Ever since starting to learn about the concept of inclusive fitness during my BSc, I've been bugged by this questions: if inclusive fitness can account for many ‘altruistic’ behaviors, why don't the genetic similarities between ‘unrelated’ individuals of the same and different species outweigh the differences? In other words: when we consider (from a gene-centric perspective) that individuals are temporary vehicles for selfish genes, and if we also consider that the fitness effect of different genes should not be measured in the reproductive success of just single individual, should we not expect to see that genes that promote more broad-scale altrustic behaviour since we share most of our genetic heritage with other individuals?
I understand that my confusion arises from the fact that I fail to sufficiently distinguish between behavioral genes that influence altruism and whose distribution in the population is differentially influenced by different game strategies and genes that have no direct influence on this type of behaviour and are thus not under the same type of selection pressure. However, I cannot seem to manage to produce a clear-cut argument from this distinction.
I'm asking because, when I find myself in the position of having to explain inclusive fitness, I feel a bit awkward when I know in the back of my head that I would not be able to answer this basic question if it comes up.