Final answer:
The maximum fitness cost a warning allele can impose on a beaver while still spreading in the population equates to the reproductive success it enables in its relatives (such as cousins and half-siblings) through kin selection.
Step-by-step explanation:
The question is asking about how a certain allele that causes a behavior (beavers warning relatives of predators by slapping their tails) can spread in a population despite it possibly reducing the fitness of the individual performing the behavior. This relates to the concept of kin selection, which is a form of natural selection that favors the reproductive success of an organism's relatives, even at a cost to the organism's own survival and reproduction. When a beaver warns relatives, it increases their chance of survival and reproduction, potentially passing on the warning allele. For an allele to spread in a population, the overall genetic contribution of that allele to the next generation must be positive. Given that the behavior helps 4 cousins and 2 half-siblings reproduce, which collectively may share a significant portion of their genes with the signaling beaver, the allele could afford up to six times the fitness loss in the signaling individual before it would fail to spread.