A prairie dog gives an alarm call when it spots a predator, attracting attention to itself and increasing its own predation risk. Under what condition does Hamilton's rule predict this behavior will be favored by natural selection?
AWhen the prairie dog is the dominant individual in the group and can afford the risk
BWhen the relatedness to recipients (r) times the benefit to them (B) exceeds the cost to the alarm-caller (C): rB > C
CWhen the group is large enough that one individual's loss is statistically negligible
DWhen the predator is likely to fail regardless, making the actual risk to the caller low
Hamilton's rule states that an altruistic act is favored when rB > C — relatedness to recipients times benefit to them exceeds cost to the actor. Prairie dog colonies are typically family groups (high r). The benefit B is the increased survival probability of multiple relatives. If this weighted benefit exceeds the cost C (increased predation risk to the caller), the alarm-calling gene spreads. Natural selection is operating on gene copies across all individuals, not just on the caller's individual survival.
Question 2 Multiple Choice
Worker bees in a hive are infertile females who spend their lives raising sisters rather than reproducing. In hymenoptera (bees, ants, wasps), females are diploid and males are haploid. How does this haplodiploidy system help explain why extreme worker altruism could evolve?
AIt makes queen bees genetically more valuable, so workers are selected to protect them at any cost
BDue to haplodiploidy, a female worker shares 75% of her genes with sisters — more than with her own offspring (50%) — so raising sisters propagates more gene copies than reproducing directly
CHaplodiploidy causes developmental sterility in workers, so eusociality is a byproduct, not an adaptation
DIt maximizes colony genetic diversity, which benefits resistance to disease rather than individual altruism
Under haplodiploidy, the father contributes identical genes to all daughters (he's haploid, so has only one set to give). This makes sisters share r = 0.75 — more than the r = 0.5 relatedness between a mother and her offspring. By Hamilton's rule, if rB > C, helping produce sisters can be more efficient at spreading gene copies than direct reproduction. This provided a genetic pre-condition that may have facilitated eusociality evolving multiple times in hymenoptera.
Question 3 True / False
From the perspective of gene-level selection, a worker bee that forgoes reproduction to help raise 10 sisters may propagate more copies of its genes than if it had raised 4 of its own offspring.
TTrue
FFalse
Answer: True
Under haplodiploidy, relatedness to sisters = 0.75 vs. relatedness to own offspring = 0.5. Helping raise 10 sisters yields 10 × 0.75 = 7.5 gene-copy-equivalents; raising 4 own offspring yields 4 × 0.5 = 2.0. The gene-level calculus strongly favors helping sisters in this case. This is the core insight: what appears to be individual sacrifice is, from the gene's perspective, a profitable investment in alternative carriers of the same gene.
Question 4 True / False
Kin selection explains altruism by showing that individuals benefit directly from helping relatives, since relatives tend to reciprocate the help.
TTrue
FFalse
Answer: False
This confuses kin selection with reciprocal altruism, which is a distinct mechanism. Kin selection does not require reciprocation or any direct benefit to the actor. The benefit is indirect — the actor's genes propagate through the relatives it helps. A ground squirrel that gives an alarm call and is killed immediately has received no direct benefit, yet the behavior can be favored by kin selection if relatives share enough gene copies. Reciprocal altruism operates among non-relatives through repeated interactions with the expectation of future returns.
Question 5 Short Answer
What does Haldane's quip 'I would lay down my life for two brothers or eight cousins' reveal about the logic of kin selection?
Think about your answer, then reveal below.
Model answer: It encodes Hamilton's rule in arithmetic. A full sibling shares r = 0.5 genes by descent; two brothers together carry on average 2 × 0.5 = 1.0 gene-copy-equivalents — the same as the actor surviving. Eight first cousins each share r = 0.125; together they represent 8 × 0.125 = 1.0. In both cases, the gene copies propagated through relatives equal those lost by the actor's death, making the sacrifice gene-level break-even. The quip reveals that 'altruism' from the gene's perspective is a calculation about where to locate copies of itself — in the actor's body or in relatives' bodies.
This gene-centered view resolves the apparent paradox of altruism without invoking group benefit or conscious motivation. The 'selfish gene' can produce selfless behavior whenever rB > C. It also predicts where altruism should be common (close kin, high benefit-to-cost ratios) and where it should be absent or weak (distant relatives, low relatedness, high cost), generating testable predictions confirmed across many animal taxa.