An organism produces 2 offspring of its own, and its help allows a full sibling (r = 0.5) to raise 3 additional offspring beyond what the sibling would have raised anyway. What is the organism's inclusive fitness?
A1.5 — only the indirect component counts (3 × 0.5)
B2 — only direct reproduction counts toward fitness
C3.5 — direct fitness (2) plus indirect fitness (3 × 0.5 = 1.5)
D5.5 — direct fitness (2) plus the sibling's total output (3 + 5 baseline) × 0.5
Inclusive fitness = direct fitness + indirect fitness. Direct fitness is 2 (own offspring). Indirect fitness is the *additional* offspring caused by the helper's assistance, discounted by relatedness: 3 × 0.5 = 1.5. Total = 3.5. Option D is the classic error — including the sibling's baseline reproduction (offspring she would have raised anyway) inflates indirect fitness by counting genes the actor had no causal role in propagating.
Question 2 Multiple Choice
A worker bee in a haplodiploid colony (sisters share r = 0.75) produces no offspring but helps her mother raise hundreds of sisters. Which statement best describes her fitness?
AHer fitness is zero because she has no direct reproductive output — she contributes nothing to the next generation
BHer inclusive fitness can be very high, because indirect fitness through highly related sisters contributes to gene propagation
CHer classical Darwinian fitness equals her inclusive fitness because both capture the same thing
DHer fitness depends only on how many offspring her mother produces, regardless of her own contribution
Classical Darwinian fitness counts only direct offspring — which gives the worker zero. But inclusive fitness adds the indirect component: the sisters she helps raise, each sharing r = 0.75 of her genes. Helping raise 100 extra sisters contributes 100 × 0.75 = 75 units of indirect fitness. Natural selection can strongly favor the worker phenotype even with zero direct reproduction. This is why inclusive fitness — not classical fitness — is needed to explain eusociality.
Question 3 True / False
A gene that causes an organism to sacrifice its own reproductive success could still spread by natural selection if it sufficiently boosts the reproduction of relatives carrying that gene.
TTrue
FFalse
Answer: True
This is Hamilton's foundational insight formalized in inclusive fitness theory. Natural selection tracks gene frequencies, not individual survival. If an allele that causes costly helping behavior propagates enough copies of itself through relatives to offset the direct cost, it will increase in frequency. The 'altruistic' gene can spread even while the individual expressing it reproduces less — as long as relatives carrying the same gene reproduce more.
Question 4 True / False
Inclusive fitness measures the total reproductive output of an organism and most its relatives combined.
TTrue
FFalse
Answer: False
This is a common misreading. Inclusive fitness is measured strictly from the *actor's* perspective and counts only the additional reproduction in relatives that the actor caused, weighted by relatedness. If a sibling would have raised 5 offspring anyway but raises 7 with your help, your indirect fitness contribution is (7 − 5) × r = 2r, not 7r. Counting baseline reproduction the actor had no role in producing creates double-counting and breaks the accounting logic.
Question 5 Short Answer
Why is it important to count only the *additional* offspring that an organism's help causes (rather than all of a relative's offspring) when calculating indirect fitness?
Think about your answer, then reveal below.
Model answer: Because inclusive fitness is a causal measure of the actor's contribution to gene propagation. A relative's baseline offspring would exist regardless of the actor's behavior, so counting them attributes fitness gains to the wrong source. Only the increment caused by the actor's assistance represents genes entering the next generation *because of* that actor. Counting total offspring would inflate indirect fitness, making every organism appear to have massive fitness through relatives it had nothing to do with.
The counterfactual baseline ('what would the relative have produced without my help?') is essential to keeping the accounting honest. Inclusive fitness theory is designed to explain *why* helping behavior evolved — which means tracking what the helping behavior actually causes. This precision is also what makes Hamilton's rule mathematically coherent: rB > C compares the causal benefit to the causal cost, not inflated totals.