I believe that (in some way) genius is group-selected, which means it is for the reproductive benefit of the group, i.e. to expand the size of the group - and not for the reproductive benefit of the genius himself.
Some geniuses have been reproductively successful, such as JS Bach with his nineteen children; but it is not necessary - and almost geniuses on average have relatively lower reproductive success for the simple reason that (unlike normal people) they put most of their life-effort into their 'work'.
So, what are the benefits of genius to the group?
When it is a matter of the genius inventing a better weapon or a more productive tool, the benefits are obvious; but I think the usual benefit of a genius comes in the form of group cohesion.
This could be the explanation for artistic genius, or the genius of a storyteller - and the neglected topic of religious genius.
This group benefit of a religious innovation can be seen in the growth of the group of religious adherents, perhaps in the growth of new forms of social organization, and the results increase in (for example) economic activity or military prowess.
I got this idea from considering the difference between the Kalahari Bushmen, and Australian Aborigines. The main social difference is in group size - the Aborigines have significantly larger groups, which means that they cohere better and could assemble larger fighting forces.
And the Aborigine groups are based upon their Totemic religion, which is more fixed and more complex than the Animism of the Kalahari Bushmen (which seems to be a version of the spontaneous religion of natural men).
The Aborigine religion both requires and benefits from a more elaborate social structure of authority and learning of the legends - which must be transmitted through the generations by songs and chants.
Presumably (of course there is no direct evidence) some (or more than one) Aborigine religious genius created this Totemic social structure - and the group who adopted it was rewarded by improved cohesion, which enabled them to displace rival groups.
This is conjectural, albeit plausible - my point is that some body at some time made these religious innovations and enabled more powerful social cohesion - and could be termed a genius (on that smaller scale); and that perhaps most example of genius could be regarded a creative breakthroughs of a cohesion-generating type.
(Of course, the works of some/most modern geniuses are cohesion-destroying; and (when adopted) such breakthroughs would tend to lead to the group becoming extinct, rather than expanding - for example fertility reducing technologies, or secular ideologies. indeed, this applies to modernity itself. But until the past 100-150 years, societies containing many geniuses seem to have expanded, and gained reproductive benefit from the presence of geniuses.)