = 0.5 to their own daughters).
Reciprocal altruism
Explains altruism between unrelated individuals.
The actor pays a cost now but expects a return benefit in the future.
Five conditions make reciprocal altruism more likely to evolve:
The benefit to the recipient outweighs the cost to the actor.
Individuals live in stable groups and encounter each other repeatedly.
There are repeated opportunities to reciprocate.
Cheaters (individuals who accept help but never return it) can be identified and punished.
The relationship is symmetrical: neither individual is permanently the donor or permanently the recipient.
Hamilton's Rule explains why worker bees sacrifice reproduction to serve the queen: they share more genes with their sisters (r = 0.75) than they would with their own daughters (r = 0.5). Reciprocal altruism explains cooperative behaviours in species like vampire bats, which regurgitate blood meals for roostmates who failed to feed, with the expectation that the favour will be returned.
Students often confuse group selection with kin selection. Group selection says selection acts on whole groups. Kin selection says individuals help relatives because they share alleles, and is the accepted explanation. Group selection is largely rejected as a primary driver of altruism.
A common error is thinking altruism is "unselfish" in the everyday sense. In evolutionary terms, altruistic behaviour reduces the actor's direct fitness but can increase inclusive fitness. The genes behind the behaviour can still spread.
Students sometimes forget that reciprocal altruism does not require relatedness. That is its distinguishing feature from kin selection.
Hamilton's Rule is often misapplied by forgetting that r represents the probability of sharing alleles by descent, not the proportion of the genome shared overall.
Be able to apply Hamilton's Rule to a scenario: given B, r, and C, determine whether altruism is favoured.
Know the three defining features of eusociality.
Understand the five conditions required for reciprocal altruism to evolve.
Expect a question asking you to classify a social interaction as selfish, cooperative, spiteful, or altruistic based on its fitness effects on actor and recipient.
True or false: Altruistic behaviour increases both the actor's and the recipient's fitness.
Answer: False. Altruism increases the recipient's fitness at a cost to the actor's direct fitness.
Fill in the blank: Hamilton's Rule states that altruism is favoured when ______ > C.
Answer: Br (benefit times relatedness).
True or false: Reciprocal altruism requires that the individuals be closely related.
Answer: False. Reciprocal altruism operates between unrelated individuals.
Fill in the blank: The three hallmarks of eusociality are overlapping generations, ______, and cooperative care of young.
Answer: Reproductive division of labour (specialised castes).
Q: A ground squirrel gives an alarm call when a predator approaches, putting itself at risk. Under what conditions would this behaviour be favoured by natural selection?
A: The behaviour is favoured if the nearby individuals who benefit from the warning are close relatives. By Hamilton's Rule (Br > C), the indirect fitness gained through the survival of kin, weighted by relatedness, must exceed the cost of increased predation risk to the caller.
Q: Vampire bats share blood meals with roostmates who failed to feed. These bats are not always closely related. What type of altruism is this, and what conditions support it?
A: Reciprocal altruism. It is supported because bats live in stable groups, encounter the same individuals repeatedly, the cost of sharing a meal is low relative to the benefit of receiving one when starving, cheaters who refuse to share can be identified and excluded, and the donor-recipient roles alternate over time.
Q: Why is group selection generally rejected as an explanation for altruism?
A: Within any group, selfish individuals outcompete altruists because they reap the benefits of others' altruism without paying the costs. Selfish traits therefore spread within the group faster than between-group selection can favour altruistic groups. Kin selection and reciprocal altruism provide individual-level explanations that are more consistent with observations.
Social behaviour ties directly to natural selection and fitness. Hamilton's Rule is an extension of the logic of fitness to include relatives. The concept of inclusive fitness also connects to sexual selection, because parental investment decisions (how much each sex invests) shape both mating strategies and kin-directed behaviours. Reciprocal altruism connects to game theory and the evolution of cooperation, topics that appear in more advanced ecology and behaviour courses.
Social behaviour, altruism, selfish behaviour, cooperation, spite, inclusive fitness, kin selection, Hamilton's Rule, Br > C, relatedness, coefficient of relatedness, eusociality, reciprocal altruism, group selection, vampire bats, alarm calls, haplodiploid, BBIO 180, intro biology