A ground squirrel gives an alarm call when it spots a predator. The call attracts the predator's attention, reducing the caller's survival probability by 10%, while increasing the survival of 4 full siblings by 8% each. Does the alarm call gene spread?
ANo — the individual squirrel's fitness decreases, so natural selection eliminates the gene
BNo — natural selection only favors behaviors that benefit the whole population, which this does not
CYes — because rB (0.5 × 32%) exceeds C (10%), so the gene increases in frequency through relatives
DYes — but only because the squirrel consciously calculates the benefit to its relatives
Applying Hamilton's rule: r = 0.5 (full siblings), B = 4 × 8% = 32% total benefit to beneficiaries, C = 10% cost to actor. rB = 0.5 × 32% = 16% > C = 10%, so the alarm call gene spreads. The gene's copies in the caller's siblings benefit more than the gene's copy in the caller itself loses — the gene increases in frequency overall even though the individual pays a cost. No conscious calculation is required; the gene spreads statistically through kin-structured populations.
Question 2 Multiple Choice
In haplodiploid insects like bees and ants, full sisters share a relatedness coefficient of r = 0.75 rather than the 0.5 expected for diploid siblings. What explains this?
ASisters share both parents, while brothers share only the mother, raising the sisters' relatedness
BHaplodiploid females have twice as many chromosomes as males, increasing shared genetic material
CAll sisters inherit an identical haploid genome from their haploid father, making them share all paternal alleles with certainty
DWorker bees suppress recombination during meiosis, preventing allele shuffling among sisters
In haplodiploid systems, males (drones) are haploid — they have only one copy of each chromosome. Every egg a drone produces is genetically identical (no meiotic recombination can occur in a haploid). Therefore, all daughters of the same father share 100% of their paternal alleles (r = 1 for the paternal contribution) and share 50% of their maternal alleles on average (r = 0.5). The weighted average: (1 × 0.5) + (0.5 × 0.5) = 0.75. This elevated relatedness helps explain why worker sterility — an extreme form of altruism — evolved repeatedly in haplodiploid Hymenoptera.
Question 3 True / False
According to Hamilton's rule, an altruistic behavior can be favored by natural selection even if the actor's direct reproductive success decreases, provided that rB exceeds C.
TTrue
FFalse
Answer: True
This is the central insight of kin selection theory. Natural selection does not maximize individual reproductive success — it maximizes the spread of genes. If an altruistic act reduces the actor's own reproduction (cost C) but increases a relative's reproduction by B, and the relative shares the altruism gene with probability r, then the gene increases in frequency whenever rB > C. The actor's personal fitness declines, but the gene's overall representation in the population increases via the relative. Inclusive fitness, which sums the actor's direct fitness and the indirect fitness gained through relatives, is the quantity being maximized.
Question 4 True / False
Kin selection requires that organisms consciously recognize and deliberately favor their genetic relatives — it cannot operate in species without social cognition or memory.
TTrue
FFalse
Answer: False
Kin selection is a population-genetic process, not a psychological one. It operates whenever genes promoting kin-directed behavior increase in frequency because those genes are statistically more likely to be present in relatives. Organisms need not recognize kin consciously — spatial proximity to relatives, imprinting on nestmates, or simply living in kin-structured populations can all create the conditions for kin selection to work. Cellular slime molds and bacteria, which have no nervous systems, show kin-selected cooperation: cells sacrifice themselves to form spore-bearing stalks that benefit nearby clone-mates carrying the same genes.
Question 5 Short Answer
What does 'inclusive fitness' mean, and why does it provide a better account of altruism's evolution than classical individual fitness alone?
Think about your answer, then reveal below.
Model answer: Classical individual fitness counts only the offspring an organism produces directly. Inclusive fitness expands this accounting to include the extra offspring produced by relatives because of the organism's help, weighted by the coefficient of relatedness r. An organism's inclusive fitness = direct fitness + sum of (r × benefit conferred on each relative). This expanded accounting resolves the altruism paradox: a behavior that reduces direct reproduction but substantially boosts reproduction in close relatives can increase the frequency of the altruism-promoting gene in the population. The gene spreads not through the actor's own offspring but through copies of itself carried by the relatives the actor helped.
The key conceptual shift is to the gene as the unit of selection rather than the individual. A gene 'cares' (in the evolutionary sense) about all its copies in the population, regardless of which bodies they occupy. A gene that promotes self-sacrifice to save relatives propagates copies of itself via those relatives — the individual may lose, but the gene wins. This perspective, developed by Hamilton and popularized by Dawkins as the 'selfish gene' view, unifies kin selection with classical natural selection.