A ground squirrel gives an alarm call when it spots a predator, increasing the survival of nearby squirrels but drawing the predator's attention to itself. Hamilton's rule predicts this altruism is most likely to evolve when:
AThe alarm call benefits the group as a whole, regardless of the genetic relationship between caller and beneficiaries
BThe caller is the dominant individual, since dominant animals can afford to take greater risks
CThe recipients of the alarm call are close relatives, so that the coefficient of relatedness r is high enough that rB > C
DThe caller has already reproduced, since fitness costs are lower for individuals who have passed on their genes
Hamilton's rule states that altruism evolves when rB > C — the cost to the actor must be outweighed by the benefit to the recipient, scaled by their relatedness. For the alarm call, C is the increased predation risk to the caller, B is the survival benefit to hearers, and r is the genetic relatedness. If the squirrels nearby are unrelated (r ≈ 0), rB is trivially small and the altruism should not evolve even if B is large. The behavior evolves specifically when the beneficiaries are close relatives. Group benefit (option A) is the common misconception: natural selection acts on genes, not groups — the mechanism is relatedness, not group membership.
Question 2 Multiple Choice
In Hymenoptera (bees, ants, wasps), female workers are more closely related to their sisters (r = 0.75, due to haplodiploidy) than they would be to their own daughters (r = 0.5). Kin selection predicts:
AWorkers should preferentially invest in their own offspring rather than the queen's, since direct reproduction always maximizes fitness
BWorkers should be indifferent between helping sisters and producing daughters, since both are offspring of the same colony
CWorkers may gain higher inclusive fitness by rearing sisters than by producing their own daughters, because the relatedness asymmetry makes sibling-rearing genetically more productive
DHaplodiploidy should cause eusociality to collapse, since workers are exploited by the queen who has higher fitness
This is Hamilton's original explanation for the extreme eusociality of Hymenoptera. Since r(sister) = 0.75 > r(daughter) = 0.5, a worker can propagate her genes more efficiently per offspring-equivalent by rearing sisters than by producing her own offspring — provided the other conditions of Hamilton's rule are met. This is why worker bees forego direct reproduction: inclusive fitness through sibling-rearing can exceed inclusive fitness through own reproduction when relatedness is high enough. Option A reflects individual-selection thinking (maximize own reproduction) without accounting for inclusive fitness.
Question 3 True / False
Kin selection requires that organisms consciously recognize and calculate their genetic relatedness to potential recipients before deciding whether to help.
TTrue
FFalse
Answer: False
Kin selection operates through natural selection on genes — no cognitive calculation is required. Selection simply favors genes that produce helping behavior in contexts where relatives are statistically likely to be nearby. A ground squirrel doesn't calculate r; it just has inherited tendencies that correlate with directing alarm calls toward relatives (often because relatives live nearby). The Explainer explicitly states: 'Kin selection does not require organisms to consciously recognize and calculate their relatedness — selection simply favors genes that produce helping behavior in contexts where relatives are statistically likely to be nearby.'
Question 4 True / False
According to Hamilton's rule, an altruistic act that is too costly to evolve between distant relatives might still be favored by selection between close relatives.
TTrue
FFalse
Answer: True
This follows directly from rB > C. For a given benefit B and cost C, whether the inequality holds depends on r. If r is small (distant relatives), rB may fall below C and the act is disfavored. If r is large (close relatives), rB may exceed C and the act is favored. Haldane's quip about 'two brothers or eight cousins' captures this: with r = 0.5 for siblings and r = 0.125 for first cousins, the same amount of gene copying requires different numbers of relatives, making some altruistic acts selectively neutral or negative for distant kin but clearly beneficial for close kin.
Question 5 Short Answer
Kin selection theory resolves a puzzle that standard natural selection cannot explain. What is that puzzle, and why does shifting the unit of selection from the individual to the gene resolve it?
Think about your answer, then reveal below.
Model answer: The puzzle is the evolution of altruism: behaviors that reduce an individual's own reproductive success but increase others'. Standard natural selection, focused on individual reproductive success, predicts such behaviors should be eliminated — individuals who help at personal cost should be outcompeted by those who don't. The resolution requires shifting to gene-level thinking: what matters is not whether the individual reproduces, but whether copies of its genes reach the next generation. An altruistic individual shares a fraction r of its genes with each relative. Helping a relative reproduce is therefore equivalent — at the genetic level — to partially reproducing oneself. A gene that programs altruism toward relatives can spread if the indirect genetic benefits (rB) exceed the direct cost (C), because the gene propagates itself through relatives' reproductive success rather than (or in addition to) the actor's own.
This reframing — from organism fitness to gene propagation — is what makes kin selection theoretically coherent. The gene 'for' altruism is present in the relatives being helped; by helping them reproduce, the altruist is helping copies of that very gene spread. Selection acts on genes across multiple bodies, not just on the individual carrying a trait. This is why W.D. Hamilton defined inclusive fitness to capture effects both through the actor's own offspring and through the extra reproduction of relatives weighted by relatedness.