Permit a humble supplicant to pose a question to the biologists of this outfit. In regards to warning calls, it was said the caller announcing his location to the predator by making noise isn't too much of a disadvantage. I would agree. Even if it is a disadvantage, it probably isn't so much of a disadvantage that the trait would be deselected. Callers rarely are killed as a result of their behaviour. Therefore, even if the behaviour isn't advantageous to the individual (while it is undoubtedly advantageous to the group) would not the trait still be passed down because it is not so disadvantageous as to warrant deselection - especially if it confers a general advantage to the whole?
Cannot this logic be applied to those species that produce sterile offspring? If giving birth to some sterile offspring does not destroy utterly the parent's prospects of reproductive success, since they may produce other offspring that are fertile, why should a variation with the tendency to produce some sterile offspring be wiped out of existence if producing sterile offspring does not hinder their fitness as an individual and enhances the fitness of the family or group?
I throw this question out into the void.