Why don't the genetic similarities between ‘unrelated’ individuals of the same and different species outweigh the differences?
Relatedness measures a specific type of genetic similarity. In particular, it measures the genetic similarity relative to the rest of the population. When $r&gt;0$, the recipient carries the actor's genes at a higher frequency than the population. When $r=0$ it doesn't mean that the actor and the recipient do not share genes; it only says that the recipient carries the genes of the actor at the same frequency as the population.